Tag Archives: exoplanet

New planet found “next door” to our solar system

At one quarter the mass of the Earth, the newly-discovered planet is not only one of the closest planets we know of, but also one of the lightest. The planet is named Proxima d.

Artist’s impression of the newly-discovered planet. Image credits: Credit: ESO/L. Calçada.

Hey planet, here’s an ESPRESSO

In 1915, the Scottish astronomer Robert Innes discovered a new star. He called it Proxima Centauri (or rather, Proxima Centaurus).

Proxima Centauri is the closest star to Earth, lying just over four light-years away — and will continue to be so for about 25,000 years, after which Alpha Centauri A and Alpha Centauri B will move closer to our solar system and will take alternating turns as the “closest star to Earth” (for about 80 years each).

But it took another hundred years after the star was named for the first planet in the Proxima Centauri solar system to be discovered. Astronomers are nothing if not methodical, so in 2016, when they discovered a planet, they called it Proxima b. They found another planet candidate in 2019 which they called Proxima c. Now, they’ve discovered a new planet and named it (you’ve guessed it) Proxima d.

“The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration,” explains João Faria, a researcher at the Instituto de Astrofísica e Ciências do Espaço, Portugal and lead author of the study published today in Astronomy & Astrophysics.

The planet was first discovered in 2020, and was now confirmed with the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO).

“After obtaining new observations, we were able to confirm this signal as a new planet candidate,” Faria says. “I was excited by the challenge of detecting such a small signal and, by doing so, discovering an exoplanet so close to Earth.”  

The planet was discovered using a less common method. Because planets don’t emit their own light, researchers rely on indirect information to find them. Most commonly, they use a method called the transit method — basically, they measure the luminosity coming from a star and look for dips in luminosity caused by planets passing in front of that star. But Proxima d was discovered using the radial velocity technique.

The technique works by detecting tiny wobbles in the motion of the star — wobbles created by a planet’s gravitational pull. With this, they can not only detect the presence of a star but also calculate its mass.

A depiction of the radial velocity method showing how a smaller object (such as an extrasolar planet) orbiting a larger object (such as a star) could produce changes in position and velocity of the latter as they orbit their common center of mass (red cross). Image via Wiki Commons.

“This achievement is extremely important,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile. “It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it.”

“This result clearly shows what ESPRESSO is capable of and makes me wonder about what it will be able to find in the future,” Faria adds.

The gravitational effect of Proxima d is pretty small — it only causes Proxima Centauri to wobble by around 40 centimeters per second (1.44 km/hour) — and it’s striking that astronomers can detect these small differences from 4 light-years away. Based on this, researchers calculated that the planet is around one-quarter the mass of the Earth and one of the lightest exoplanets ever found.

This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from pictures forming part of the Digitized Sky Survey 2. The blue halo around Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in color like the Sun. Image credits: Digitized Sky Survey 2. Acknowledgment: Davide De Martin/Mahdi Zamani.

The planet does not lie in the habitable zone. Although the star is a red dwarf star with a mass around 8 times lower than that of the Sun, the planet simply orbits the star too closely. Assuming an Earth-like reflectivity of the planet, the surface temperature would be 87 °C (188 °F) — too hot to support life as we know it. Another Proxima Centauri planet (Proxima b) could lie in the habitable zone, but this is still disputed by astronomers.

Researchers expect more intriguing data to come from ESPRESSO’s search for other worlds, especially as it will soon be complemented by ESO’s Extremely Large Telescope (ELT), currently under construction in the Atacama Desert. Together, these two will enable researchers to discover and study many more planets around nearby stars.

The study was published in the journal Astronomy and Astrophysics.

Astronomers spy oblong-shaped planet

WASP-103b bucks the norm when it comes to planet shapes.
Artist impression of WASP-103b and its host star. (Image: ESU)

Most planets in this universe are spherical, and for good reason. Forces of gravity generally pull equally from all sides, from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere. Some can be more spherical than others (Earth is flattened at the poles and bulges at the equator) depending on their distance from the sun and speed of rotation, but generally all are alike.

However, the European Space Agency’s exoplanet mission Characterizing Exoplanets Satellite (Cheops) has revealed a planet with a deformed shape more like that of a rugby ball than a sphere. This planet, coined WASP-103b and which has a density 1.5 times that of Jupiter, is located in the Hercules constellation approximately 1,225 light-years away from our oblate spheroid home.

“It’s incredible that Cheops was actually able to reveal this tiny deformation,” says Jacques Laskar of Paris Observatory, Université Paris Sciences et Lettres, and co-author of the research. “This is the first time such analysis has been made, and we can hope that observing over a longer time interval will strengthen this observation and lead to a better knowledge of the planet’s internal structure.”

The deformation is caused by gravitational tugs from WASP-103, its host star. The planet lies a mere 1.8 million miles away from WASP-103, which is both hotter and 1.5 times larger than our Sun (by comparison, Earth is around 93 million miles from the Sun). Astronomers have suspected that such close proximity would cause monumental tides, but up until now they haven’t been able to measure them.

Using new data from Cheops, which measures exoplanet transits — the dip in light caused when a planet passes in front of its star from our point of view — along with data already obtained by the Hubble Space Telescope and Spitzer Space Telescope, researchers were able to detect how tidal forces deform the exoplanet from a usual sphere into the rugby ball shape.

The team was able to utilize the transit light curve of WASP-103b to obtain a boundary—the Love number—that determines how mass is distributed within a planet. Understanding how mass is distributed can reveal details on the internal structure of the planet.

“The resistance of a material to being deformed depends on its composition,” explains Susana Barros of Instituto de Astrofísica e Ciências do Espaço and University of Porto, Portugal, and lead author of the research. “For example, here on Earth we have tides due to the Moon and the sun but we can only see tides in the oceans. The rocky part doesn’t move that much. By measuring how much the planet is deformed we can tell how much of it is rocky, gaseous or water.”

When they calculated the Love number for WASP-103b, the researchers discovered an exoplanet larger than our Solar System’s largest inhabitant, which suggested that the internal structure is similar, despite the exoplanet having almost twice the radius.

“In principle, we would expect a planet with 1.5 times the mass of the Jupiter to be roughly the same size, so WASP-103b must be very inflated due to heating from its star and maybe other mechanisms,” Susana said. “If we can confirm the details of its internal structure with future observations maybe we could better understand what makes it so inflated. Knowing the size of the core of this exoplanet will also be important to better understand how it formed.”

While it’s fairly certain that WASP-103b isn’t the only deformed planet out there, it is the most conclusive evidence to date. Researchers hope that further work, including from the James Webb Space Telescope, can provide more certainty on where those might lie within the vast universe.

The research was published in Astronomy & Astrophysics.

AI is so good it can spot molecules on exoplanets. One day, it may discover new laws of phyics

Artist’s impression of exoplanet KELT-11 b. Impression by Léa Changeat., Author provided

Do you know what the Earth’s atmosphere is made of? You’d probably remember it’s oxygen, and maybe nitrogen. And with a little help from Google you can easily reach a more precise answer: 78% nitrogen, 21% oxygen and 1% argon gas. However, when it comes to the composition of exo-atmospheres – the atmospheres of planets outside our solar system – the answer is not known. This is a shame, as atmospheres can indicate the nature of planets, and whether they can host life.

As exoplanets are so far away, it has proven extremely difficult to probe their atmospheres. Research suggests that artificial intelligence (AI) may be our best bet to explore them – but only if we can show that these algorithms think in reliable, scientific ways, rather than cheating the system. Now our new paper, published in the Astrophysical Journal, has provided reassuring insight into their mysterious logic.

Astronomers typically exploit the transit method to investigate exoplanets, which involves measuring dips in light from a star as a planet passes in front of it. If an atmosphere is present on the planet, it can absorb a very tiny bit of light, too. By observing this event at different wavelengths – colours of light – the fingerprints of molecules can be seen in the absorbed starlight, forming recognisable patterns in what we call a spectrum.

A typical signal produced by the atmosphere of a Jupiter-sized planet only reduces the stellar light by ~0.01% if the star is Sun-like. Earth-sized planets produce 10-100 times lower signals. It’s a bit like spotting the eye colour of a cat from an aircraft.

In the future, the James Webb Space Telescope (JWST) and the Ariel Space Mission, both probes that will investigate exoplanets from their orbit in space, will help by providing high-quality spectra for thousands of exo-atmospheres. But while scientists are excited about this, the latest research suggests it may be tricky. Due to the complex nature of atmospheres, the analysis of a single transiting planet may take days or even weeks to complete.

Naturally, researchers have started to look for alternative tools. AI are renowned for their ability to assimilate and learn from a large amount of data and their superb performance on different tasks once trained. Scientists have therefore attempted to train AI to predict the abundance of various chemical species in atmospheres.

Current research has established that AIs are well-suited for this task. However, scientists are meticulous and sceptical, and to prove this is really the case, they want to understand how AIs think.

Peeking inside the black box

In science, a theory or a tool cannot be adopted if it is not understood. After all, you don’t want to go through the excitement of discovering life on an exoplanet, just to realise it is simply a “glitch” in the AI. The bad news is that AIs are terrible at explaining their own findings. Even AI experts have a hard time identifying what causes the network to provide a given explanation. This disadvantage has often prevented the adoption of AI techniques in astronomy and other scientific fields.

We developed a method that allows us a glimpse into the decision-making process of AI. The approach is quite intuitive. Suppose an AI has to confirm whether an image contains a cat. It would presumably do this by spotting certain characteristics, such as fur or face shape. To understand which characteristics it is referencing, and in what order, we could blur parts of the cat’s image and see if it still spots that it is a cat.

How an AI’s predictions works for blurred cat image (please click on the image to enlarge it). Author provided

This is exactly what we did for an exoplanet-probing AI by “perturbing”, or changing, regions of the spectrum. By observing how the AI’s predictions on the abundances of exoplanet molecules changed (say water in the atmosphere) when each region was doctored, we started to build a “picture” of how the AI thought, such as which regions of the spectrum it used for deciding the level of water in the atmosphere.

We can combine features highlighted by the AI together with the original image to produce what we called a sensitivity map which outlines the areas it is looking closely at. Author provided

Reassuringly for us astronomers, we found that a well-trained AI relies heavily on physical phenomena, such as unique spectroscopic fingerprints – just like an astronomer would. This may come as no surprise, after all, where else can the AI learn it from?

In fact, when it comes to learning, AI is not so different from a cheeky high-school student – it will try its best to avoid the hard way (such as understanding difficult mathematical concepts) and find any shortcuts (such as memorising the mathematical formulae without understanding why) in order to get the correct answer.

If the AI made predictions based on memorising every single spectrum it had come across, that would deeply undesirable. We want the AI to derive its answer from the data, and perform well on unknown data, not just the training data for which there is a correct answer.

This finding provided the first method to have a sneaky peek into so-called “AI black-boxes”, allowing us to evaluate what the AIs have learnt. With these tools, researchers now can not only use AIs to speed up their analysis of exo-atmospheres, but they can also verify that their AI uses well-understood laws of nature.

That said, it’s too early to claim that we fully understand AIs. The next step is to work out precisely how important each concept is, and how it gets processed into decisions.

The prospect is exciting for AI experts, but even more so for us scientists. AI’s incredible learning power originates from its ability to learn a “representation”, or pattern, from the data – a technique similar to how physicists have discovered laws of nature in order to better understand our world. Having access to the minds of AI may therefore grant us the opportunity to learn new, undiscovered laws of physics.

Kai Hou (Gordon) Yip, Postdoctoral Research Fellow at ExoAI, UCL and Quentin Changeat, Postdoctoral Research Fellow in Astronomy, UCL

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Stellar pollution shows exoplanets are more diverse than we thought — but few are like Earth

Artist’s impression of an exoplanet. Image credits: IAU/L. Calçada.

For Earth to be able to support life, a lot of things needed to be just right. The Sun had to be the right size and brightness, and just at the right distance from the Earth; the atmosphere that protects the planetary surface from harmful radiation; the chemistry needed for water and the seeds of life; and a crust and plate tectonics. We don’t often think about plate tectonics as a key ingredient for life, but it is. Without a crust, plate tectonics couldn’t exist, and without plate tectonics, life as we know it couldn’t exist.

The crust is where dry, hot rock from the deeper parts of the Earth interact with the water and air on the surface, producing new minerals and rocks. New crust is constantly being produced and destroyed, and if this didn’t happen, the seafloor could become rigid and much more unfriendly to life. New research is suggesting that plate tectonics is essential to life as we know it.

There are also two types of crust on Earth: basaltic and granitic. The basaltic crust is dark and heavy, and sometimes called oceanic crust. Meanwhile, granite crust is light and accumulates into continent-sized rafts that move in this “sea of basalt.” It’s sometimes called continental crust. When an oceanic crust moves against the continental crust, the heavier and denser oceanic one subducts (or goes under the other one).

But our planet’s crust may be a rarity, at least in our corner of the universe.

Image credits: USGS.

Keith Putirka from California State University, Fresno, and Siyi Xu of the Gemini Observatory analyzed the atmospheres of 23 nearby white dwarfs, looking for signs of so-called “pollution” — chemical traces that stars can pick up from nearby planets as they explode into red giants.

“White dwarfs start out like our Sun, and late in life expand to become a red giant, and then collapse to a very small size – about the size of Earth,” Putirka told ZME Science. “As the star collapses, planets orbiting the star (at least those not obliterated during the red giant phase) can orbit close enough to the star that they are destroyed by tidal forces. The debris that results can fall into the star’s atmosphere. This infalling debris is the “pollution” that is measured by astronomers, and records the composition of the formerly orbiting planetary objects.”

The researchers found that some contain high amounts of calcium (Ca), but all have very low silicon (Si) and high magnesium (Mg) and iron (Fe) amounts. This suggests a composition closer to the mantle of exoplanets, and not at all what you’d expect to see from a planetary crust.

Roughly speaking, the Earth consists of a crust, a mantle, and a core. Although the movement of plate tectonics is driven by movement from the mantle, it’s the crust that is fragmented into rigid plates (hence the “plate” tectonics). But there seems to be no sign of a granitic crust — or even other crust types. So plate tectonics may have not existed on these planets, or if it did, it was very different from what we see on Earth.

Magma circulation driving the movement of plate tectonics.

“It’s hard to say whether granitic crusts might exist on other planets, or not,” Putirka explained to ZME Science. “In our Solar System, granitic crust only exists in any great abundance on Earth, and its abundance is probably related to our abundant surface water and plate tectonics, which are also unique to Earth. The exoplanets that once orbited white dwarfs have silicate mantles (all the rocky material between the iron core and the crust) that are very different from Earth – so different that plate tectonics and crust formation might occur very differently.”

“Some exoplanets may have mantle compositions that might yield very thick granitic crusts and more abundant continental material than on Earth. Others have mantle compositions that might not produce any continental crust at all. Many of these planets may look totally unlike anything we see in our inner Solar System.”

More questions

The findings have important implications for potential life on other planets — but it’s hard to interpret just what this means. But what is clear is that we need to have a broader view when we consider exoplanet environments.

“I think it’s fair to say that the trajectory of biological evolution is dependent upon geologic history. For example, if a planet has abundant water, but no granitic crust, then nothing like the terrestrial life as we see on Earth could possibly evolve – because there would be no terra firma for such evolution to take place. But we don’t yet know how such odd exoplanets (in the white dwarf database) might evolve from a geologic standpoint because, up to now, we have focused our laboratory experiments (on how rocks melt or deform) based on questions about how Earth-like (or Mars-like or Moon-like) planets might evolve. But the compositions we see in the white dwarf data indicate planets that are mineralogically very different, and so require new experimental studies.”

Ultimately, this study still shows that there’s plenty we don’t know about the Earth — if we did, we’d have a better idea of what makes it so special (if anything). There are still plenty of questions we need to answer about our planet, and only once we do (and once we study our solar system neighborhood more closely) will we be able to understand planets outside of our solar system as well.

“My take on our findings is that it reflects back on what we still don’t know about Earth and our rocky planetary neighbors.,” Putirka concludes “Earth not only has abundant water and life, but two very distinct kinds of crust – one of which (granitic, continental crust) is effectively unique in our solar system, and is essential for human evolution. Earth is also the only planet that has a long history of plate tectonics.”

“How and why did these features appear on Earth, and what inhibited their development elsewhere in our own Solar System? Tectonics and crust formation are surely sensitive to planet size, orbital radius, and planet composition. To what extent can we change any of these parameters and still end up with a habitable planet – and/or something that, geologically, looks roughly like Earth? We’ll have better answers to these questions when we conduct new experiments, and when we start exploring Venus, and when we shift our focus from trying to find life on Mars to instead better understanding the geological conditions that limited evolution there in the first place.”

The study “Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood” was published in Nature Communications.

Astronomers find hidden planets in distant radio signals

Radiowave telescopes have the advantage that sunlight, clouds, and rain do not affect observations. (Photo: Pixabay)

Benjamin Pope and his Dutch team at the observatory ASTRON, home to the world’s most powerful radio antenna, found some unexpected radio waves coming from distant stars. Does this mean that there are some hidden planets out there? Turns out there could be.

While searching for red dwarf stars using the Low Frequency Array, the world’s most powerful radio telescope, the team discovered four magnetically inactive stars, a finding which bucked the conventional understanding of what astronomers can find with radio waves.

“We’ve discovered signals from 19 distant red dwarf stars, four of which are best explained by the existence of planets orbiting them,” Pope said. “We’ve long known that the planets of our own solar system emit powerful radio waves as their magnetic fields interact with the solar wind, but radio signals from planets outside our solar system had yet to be picked up.”

Leiden University professor Joseph Callingham, lead author of the report, said that the team is confident these signals are coming from the magnetic connection of the stars and unseen orbiting planets, similar to the interaction between Jupiter and its moon, Io. The duo contains strong aurorae due to Io’s volcanic activity which blasts material out into space, material that fills Jupiter’s environment and creates a strong magnetic pull between the two bodies, not unlike aurorae here on Earth.

“Our own Earth has aurorae, commonly recognized here as the northern and southern lights, that also emit powerful radio waves – this is from the interaction of the planet’s magnetic field with the solar wind,” said Callingham. “Our model for this radio emission from our stars is a scaled-up version of Jupiter and Io, with a planet enveloped in the magnetic field of a star, feeding material into vast currents that similarly power bright aurorae.”

Radio telescopes study the radio waves originating from planets, comets, giant clouds of gas and dust, stars and galaxies. Astronomy using radio waves has the advantage that sunlight, clouds, and rain do not affect observations. The method of discovery has found many new types of objects including pulsars, the rapidly spinning neutron stars that are collapsed cores of massive stars that have exhausted their fuel.

Since radio waves are longer than optical waves, radio telescopes are made differently than the telescopes used for visible light. Radio telescopes must be physically larger than an optical telescopes in order to make images of comparable resolution.

Next on the team’s docket is verification that planets are indeed there and the observed signals aren’t some anomaly.

“We can’t be 100% sure that the four stars we think have planets are indeed planet hosts, but we can say that a planet-star interaction is the best explanation for what we’re seeing…This discovery is an important step for radio astronomy and could potentially lead to the discovery of planets throughout the galaxy.”

Astronomers find moon-forming disk around exoplanet

Artist impression of the circumplanetary disk surrounding PDS 70c. Credit: ESO/ALMA.

In the early 1990s, scientists knew about a handful of exoplanets (planets outside the solar system). Since then, more than 4,000 exoplanets have been confirmed with thousands more up for investigation. Indeed, technology and astronomers’ skills have grown tremendously. So much so that we can now peer inside certain exoplanets and determine their composition or atmosphere, as well as tell whether some have moons orbiting them. Now, astronomers have upped their game once more, reporting the discovery of a disk of gas and matter surrounding a planet that is supposed to coalesce into a new moon.

The novel discovery was made in the PDS70 star system, located relatively closeby, about 370 light-years from Earth in the constellation Centaurus. Astronomers working with the European Southern Observatory’s (ESO) Atacama Large Millimeter/submillimeter Array (ALMA) found that the system consists of at least two huge Jupiter-sized planets, along with a dust-rich circumstellar disk about as large in width as the distance from the Sun to Earth’s orbit.

Both gas giants feed on the dust disk, funneling material towards them by gravity. So, essentially, these young planets, unceremoniously named PDS 70b and PDS 70c, are still a work in progress.

“More than 4,000 exoplanets have been found until now, but all of them were detected in mature systems,” says Miriam Keppler, co-author of the new study and researcher at the Max Planck Institute for Astronomy in Germany. “PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed.”

This image shows wide (left) and close-up (right) views of the moon-forming disk surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away. The close-up view shows PDS 70c and its circumplanetary disk center-front, with the larger circumstellar ring-like disk taking up most of the right-hand side of the image. Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

But the researchers noticed something else too. When they zoomed in on the high-resolution observations in submillimeter light performed by ALMA, the astronomers uncovered a circumplanetary disk surrounding PDS 70c. The disk was so well defined that its size could be ascertained, being roughly 500 times larger than Saturn’s rings.

This moon-making disk is most likely made of the same material as the much larger looming circumstellar disk that was collected by PDS 70c as the planet swept its orbit. Over millions of years, the researchers believe all of this matter will join together to form a new satellite, similar to how planets form around the sun from the much larger circumstellar disk. In fact, there may be enough material to make three satellites the size of Earth’s Moon.

Subsequent observations should serve to confirm that the object in question around PDS 70c is indeed a circumplanetary disk. If that’s the case, these observations could prove invaluable in clarifying how exomoons form and validating existing theories concerning their formation. ESO’s Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Chilean Atacama desert, will be ideal for this task.

“The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail,” says co-author Richard Teague, a co-author and Submillimeter Array (SMA) fellow at the CfA.

The findings were reported in The Astrophysical Journal Letters.

The largest planetary ring system we’ve found would dominate the sky — if it was in our solar system

When talking about planetary ring systems, Saturn and Jupiter likely spring to mind — they are our closest ringed neighbors, after all. But although impressive, their rings aren’t that large, in the grand scheme of things. Jupiter’s aren’t that large even when judging only by our Solar System. Neptune and Uranus also have rings, but they’re tinier.

A picture of Saturn’s ring structure created using data from the Cassini craft on April 19 2017. Image credits Kevin Gill / Flickr.

Luckily, the Universe is a huge place, and there’s no shortage of beautiful ring systems to enjoy. There are also plenty of grand, sprawling ones to take your breath away. So, today, let’s take a look at what these ring systems actually are, how they form, and the biggest ones we’re spotted so far.

What even are planetary ring systems?

Every stellar body generates a gravitational field. Large, dense ones create a strong pull; giant, ponderous planets generate an immensely strong pull. It’s these large planets, typically gas giants, which sport planetary ring systems. It’s not all that different from how our Earth sports a Moon.

Ring systems are sprawling fields of material such as rocks, minerals, and ice. They look like wispy sheets of material, but up close, these are massive structures. They’re not particularly thick (Saturn’s rings, for example, are probably only 50 meters high) but they go all the way around a planet, most often in several different ‘rings’ each at different distances from the planet.

The exact size of the particles in a ring system is dependent on several factors: the material these particles are made of (mainly its specific strength and density), how far it is from the planet, and the strength of tidal forces at that altitude. In other words, rings are made up of particles in all kinds of shapes and sizes; the planet’s gravitational pull and rotation will try to grind them down, while a material’s toughness and its distance from the planet will help it survive in larger chunks. The materials they’re made of aren’t consistent — it’s all related to how the system and planet formed, and their history since. It’s generally gas, dust, and ice, but according to NASA, such particles can be “as large as mountains”.

Lastly, know that although we call them ‘planetary’ ring systems, they don’t only form around planets. Minor planets, moons, unignited stars (brown dwarfs) can also sport ring systems. There is even some evidence of a similar structure residing in the void between Venus and Mercury.

How do they form?

As far as we understand, there are three main ways for a planet to get the material that makes up its rings.

The first one is that they simply ‘gathered’ it in the early days of the system they inhabit — the so-called accretion stage. In this phase, a star system resembles a disk churning around its star. In time, pockets of matter come together (accrete), gaining gravitational strength, which keeps drawing more material in. This is how stars and planets form (the star just forms first and can thus grab most of the matter in the disk).

If a planet forms early and gets large enough, it can start drawing in material from around it, preventing it from accreting into the forming planets. Instead, it forms a ring system under the hosts’ tidal forces (gravitational pull + rotational movement). This is exactly the same process that builds planets around a star from the primordial dust, only at a much smaller scale.

Detail of Saturn’s shepherd moons, created using data from Cassini. Image credits NASA-JPL / Caltech.

The second way is to use your gravity to capture asteroids or moons or let some form near you and pull them inside your Roche limit. This is a theoretical boundary beyond which a planet’s tidal forces will break apart any other stellar body. Inside this range, moons and asteroids will be ground into dust. The Roche limit also dictates how far a body’s influence extends during the accretion phase — nothing can form within this boundary due to the extreme tidal forces present.

The third way generally doesn’t form very large ring systems by itself. It involves a planet capturing any material produced by asteroids crashing into its moons, or materials produced by volcanic processes that make it into space (from traditional or cryovolcanoes). Compared to the previous two, such capture processes involve minute amounts of matter.

One of the final elements that can help produce and maintain a planetary ring system is shepherd (or ‘watcher’) moons. These are larger bodies that orbit through or on the edges of rings. Their gravity pulls at particles in the ring as they orbit, which helps to maintain the shape of the rings — they ‘herd’ the ring particles, hence their names. If you see any empty strips in a ring system, it’s very likely that a shepherd moon created it.

The movements of such a moon through the ring are truly a thing of beauty. As they orbit, shepherd moons form ripples through the ring, like the wake of a boat traveling over a calm lake. This only makes the thought that such moons tend to be short-lived that much sadder. They’re generally inside their host’s Roche limit, so they will eventually be broken apart and ground down.

Now that we have a better understanding of what they are and how they form, let’s take a look at the biggest, most impressive ring system we’ve found so far.


Artistic rendering of the exoplanet and its impressive rings. Image via Wikimedia.

Discovered way back in 2015, J1407b boasts the largest ring system we’re ever seen — around 200 times larger than Jupiter’s (the largest in our solar system). The planet that hosts it is equally immense: we don’t exactly know whether it’s a gas giant or a brown dwarf. So far, it’s been referred to as a super-Jupiter type of stellar body.

To give you an idea of just how stupidly massive this ring system is, if Saturn had the same rings, they would be many times larger in diameter than the moon in the night’s sky. It’s not only that you could see it easily with the naked eye — it would pretty much dominate the view. All in all, the exoplanet boasts some 30-odd layers of rings.

“It’d be huge. You’d see the rings and the gaps in the rings quite easily from Earth,” said Matthew Kenworthy of the Leiden Observatory in the Netherlands, one of the co-authors on the paper describing the findings, at the time. “It’d be several times the size of the full moon.”

Maybe the size of its rings helped too because J1407b was the first confirmed case of an exoplanet with a ring system. So far, it’s also the only exoplanet with rings that we’ve spotted.

Still, in cosmological terms, such lush manes of rings do not last for long. Researchers expect them to get progressively thinner and disappear in the next several million years as new moons form from the sheer quantity of material zipping and zapping through J1407b’s rings. Compared to planets in our solar system, J1407b is also very young, at only about 16 million years old. The Sun and Earth are 4.5 billion years old.

So it might be just youthful energy that makes large ring systems possible. Right now, we simply don’t know. The methods we use to spot exoplanets (planets outside our solar system) aren’t very good at all at picking up ring systems — they can do it, but there’s a lot of luck involved.

For now, our best knowledge of planetary ring systems come from our neighboring planets. There may well be larger rings than those boasted by J1407b out there, but until we can get a better view into deep space — or, even better, make our way there — they will likely remain undiscovered.

An artist's conception of HD 209458 b, an exoplanet whose atmosphere is being torn off at more than 35,000 km/hour by the radiation of its close-by parent star. This hot Jupiter was the first alien world discovered via the transit method, and the first planet to have its atmosphere studied. [NASA/European Space Agency/Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS)]

Stellar flares can strip away the atmosphere of planets, make them less habitable

As humanity continues to explore planets beyond the solar system — exoplanets — investigations into conditions on these worlds become increasingly complex. This includes the question of whether these exoplanets can support life. 

New research has identified which stars would be most likely to host planets with the necessary conditions for habitability, based upon that star’s stellar activity and crucially the rate at which such activity strips away a planet’s atmosphere. 

“We wanted to figure out how planets lose their atmospheres from extreme ultraviolet radiation and estimate their impact on their potential to host life,” Dimitra Atri, a researcher from the Space Science at NYU Abu Dhabi (NYUAD), tells ZME Science. “We focused on a channel of escape called hydrodynamic escape where stellar radiation heats up the planet’s atmosphere and a part of it escapes into space.”

An artist's conception of HD 209458 b, an exoplanet whose atmosphere is being torn off at more than 35,000 km/hour by the radiation of its close-by parent star. This hot Jupiter was the first alien world discovered via the transit method, and the first planet to have its atmosphere studied. [NASA/European Space Agency/Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS)]
An artist’s conception of HD 209458 b, an exoplanet whose atmosphere is being torn off at more than 35,000 km/hour by the radiation of its close-by parent star. This hot Jupiter was the first alien world discovered via the transit method, and the first planet to have its atmosphere studied. [NASA/European Space Agency/Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS)]

Atri is the author of a paper published in the journal Monthly Notices of Royal Astronomical Society: Letters, which analyzes flare emissions using data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) observatory ultimately helping to determine where else in the Universe life is most likely to prosper.

Harbouring Life: A Question of Water Retention

Planet habitability is closely associated with that world’s ability to hold liquid water. That means that factors which can boil away that water or cause it to be lost to space reduce that habitability. The habitable zone of a star’s environment is defined as the range at which a planet can orbit and still possess liquid water. This means not too hot or too cold — criteria that led to the alternative name for such regions, the Goldilocks zone.

Yet, distance and a star’s luminosity are not the only factors which can affect a planet’s ability to hold liquid water. Space weather — including solar flares — is another determining element, one that as of yet is not well understood. “Flares erode planetary atmospheres,” Atri says. “A substantial atmosphere is needed to sustain liquid water on a planet’s surface. Flares reduce those chances and make planets less habitable.”

Betelgeuse a type H 1-2 star similar to those Atri found jettison frequent XUV flares which can strip an exoplanet’s atmosphere reducing conditions for habitability (NASA/SDO)

What Atri, alongside coauthor and graduate student Shane Carberry Mogan, discovered was that whilst luminosity from a star was still the primary driving factor in atmosphere stripping, flares were a more important factor for some stars than others. In particular, they discovered that flares from M0-M4 stars — cool, red stars like Betelgeuse — were more likely to strip an orbiting planet’s atmosphere. 

The duo determined that more frequent, lower energy flares in the extreme ultraviolet region (XUV) of the electromagnetic spectrum were more effective at stripping a planet’s atmosphere and thus reducing its habitability than less frequent, higher energy outbursts. XUV radiation strikes a planet’s atmosphere heating it. This causes hydrodynamic escape, pushing out light atoms first, which through collision and other drag effects also pull out heavier molecules. 

“We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares,” Atri explains. “However, flares dominate the loss mechanism of around 20 per cent of M4–M10 stars.

“M0–M4 stars are most likely to completely erode both their proto- and secondary atmospheres, whilst M4–M10 stars are least likely to erode secondary atmospheres.”

The study also highlights the fact that better modelling of the factors that affect an exoplanet’s atmosphere is needed. Determining the systems and planets most likely to harbour life will play an important factor in selecting targets for the upcoming James Webb Space Telescope — set to launch on October 31st 2021 — and the ESO’s Extremely Large Telescope (ELT) currently under construction in the Acatma desert, Chile. 

“The next research step would be to expand our data set to analyze stellar flares from a larger variety of stars to see the long-term effects of stellar activity, and to identify more potentially habitable exoplanets,” adds Atri.

The researcher also points out that the continued investigation of how planets lose their atmosphere could also focus on a target closer to home, our nearest neighbour, Mars. “Since it is extremely difficult to observe the escape process in exoplanets, we are planning to study this phenomenon in great detail on Mars with the UAE’s Hope mission,” the researcher says, explaining how observations from Mars missions can be used to better understand atmospheric escape and how this knowledge can be applied to exoplanets.“We will then apply our understanding of atmospheric escape to exoplanets and estimate the impact of extreme UV radiation on planetary habitability.”

An artist’s illustration of UAE’s Tess probe approaching the surface of Mars. Atri believes this investigation could yeild important data about exoplanet habitability.
(Mohammed bin Rashid Space Centre)

Further to the question of habitability, the study begins to address the wider question of the dynamics of stars and their planetary systems and the evolution of such arrangements. “Given the close proximity of exoplanets to host stars, it is vital to understand how space weather events tied to those stars can affect the habitability of the exoplanet,” Atri concludes. “Stars and planets are very tightly coupled in a number of ways and an improved understanding of this coupling are absolutely necessary to find habitable planets in our Galaxy and beyond.”

Atri. D., Carberry Morgan. S. R., [2020], ‘Stellar flares versus luminosity: XUV-induced atmospheric escape and planetary habitability,’ Monthly Notices of Royal Astronomical Society: Letters.

Astronomers stunned by hellish world where it rains rocks on oceans of molten lava

Artist impression of K2-141b. Julie Roussy, McGill Graphic Design.

With its clouds of sulfuric acid and surface temperatures exceeding 400 degrees Celsius, Venus is often referred to as a sort of incarnation of hell. It could be worse, though. In a new study, astronomers have zoomed in on K2-141b, a planet that is so hot it is covered in oceans of molten lava and rocks rain down from its atmosphere.

This is truly one of the most extreme worlds scientists have found out of the more than 4,000 exoplanets identified to date. In a new study, researchers from McGill University, York University, and the Indian Institute of Science Education examined the scorching planet’s atmosphere and weather system, revealing new insights about the formation and dynamics of so-called “lava planets”.

“The study is the first to make predictions about weather conditions on K2-141b that can be detected from hundreds of light-years away with next-generation telescopes such as the James Webb Space Telescope,” says lead author Giang Nguyen, a PhD student at York University 

K2-141b, which is located hundreds of light-years away from Earth, owes its bizarre weather to its close proximity to its parent star. Being so close to the star also causes the planet to be gravitationally locked in its place — meaning the same side always faces the star just like the moon does Earth. As a result, two-thirds of the distant exoplanet experiences perpetual daylight, where surface temperatures 3,000 degrees Celsius (5,400 degrees Fahrenheit).

That’s so hot that rocks melt, covering the planet in a 96-km (60-mile) ocean of magma. It’s actually so hot that some of the molten rock is vaporized into the atmosphere.

On Earth, liquid water evaporates, rising up into the atmosphere where it condenses, ultimately returning to the surface in the form of rain. A similar cycle also occurs on K2-141b, only instead of water there’s sodium, silicon monoxide, silicon dioxide, and other vaporized rocky substances, which are carried by supersonic winds blowing in excess of 3,000 mph to the planet’s dark side.

In the part of the planet shrouded in eternal darkness, temperatures are frigid, hovering at -200 degrees Celsius (-424 degrees Fahrenheit). The cold atmosphere condenses the rocky substances, which rain back into the magma ocean, restarting the cycle.

However, unlike the water cycle on Earth, this rocky cycle is not in equilibrium since the flow of material from the dark side to the dayside is slower. Eventually, the researchers predict that the planet’s surface and atmospheric composition will be altered dramatically.

“All rocky planets­, including Earth, started off as molten worlds but then rapidly cooled and solidified. Lava planets give us a rare glimpse at this stage of planetary evolution,” said Nicolas Cowan, a professor in the Department of Earth & Planetary Sciences at McGill University.

The findings appeared in the Monthly Notices of the Royal Astronomical Society.

An artist's impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)

‘Lonely’ Rogue Planet Discovered Wandering the Milky Way

Researchers believe that our galaxy is teeming with cosmic orphans, planets wandering free of a parent star. Though common, these rogue planets are difficult to spot, especially when they are in the size range of the earth. 

Despite this difficulty; an international team of astronomers including Przemek Mróz, a postdoctoral scholar at the California Institute of Technology (Caltech) and Radosław Poleski from the Astronomical Observatory of the University of Warsaw, have spotted what they believe to be a free-floating planet with a size and mass somewhere in the range of Mars and Earth, wandering the Milky Way. 

The discovery represents a major step forward in the field of exoplanet investigation as it is the first earth-sized ‘rogue planet’ ever observed.

An artist's impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)
An artist’s impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)

“We found a planet that seems extremely lonely and small, far away in the Universe,” Poleski tells ZME Science. “If you can imagine, Earth is in a sandbox surrounded by lots of other planets, and light from the Sun. This planet isn’t. It’s truly alone.”

The rogue planet the team found — OGLE-2016-BLG-1928 — is believed to be the smallest free-floating planet ever discovered. It was found in data collected by Optical Gravitational Lensing Experiment (OGLE), a Polish astronomical project based at the University of Warsaw. Previously discovered rogues — such as the first-ever recorded free-floating planet also found by OGLE in 2016 — are closer in size to Jupiter.

The gravity of a free-floating planet may deflect and focus light from a distant star when passing close in front of it. Due to the distorted image, the star temporarily seems much brighter. (v)

“We discovered the smallest free-floating planet candidate to date. The planet is likely smaller than Earth, which is consistent with the predictions of planet-formation theories,” Mróz — lead author of the team’s study published in Astrophysical Journal Letters — explains to ZME Science. “Free-floating planets are too faint to be observed directly — we can detect them using gravitational microlensing via their light-bending gravity.”

The Gravity of the Situation

The team spotted this wandering planet using the technique of gravitational microlensing, often utilised to spot exoplanets — planets outside our solar system. Exoplanets can’t often be observed directly, and when they can it’s a result of interaction with radiation from their parent star — for example, the dimming effect exoplanets have when they cross in front of their star and block some of the light it emits. Clearly, as rogue planets don’t have a parent star, they don’t have these interactions, making micro-lensing events the only way of spotting them.

“Microlensing occurs when a lensing object — a free-floating planet or star — passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source,” Mróz explains to ZME Science. “The observer will measure a short brightening of the source star, which we call a gravitational microlensing event.”

When the gravity of a free-floating planet deflects and focuses light from a distant star, we can observe temporary changes in star brightness. (temporary changes in star brightness.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw.)

Mróz continues by explaining that the duration of microlensing events depends on the mass of the object acting as a gravitational lens. “The less massive the lens, the shorter the microlensing event. Most of the observed events, which typically last several days, are caused by stars,” Mróz says. “Microlensing events attributed to free-floating planets usually last barely a few hours which makes them difficult to spot. We need to very frequently observe the same part of the sky to spot brief brightenings caused by free-floating planets.”

Changes of brightness of the observed star during the gravitational microlensing event by a free-floating planet. (Credit: Jan Skowron / Astronomical Observatory, University of Warsaw/ Robert Lea)

By measuring the duration of a microlensing event and shape of its light curve astronomers can estimate the mass of the lensing object. That is how the team were able to ascertain this free-floating planet is approximately Earth-sized. “Hence, we can discover very dim objects, like black holes, or free-floating planets,” says Poleski. “We found it an event, which has a timescale of 41 minutes. And it’s the shortest event ever discovered.”

Poleski explains that the lack of any other lensing body in the system told the team that it is a very strong candidate for a free-floating planet. He adds: “We know it’s a planet because of the very short timescale and we think it’s free-floating because we don’t see any star next to it.”

Going Rogue. How Free-Floating Planets Come to Wander the Universe Alone

Astronomers believe that free-floating planets actually formed in protoplanetary disks around stars in the same way that ‘ordinary’ planets are. At some point, they are ejected from their parent planetary systems, probably after gravitational interactions with other bodies, for example, with other planets in the system.

“Some low-mass planets are expected to be ejected from their parent planetary systems during the early stages of planetary system formation,” says Mróz. “According to planet formation theories, most of the ejected planets should be smaller than Earth. Theories of planet formation predict that typical masses of ejected planets should be between 0.3 and 1.0 Earth masses. Thus, the properties of this event fit the theoretical expectations.”

These free-floating rogue planets are believed to be fairly common, but researchers can’t be certain because they are so difficult to spot. “Our current studies indicate that the frequency of low-mass–in the Earth to super-Earth-mass range–free-floating or wide-orbit planets is similar to that of stars — there are about two-five such objects per each star in the Milky Way,” says Mróz. “These numbers are very uncertain because they are based on a few sightings of short-timescale microlensing events. However, if free-floating/wide-orbit planets were less frequent than stars, we would have observed much fewer short-timescale events than we do.”

The researcher adds that though these objects are relatively common, the chances of observing microlensing events caused by them are still extremely small. “Three objects — source, lens, and observer — must be nearly perfectly aligned,” Mróz says. “If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”

In fact, one of the extraordinary elements of the team’s study is that such a short duration lensing event wasn’t believed to be observable given the sensitivity of the current generation of telescopes.

“The surprise, in general, was that with current technology we could define such a short time event,” Poleski says. “It’s especially surprising if you beat the previous record by a factor of few.”

The Nancy Grace Roman Telescope and Future Rogue Reconnaissance

For Mróz, there are still questions that he would like to see answered about OGLE-2016-BLG-1928. Primarily, confirming that it definitely is a free-floating planet. 

“We aren’t fully sure whether our planet is free-floating or not. Our observations rule out the presence of stellar companions within 10 astronomical units–930 million miles–of the planet, but the planet may have a more distant companion,” Mróz says. “Let’s imagine that we’re observing microlensing events by a doppelganger of the Solar System. If Jupiter or Saturn caused a microlensing event, we would see a signature of the Sun in the microlensing event light curve. However, microlensing events by Uranus or Neptune would likely look like those of free-floating planets, because they are very far from the Sun.”

Fortunately, Mróz says that should be possible to distinguish between free-floating and wide-orbit planets. “The lens is moving relative to the source star in the sky and — a few years after the microlensing event — the lens and source should separate in the sky,” the researcher elaborates. “If the lens has a stellar companion, we will see some excess of light at its position. If it is a free-floating planet, we will not.” 

Whilst this method may seem simple, Mróz says we cannot apply it now, because the existing telescopes are not powerful enough. This includes the instrument that conducted the long-term observations that gave rise to the OGLE sky survey–the data from which the team found the micro-lensing event OGLE-2016-BLG-1928.

“[The discovery of OGLE-2016-BLG-1928] was part of the larger search for microlensing events in general, which we perform in a number of steps,” Poleski tells ZME. “In one step, I started looking at the wide orbit planets — planets similar to Uranus, or Neptune and on similar orbits. And while looking for those, I screened a list of candidate microlensing events in general and I found this one.”

Soon NASA’s Nancy Grace Roman Telescope will take over the search for microlensing events, but in the meantime, there is still data from OGLE and other projects to be examined. “We now have more data and other surveys are also collecting data. So we hope to analyze those,” Poleski says. “The longer-term future is the launch of the Nancy Grace Roman Space Telescope. It will be a telescope similar to the Hubble telescope, only with new infrared and infrared cameras and that camera field of view larger than the Hubble Space Telescope. 

“One of the main projects for the Raman telescope will be to observe galactic bulge in search for microlensing planets, including free-floating planets.”

Mroz, P., Poleski, R., Gould, A. et al., ‘A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event,’ Astrophysical Journal Letters, [2020] DOI: 10.3847/2041–8213/abbfad

New study lists 24 exoplanets that may be better for life than Earth

Although we’re all from here, Earth isn’t necessarily the best planet to live on in the Universe, according to new research. A new study lists two dozen such “superhabitable” planet candidates for further research.

Image via Pixabay.

It is quite an unexpected turn of events, but the planets identified in the new study all show some properties that could make it a better home for earthlings than Earth itself.

The study, published by researchers from Washington State University, also detail why these planets were chosen. Some are older, some larger, others wetter and slightly warmer than Earth. Some of them even orbit ‘better’ stars than our own Sun, and are expected to exist for longer.

Better than the original

“With the next space telescopes coming up, we will get more information, so it is important to select some targets,” said Schulze-Makuch, a professor with WSU and the Technical University in Berlin.

“We have to focus on certain planets that have the most promising conditions for complex life. However, we have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours.”

All the 24 maybe-superhabitable planets are over 100 light years away from Earth, so for the time being, they’re far beyond our grasp. However, with concentrated effort and data pooled from current and future telescopes, such as from NASA’s James Webb Space Telescope, the LUVIOR space observatory, and the European Space Agency’s PLATO space telescope, we could glean enough data to see whether they’d be nice places to live.

Schulze-Makuch, a geobiologist with expertise in planetary habitability, worked with astronomers René Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University to first determine which criteria would make a planet superhabitable. Then, they dredged through data of the over 4,500 known exoplanets to see which would fit.

A habitable planet isn’t necessarily one that has life, but one that has the right conditions to sustain life as we know it — things like pleasant temperatures, liquid water, magnetic fields, and breathable atmosphere.

The team looked at systems with stars similar to our Sun (G-class stars) and planets orbiting them in the liquid water zone — not too far nor too close. But since G-class stars have a lifespan of just 10 billion years, and it took life on Earth 4 billion years to evolve, this means they’re not the best candidates for life, as they can consume their fuel before anything spawns on the planets that orbit them. That’s why the team also looked at the cooler, dimmer K-class dwarf stars, which have lifespans of 20 billion to 70 billion years.

While this gives the planets more time to develop life, the authors argue that we shouldn’t be looking at ones that are too old. In order to be habitable, a planet needs a protective magnetic field — the ones that don’t end up like Mars is today. Those magnetic fields are generated in their core, and they’re powered by geothermal energy — which itself is produced by radioactive decay inside the planet. The Earth is around 4.5 billion years old, meaning it hasn’t yet expended its cache of geothermal energy. The authors thus argue that we should be looking for exoplanets 5 billion to 8 billion years old.

As far as size is concerned, we should look for something slightly larger and heavier than our own. Exoplanets that are at least 10% larger than Earth would have more habitable land but provide almost the same experience as living here. One around 1.5 times heavier should be able to better retain its geothermal energy and have enough gravity to keep its atmosphere intact.

Size and mass also matter. A planet that is 10% larger than the Earth should have more habitable land. One that is about 1.5 times Earth’s mass would be expected to retain its interior heating through radioactive decay longer and would also have a stronger gravity to retain an atmosphere over a longer time period.

Another element they’d look for is more water, especially in the form of moisture, clouds, and atmospheric humidity. A surface temperature of around 5 degrees Celsius (8 Fahrenheit) higher than on Earth, alongside extra moisture, would probably be better suited to life as we know it today. They argue that these elements make rainforests on Earth more biodiverse than forests in colder, drier areas.

None of the 24 exoplanets meet all the criteria, but one of the candidates has four.

“It’s sometimes difficult to convey this principle of superhabitable planets because we think we have the best planet,” said Schulze-Makuch.

“We have a great number of complex and diverse lifeforms, and many that can survive in extreme environments. It is good to have adaptable life, but that doesn’t mean that we have the best of everything.”

The paper “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World” has been published in the journal Astrobiology.

Machine learning helps identify 50 new exoplanets, more to come

Researchers at the University of Warwick have identified a host of new exoplanets from old NASA data with the use of machine learning.

Artist’s impression of exoplanet orbiting two stars.
Image credits NASA, ESA, and G. Bacon (STScI).

Identifying planets far away from our own isn’t easy. It involves a painstaking process of waiting for the planet to come between its star and our telescope, which will temporarily block or reduce its brightness. Based on how much of the light is obscured, we can tell whether they’re caused by a planet or something else in the huge expanse of space. This is called the transit method.

We’re far from getting this process down to a T, simply because human beings aren’t very good at processing massive amounts of data at the same time. But machine learning is.

Planet App

“In terms of planet validation, no-one has used a machine learning technique before,” said David Armstrong of the University of Warwick, one of the study’s lead authors, in a news release.

“Machine learning has been used for ranking planetary candidates but never in a probabilistic framework, which is what you need to truly validate a planet.”

The team trained their algorithm using data from the Kepler Space Telescope, retired in 2018 after a nine-year mission. From this wealth of data, it learned to identify planets and to weed out false positives using feedback from the researchers. After it was trained, the team fed it older data sets, and the program found 50 exoplanets ranging from Neptune-sized gas giants to rocky worlds smaller than Earth. Their orbits (how long it takes them to go around their stars) range from around 200 days to some as short as a single day.

The team notes that smaller planets are particularly hard to spot with the transit method, so finding such planets showcases the ability of the AI. The next step is to take our existing tools and give these 50 planets a more thorough look-over.

Such an AI however will surely be used again. The ability to monitor huge areas of the night’s sky quickly and reliably could speed up our efforts to identify planets by a huge degree. It’s likely far from perfect now, but algorithms can be improved as our knowledge improves, the team notes.

“We still have to spend time training the algorithm, but once that is done it becomes much easier to apply it to future candidates,” Armstrong said.

The paper “Exoplanet Validation with Machine Learning: 50 new validated Kepler planets” has been published in the journal Monthly Notices of the Royal Astronomical Society.

This artist's conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. (NASA)

Rogue Planets Could Outnumber Stars in the Milky Way

Our galaxy is teeming with rogue planets either torn from their parent stars by chaotic conditions or born separate from a star. These orphan planets could be discovered en masse by an outcoming NASA project — Nancy Grace Roman Space Telescope. 

The Milky Way is home to a multitude of lonely drifting objects, galactic orphans — with a mass similar to that of a planet — separated from a parent star. These nomad planets freely drift through galaxies alone, thus challenging the commonly accepted image of planets orbiting a parent star. ‘Rogue planets’ could, in fact, outnumber stars in our galaxy, a new study published in the Astronomical Journal indicates. 

“Think about how crazy it is that there could be an Earth, a Mars, or a Jupiter floating all alone through the galaxy. You would have a perfect view of the night sky but stuck in an eternal night,” lead author of the study, Samson Johnson, an astronomy graduate student at The Ohio State University, tells ZME Science. “Although these planets could not host life, it is quite a place to travel to with your imagination. The possibility of rogue planets in our galaxy had not occurred to me until coming to Ohio State.”

This artist’s conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. (NASA)

Up to now, very few very of these orphan planets have actually been spotted by astronomers, but the authors’ simulations suggest that with the upcoming launch of NASA’s Nancy Grace Roman Space Telescope in the mid-2020s, this situation could change. Maybe, drastically so.

“We performed simulations of the upcoming Nancy Grace Roman Space Telescope (Roman) Galactic Exoplanet Survey to determine how sensitive it is to microlensing events caused by rogue planets,” Johnson says. “Roman will be good at detecting microlensing events from any type of ‘lens’ — whether it be a star or something else — because it has a large field of view and a high observational cadence.”

The team’s simulations showed that Roman could spot hundreds of these mysterious rogue planets, in the process, helping researchers identify how they came to wander the galaxy alone and indicating how great this population could be in the wider Universe.

Rogue by Name, Rogue by Nature: Mysterious and Missing

Thus far, much mystery surrounds the process that sees these planets freed from orbit around a star. The main two competing theories suggest that these stars either are thrown free of their parent star, or form in isolation. Each process would likely lead to rogue planets with radically different qualities. 

ESO’s New Technology Telescope at the La Silla Observatory captures the rogue planet CFBDSIR J214947.2-040308.9 in infrared light–appearing as a faint blue dot near the centre of the picture. This is the closest such object to the Solar System discovered thus far. (ESO/P. Delorme)

“The first idea suggests that rogue planets form like planets in the Solar System, condensing from the protoplanetary disk that accompanies stars when they are born,” Johnson explains. “But as the evolution of planetary systems can be chaotic and messy, members can be ejected from the system leading to most likely rogue planets with masses similar to Mars or Earth.”

 Johnson goes on to offer an alternative method of rogue planet formation that would see them form in isolation, similar to stars that form from giant collapsing gas clouds. “This formation process would likely produce objects with masses similar to Jupiter, roughly a few hundred times that of the Earth.”

“This likely can’t produce very low-mass planets — similar to the mass of the Earth. These almost certainly formed via the former process,” adds co-author Scott Gaudi, a professor of astronomy and distinguished university scholar at Ohio State. “The universe could be teeming with rogue planets and we wouldn’t even know it.”

The question is if these objects are so common, why have we spotted so few of them? “The difficulty with detecting rogue planets is that they emit essentially no light,” Gaudi explains. “Since detecting light from an object is the main tool astronomers use to find objects, rogue planets have been elusive.”

Astronomers can use a method called gravitational microlensing to spot rogue planets, but this method isn’t without its challenges, as Gaudi elucidates:

“Microlensing events are both unpredictable and exceedingly rare, and so one must monitor hundreds of millions of stars nearly continuously to detect these events,” the researcher tells ZME Science. “This requires looking at very dense stellar fields, such as those near the centre of our galaxy. It also requires a relatively large field of view.”

Additionally, as the centre of the Milky Way is highly obscured by requiring us to look at it in the near-infrared region of the electromagnetic spectrum — a task that is extremely difficult as the Earth’s atmosphere makes the sky extremely bright in near-infrared light.

“All of these points argue for a space-based, high angular resolution, wide-field, near-infrared telescope,” says Gaudi. “That’s where Roman — formally the Wide Field InfraRed Survey Telescope (WFIRST) — comes in.” 

Nancy Grace Roman Space Telescope (and Einstein) to the Rescue!

The Roman telescope — named after Nancy Grace Roman, NASA’s first chief astronomer, who paved the way for space telescopes focused on the broader universe–will launch in the mid-2020s. It is set to become the first telescope that will attempt a census of rogue planets — focusing on planets in the Milky Way, between our sun and the centre of our galaxy, thus, covering some 24,000 light-years.

An artist’s redition of the Nancy Grace Roman Space Telescope, named after ‘the Mother of Hubble’ (NASA)

The team’s study consisted of simulations created to discover just how sensitive the Roman telescope could be to the microlensing events that indicate the presence of rogue planets, finding in the process, that the next generation space telescope was 10 times as sensitive as current Earth-based telescopes. This difference in sensitivity came as a surprise to the researchers themselves. “Determining just how sensitive Roman is was a real shock,” Johnson says. “It might even be able to tell us about moons that are ejected from planetary systems! We also, found a new ‘microlensing degeneracy’ in the process of the study — the subject another paper that will be coming out shortly.”

Johnson’s co-author Gaudi echoes this surprise. “I was surprised that Roman was sensitive to rogue planets with mass as low as that of Mars and that the signals were so strong,” the researcher adds. “I did not expect that before we started the simulations.”

The phenomenon that Roman will exploit to make its observations stems from a prediction made in Einstein’s theory of general relativity, that suggests that objects with mass ‘warp’ the fabric of space around them. The most common analogy used to explain this phenomenon is ‘dents’ created in a stretched rubber sheet by placing objects of varying mass upon it. The heavier the object — thus the greater the mass — the larger the dent. 

This warping of space isn’t just responsible for the orbits of planets, it also curves the paths of light rays, the straight paths curving as they pass the ‘dents’ in space. This means that light from a background source is bent by the effect of the mass of a foreground object. The effect has recently been used to spot a distant Milky Way ‘look alike’. But in that case, and in the case of many gravitational lensing events, the intervening object was a galaxy, not a rogue planet, and thus was a much less subtle, more long-lasting, and thus less hard to detect effect than ‘microlensing’ caused by a rogue planet. 

This animation shows how gravitational microlensing can reveal island worlds. When an unseen rogue planet passes in front of a more distant star from our vantage point, light from the star bends as it passes through the warped space-time around the planet. The planet acts as a cosmic magnifying glass, amplifying the brightness of the background star. (NASA’s Goddard Space Flight Center/CI Lab)

“Essentially, a microlensing event happens when a foreground object — in this case, a rogue planet — comes into very close alignment with a background star. The gravity of the foreground object focuses light from the background star, causing it to be magnified,” Gaudi says. “The magnification increases as the foreground object comes into alignment with the background star, and then decreases as the foreground object moves away from the background star.”

As Johnson points out, microlensing is an important and exciting way to study exoplanets — planets outside the solar system — but when coupled with Roman, it becomes key to spotting planetary orphans.

“Roman really is our best bet to find these objects. The next best thing would be Roman 2.0 — with a larger field of view and higher cadence,” the researcher tells ZME, stating that rogue planets are just part of the bigger picture that this forthcoming space-based telescope could allow us to see. “I’m hoping to do as much work with Roman as possible. The next big project is determining what Roman will be able to teach us about the frequency of Earth-analogs — Earth-mass planets in the habitable zones of Sun-like stars.”

Original Research

Johnson. S. A., Penny. M, Gaudi. B. S, et al, ‘Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates*,’ The Astronomical Journal, [2020]. 

Astronomers may have found a twin solar system thousands of light-years away

The newly discovered planet candidate KOI-456.04 and its star Kepler-160 (second panel from above) have great similarities to Earth and sun (top panel). Credit: MPS / René Heller.

In the last 14 years, astronomers have identified more than 4,000 planets orbiting far-away solar systems, but none seems to come close to KOI-467.04. Don’t be fooled by its unceremonious designation, for this is a rare astronomical gem.

According to a new study led by researchers at the Max Planck Institute for Solar System Research in Göttingen, Germany, KOI-467.04 is less than twice the size of Earth and orbits a star that is almost like a twin to our Sun. What’s more, the exoplanet orbits its star at a distance similar to that between Earth and the Sun, suggesting that its surface temperature is conducive to life.

In other words, the scientists may have found a twin solar system — all that astronomers have been wishing for since the first exoplanet was discovered in 1992.

Billions of planets, but there’s only one place like home… or is there?

There are billions — perhaps trillions — of stars in the Milky Way galaxy alone and, on average, each of those stars has at least one planet orbiting them

“We are seeing just how diverse planets are. Planets are more common than they were thought to be before the first exoplanets were found. The number of planets in our galaxy is at least as large as the number of stars. But while planets and planetary systems are so diverse, planets like Earth may be very, very rare,” Dr. Jack Lissauer, a scientist at the NASA Ames Research Center and co-investigator of the Kepler mission, told me last year at a conference in Budapest.

The vast majority of exoplanets identified by astronomers in the past are the size of gas giants like Neptune or Jupiter and orbit their parent stars much too close for life to stand a chance.

Occasionally, astronomers will come across exoplanets that are Earth-sized and potentially rocky, but these planets either orbit too close or too far away from the star. On the extremely rare occasion that scientists discover an Earth-sized rocky exoplanet orbiting in the Goldilocks Zone — or habitable zone, where the distance from the parent star is just right for liquid water to form.

But even then, things are typically far from perfect. Almost all exoplanets less than twice the size of Earth found thus far orbit around a red dwarf, which present their own set of limitations.

Red dwarfs are by far the most common types of stars in the Universe. They’re small, dim, and relatively cool, and also have a long lifetime.

Yet, although life on an exoplanet orbiting a red dwarf would have twice as much time than that on Earth to evolve, it would have to overcome other important challenges.

One has to do with radiation. While the surface temperature might be just right for liquid water to form, exoplanets orbiting red dwarfs are mostly hit by infrared rather than visible light.

If that wasn’t enough, red dwarfs also regularly spew powerful solar flares that can fry nearby planets. What’s more, because red dwarfs are so faint, exoplanets hoping to harbor life need to orbit so close to the parent stars that they get deformed by the stellar gravity. This can result in tidal heating that can trigger global volcanism and turn the promising exoplanet into a hellish world.

So, it’s not just a question of exoplanet quality, stellar quality is vital too. Now, an international team of researchers think they’ve found one that checks all the boxes.

Mirror Earth and Sun

The Earth-like planet candidate that orbits a sun-like star is located over 3,000 light-years away, in the Kepler-170 system.

It was first identified in 2009 and since 2014, astronomers have found that it hosts two exoplanets, known as Kepler-160b and Kepler-160c, which are both much bigger than Earth and in relatively close orbits around their stars. Nothing to warrant particular attention so far.

But then the researchers combed the archival Kepler data of Kepler-160 hoping to perhaps find other planets using a novel method developed by Michael Hippke and René Heller, both from Max Planck. Their investigation was prompted by evidence that Kepler-160c’s orbit was perturbed — something was out there.

This is when they found another two planets, among them the exciting KOI-456.04.

“Our analysis suggests that Kepler-160 is orbited not by two but by a total of four planets,” Heller said in a statement.

 “The planetary signal is so faint that it’s almost entirely hidden in the noise of the data. Our new search mask is slightly better in separating a true exoplanetary signal from the noise in the critical cases,” Heller adds.

According to Heller and colleagues, KOI-456.04 has a radius of 1.9 Earth radii (almost twice as large as Earth) and orbits its parent star every 378 days — that’s mighty close to Earth’s 365 days (or 366 days during a leap year).

As for the star, the astronomers estimate that its radius is 1.1 that of the Sun, with a surface temperature of around 5,200 degrees Celsius, just 300 degrees shy of that of the sun. Its luminosity is also sun-like.

Given this information, the researchers believe that KOI-456.04 might have an average surface temperature of around 5 degrees Celsius, as long as it has an atmosphere that can support at least a mild Earth-like greenhouse gas effect.

“KOI-456.01 is relatively large compared to many other planets that are considered potentially habitable. But it’s the combination of this less-than-double the size of the Earth planet and its solar type host star that make it so special and familiar,” Heller 

Don’t get too excited, though. As a caveat, the researchers claim that they cannot entirely rule out KOI-456.01 as a statistical fluke. According to the study published in the journal Astronomy & Astrophysics, the odds of KOI-456.04 being a real planet and not some statistical aberration is 85%. Formal planetary status requires a 99% degree of confidence.

All hope is not lost, though. Astronomers will have a good chance to confirm their findings once ESA’s PLATO space mission launches in 2026. Plato will specialize in the examination of rocky exoplanets orbiting in habitable zones around Sun-like stars, particularly focusing on the potential for these planets to hold liquid water. Fingers crossed.

Scientists find extreme exoplanet raining with iron

Rain is not necessarily synonymous with water on other planets. Astronomers working with the European Southern Observatory’s Very Large Telescope (VLT) have come across a bizarre exoplanet where it rains iron in the evening.

Artist illustration showing the night-side of WASP-76b, where it rains iron. Credit: ESO/M. Kornmesser.

The exoplanet, known as WASP-76b, is located about 6400 light-years away from Earth in the constellation Pisces. The ultra-hot gas giant orbits so close to its parent star that temperatures regularly climb above 2,400°C — but only on the planet’s day-side.

Just like the moon, WASP-76b is tidally locked, meaning it only shows one face to its parent star, since the planet takes just as long to rotate around its own axis as it does to orbit around the star. As a result, the night side is shrouded in perpetual darkness and is much cooler.

The exoplanet receives thousands of times more radiation than Earth does from the Sun, making the surface of Wasp-76b’s day side so hot it vaporizes metals like iron. Vigorous winds generated by the extreme temperature difference between the planet’s two sides carry a fraction of this iron vapor to the cooler side, where temperature decreases to 1,500°C. That’s still very hot, yet cool enough for iron vapor to condense and rain down.

“One could say that this planet gets rainy in the evening, except it rains iron,” says David Ehrenreich, a professor at the University of Geneva in Switzerland, who led the new research published in the journal Nature.

“Surprisingly, however, we do not see the iron vapor in the morning,” says Ehrenreich, adding that “it is raining iron on the night side of this extreme exoplanet.”

The discovery was made possible thanks to a new instrument equipped on ESO’s VLT in the Chilean Atacama Desert. Known as the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations — or ESPRESSO — the instrument was originally designed to hunt for Earth-like planets around Sun-like stars. However, ESPRESSO has proven itself much more versatile than originally thought, allowing astronomers to detect a strong signature of iron vapor at the evening border that separates Wasp-76b’s two sides.

“We soon realised that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.

This crazy planet is not just some curious oddity. The insight gained by studying its atmosphere will help scientists better fine-tune and test climate and global circulation models. Ultimately, outlier planets like WASP-76 b will better our understanding of exoplanet atmospheres in general.

“What we have now is a whole new way to trace the climate of the most extreme exoplanets,” concludes Ehrenreich.

If you found an iron-raining planet weird, this exoplanet is actually not that peculiar. On Venus, it rains sulfuric acid, while on Neptune rainfall is in the form of diamonds.

One year on this exoplanet lasts only 18 hours

Researchers at the University of Warwick have discovered an exoplanet that boasts the shortest year yet — just 18 hours.

Image credits University of Warwick.

This hot Jupiter type planet makes a full trip around its host star in just 18 hours, giving it the shortest year out of any planet we know so far. However, this may be bad news for the planet itself: the team suspects that this planet is falling into its star.

Too close for comfort

“We’re excited to announce the discovery of NGTS-10b, an extremely short period Jupiter-sized planet orbiting a star not too dissimilar from our Sun,” says lead author Dr. James McCormac from the University of Warwick Department of Physics. “We are also pleased that NGTS continues to push the boundaries in ground-based transiting exoplanet science through the discovery of rare classes of exoplanets.

The planet lies around 1000 light-years away from Earth, and was discovered as part of the Next-Generation Transit Survey (NGTS) exoplanet survey using the transit method. Basically, it was detected by analyzing the dips in brightness it causes while passing in front of its host star.

It was immediately apparent that this planet was not like most others; it transited in front of the star way too often.

“Although in theory hot Jupiters with short orbital periods (less than 24 hours) are the easiest to detect due to their large size and frequent transits, they have proven to be extremely rare,” explains McCormac. “Of the hundreds of hot Jupiters currently known there are only seven that have an orbital period of less than one day.”

NGTS-10b is very close to its star, orbiting only two times the diameter of the star away from its surface. In our solar system, the team explains, this would make it around 27 times closer to it than Mercury is to our Sun. This would put NGTS-10b dangerously close to being ripped apart by the star’s tidal (gravitational) forces.

Temperatures on its surface are likely around 1000 degrees Celsius on average, they add, since the host star is only around 70% as big as the Sun in diameter and its surface is 1000 degrees Celsius cooler. NGTS-10b itself is around one-fifth larger than Jupiter, estimated to be over twice its mass and likely tidally locked to the star.

Massive planets typically form some distance away from their stars, and can then migrate either while they’re still forming or after they mature. This makes NGTS-10b particularly useful, as the team plans to continue observing it and determine whether it will continue falling into the star — potentially telling us more about how hot Jupiters form.

“It’s thought that these ultra-short planets migrate in from the outer reaches of their solar systems and are eventually consumed or disrupted by the star,” explains co-author Dr. David Brown.

“We are either very lucky to catch them in this short period orbit, or the processes by which the planet migrates into the star are less efficient than we imagine, in which case it can live in this configuration for a longer period of time.”

The paper “NGTS-10b: the shortest period hot Jupiter yet discovered” has been published in the journal Monthly Notices of the Royal Astronomical Society.

As long as we have known that the sun is just another star, we have dreamed of other worlds. Thanks to the exoplanet hunter's toolkit, we can now do more than dream.

The Exoplanet Hunter’s Toolkit: the science of searching for other worlds

We have all, at some point, stared at the stars and dreamed lazily of other worlds, but, fortunately, for many of us, dreaming alone was not enough. These people set about building a toolkit stocked with instruments and techniques to find planets outside our solar system — exoplanets. In turn, these tools help us better understand our place in the Universe. 

An artist’s impression of 51 Pegasi b — not the first exoplanet to be discovered, but the most important (ESO/L. Benassi)

In October the Nobel Committee awarded the 2019 Nobel Prize in Physics to Michel Mayor, Professor at the Observatory of the Faculty of Science of the University of Geneva (UNIGE), Switzerland, and his doctoral student Didier Queloz for their discovery of 51 Pegasi b in 1995 — which marked the first discovery of an exoplanet orbiting a Sun-like star. The award marked the first time that exoplanet research has scooped what is, arguably, the most prestigious prize in science. Quite fitting as even though 51 Pegasi b was not the first exoplanet to be discovered — that honour goes to Astronomers Aleksander Wolszczan and Dale Frail who discovered an exoplanet around a neutron star in 1992 — it was Mayor and Queloz’s breakthrough that really spurred on the science of exoplanet investigation. 

As extraordinary as it sounds, before the 1990s, it wasn’t entirely certain that other stars actually possessed planets of their own. Whilst there was technically no reason to suspect that the solar system was unique, the 1980s had proved a frustrating time for exoplanet hunters. By the turn of that decade, many potential candidates had come and gone evading positive confirmation. 

Despite early setbacks, since 1995, the catalogue of exoplanets has soared, with over 4,000 examples now in NASA’s catalogue. And with technology only improving, that collection is set to soar. This animation and sonification from SystemSounds is a stunning representation of how the field has exploded since the 1990s. Created by SYSTEM Sounds (Matt Russo, Andrew Santaguida)

Created by SYSTEM Sounds (Matt Russo, Andrew Santaguida)

We are becoming so confident in the discovery of exoplanets, that we are now turning our attention to much more detailed examinations of previously discovered examples. For example, many researchers are now focusing on the investigation of exoplanet atmospheres, attempting to discover if they contain traces of chemicals such as carbon monoxide and other organic and complex molecules, and, of course, water. Should these elements be observed it constitutes a clue, a tiny hint, that life may not be unique to our planet. 

Thus far, searches for exoplanets have been more effective in finding gas giants, planets similar to Jupiter. But new advances such as the James Webb Space Telescope and the Extremely Large Telescope have researchers salivating at the idea of finding and examining smaller, rocky planets. Planets just like Earth. And of course, the discovery of the Trappist-1 system — containing seven Earth-like rocky planets, three in the so-called ‘habitable zone’ capable of harbouring liquid water — has shown that these planets are definitely out there waiting for us to find them. 

An artist’s impression of the Trappist-1 system — 7 Earth-like planets orbiting a red dwarf star (NASA)

As such, exoplanet research stands on the cusp of providing an answer to the question we have all pondered at some point whilst staring at the stars, are we alone in the universe?

Of course, the fact that it took so many years of fruitless searching to begin to successfully spot exoplanets illustrates, these blighters are extremely difficult to observe. This means that astronomers have had to develop extremely precise and sensitive methods of exoplanet detection. These techniques are numerous, each with its own strengths and weaknesses. 

Wobbly Stars

It goes without saying that before we spotted the first exoplanet, our experience of observing other planets was restricted to our neighbours in the solar system. This was done exclusively through direct imaging, but this technique becomes much more difficult as the distance to an object increases. 

Spotting planets in our solar system is possible with direct imaging techniques, for exoplanets, much craftier methods are required (NASA)

The hindrances imposed on direct imaging increase exponentially when we consider the effect of attempting to spot a dim object next to a much bright one — exactly the scenario faced when attempting to spot a distant planet orbiting its parent star. But, this proximity to an extremely bright object is not always a hindrance to exoplanet detection. In fact, many methods of spotting these planets absolutely depend on it. If a dim object can have an effect of the extremely bright object — then the ability to observe this bright object is a benefit.

This interference arises from the fact that stars with planets orbiting them demonstrate a ‘wobble’ in their motion. This arises from the fact, that despite common belief, planets don’t actually orbit stars. In fact, planets and stars orbit a mutual centre of mass— or barycentre —its location based on the masses of the planets and stars involved. As the usual set-up of a planetary system involves a star that is tremendously more massive than its planets, this mutual point of orbit is usually closer to the star centre of mass — often within the star’s surface.

This huge disparity in mass means that this ‘wobble’ is tiny. As an example, consider our own solar system. As the Sun constitutes more than 99.9% of the total mass of the solar system, the barycentre for our planetary system is located very close to our star’s centre of mass. The most significant gravitational influence on the Sun arises as a result of the solar system’s most mass planet — Jupiter. 

Rather than orbiting the Sun, both Jupiter and the Sun orbit a mutual centre of mass. This centre of mass is closer to the surface of the Sun and thus results in a slight ‘wobble’ in the star’s motion. This wobble can be used by astronomers to infer the presence of a planet. (Robert Lea) 

Let’s imagine, for simplicity’s sake, that Jupiter is the only planet orbiting the Sun. An observer viewing this reduced solar system and Jupiter’s 12-year orbit from the nearest planetary system — Alpha Centauri, 4.4 light-years away — would see the Sun as a mere point of light. The shift in its position caused by Jupiter would be just 3.7 milliarcseconds. To put this shift into perspective, consider that one pixel in an image from the Advanced Camera for Surveys aboard the Hubble Space Telescope represents 50 milliarcseconds — one pixel! Thus you can see, this ‘wobble’ caused by Jupiter is a tiny, barely perceptible amount of movement, less than 1/10 of a pixel from the nearest star!

Two further things to consider in this hypothetical situation, Jupiter is the most massive planet in the solar system, the wobble caused by Earth viewed from the same position would be smaller by a factor of at least 300. Also, many of the exoplanets that we are attempting to spot are much further afield than 4.4 light-years. That means that any method using this wobble must be incredibly sensitive and precise. Incredibly, despite this tiny effect, the wobble has spawned several methods of exoplanet detection. 

The astrometry method depends on measuring the ‘wobble’ of a star caused by a planet in orbit around it. (ESA)

One of these methods is astrometry — very effective for detailing high-mass planets in wide orbits around relatively low-mass stars, and thus not well suited to tracking down Earth-like, rocky planets. For an indirect observation method, astrometry is pretty good at pinning down characteristics like mass, and orbital period, shape and width. Unfortunately, it isn’t great at actually identifying planets. 

Thankfully there are other indirect techniques that have helped astronomers help find exoplanets — one of which combines a star’s ‘wobble’ with a phenomenon familiar to drivers and pedestrians everywhere.  

Sirens, soundwaves and stars. The Doppler Wobble

I’m sure everyone reading this has been in a situation in which an ambulance with sirens blaring has raced towards them, passed their position and continued on its journey. You’ll have likely noticed that as the vehicle approaches the sound of its siren is higher pitched, switching to a lower pitch as it moves away.

This is because as the soundwaves are emitted by the approaching ambulance they are compressed and shorter wavelength sound waves mean a higher-pitched sound. As the siren recedes, the soundwaves are stretched out — resulting in a lower pitch. 

The Doppler effect as it applies to soundwaves. The soundwaves from the approaching ambulance are compressed — resulting in a higher pitch. The soundwaves as the ambulance recedes are expanded, and thus lower-pitched. (Robert Lea)

This is the Doppler effect, and the key thing for astronomers is, it applies to any kind of wave emitted by a moving object — even light which propagates as a wave. Just as the wavelengths of soundwaves correspond to different pitches, the wavelengths of light correspond to different colours. Longer wavelengths producing a reddening, shorter wavelengths produce bluer light. This is referred to as redshift and blueshift— a crucial phenomenon in astronomy. 

Instead of an ambulance, let’s think about a star moving towards us, as the light waves emitted are compressed — causing the light signature of the star to be shifted towards the blue end of the spectrum. As the star moves away, the light is stretched out again — shifting the light signature towards the red end of the electromagnetic spectrum. 

The Doppler Effect as it would appear for light-waves emitted by a star. I shouldn’t have to say, but nothing is to scale in this image! (Robert Lea)

If this is the case, why do the lights atop the ambulance as it recedes not appear redder than they were on their approach? This is because the amount of red and blue shift is determined by an object’s speed divided by the speed of light c. As c is so large, an object would have to be moving at tremendous speeds to result in a significant enough change in colour for us to notice. 

You might be wondering how exactly scientists can tell that a star’s signature has shifted towards either end of the electromagnetic spectrum. This is because stars don’t emit light in a constant ‘smear’ across the spectrum. There are notable dark bars where light is not emitted — referred to as the absorption spectrum. It is by tracing the shift of these bars that researchers can see if a star is wobbling and by how much, thus inferring the presence of exoplanets. 

The light signature of a star contains dark absorption lines. These lines not only tell us what elements the star is composed of but also, by tracking the shift in these lines we can infer the presence of an orbiting planet. (Lund Observatory)

Of course, you may well have noticed a flaw with this technique. It’s only useful in detecting exoplanets that are causing their star to wobble towards and away from Earth. This gives rise to the method’s alternative name — the radial-velocity method. In addition to this blind-spot in glimpsing planets moving perpendicular to Earth, the Doppler-wobble technique can also only tell us what a planet’s minimum mass is. 

But, knowledge of the light signature of a star gives rise to another tool in the exoplanet hunter’s arsenal, one which depends on planets crossing — or transiting — their parent star. The photometry technique. 

Don’t cross me!

The photometry technique measures the dip in brightness of a star caused as a planet crosses in front of it, thus allowing us to infer the presence of an exoplanet and even collect details about a few of its characteristics. The method clearly requires searching for rare eclipse events where a planet blocks some of its parent star’s light. 

An illustration of the photometry technique which relies on a planet crossing its parent star, blocking some of the light it emits (NASA)

You may be unsurprised to learn that like the other methods detailed above, the photometry technique has to be incredibly sensitive. In this case, that is because the disparity in size between a star and a planet orbiting it is so huge in the star’s favour that the light obscured by this transit is minuscule. 

To illustrate this, take a look at this image of Mercury transiting the Sun seen from within the solar system. 

This speck, highlighted below, is Mercury. Now imagining the tiny fraction of light this would have obscured. When you’ve done that, imagine viewing this scene from millions of light-years away!

Returning to the example of Jupiter, the largest planet in our solar system, when it transits the Sun it blocks just 1% of the star’s light — making the Sun appear 1% fainter for a period of 12 hours. As small as it is, in comparison to the phenomena exploited by the two methods above this effect is massive.

And again, like its fellow methods of exoplanet detection, the photometric technique has significant limitations that define the situations in which it can be employed. Many planets don’t transit their parent stars and those that do have to be orientated ‘just right’ for the photometry method to work. Also, transits that do occur are incredibly brief, so it takes a lot of good fortune to catch one. That means that the vast majority of exoplanets that we believe exist out there in the depths of space can’t be spotted by this method. 

The Earth’s atmosphere and the ‘twinkling effect’ it has on stars is also a major hindrance to the photometry method. This results in its reliance on space-based telescopes. By taking the atmosphere out of the equation, it is possible to not just improve the precision of our measurements but also allows for the continuous monitoring of a star’s brightness without the agony of something as mundane as a rainy-day ruining data.

The future of exoplanet research is extremely bright

With all the limits and drawbacks I’ve listed it may seem like searching for exoplanets is something of a hopeless task, like searching for a needle in a haystack. Except there are 100 billion ‘haystacks’ or stars outside of our analogy in each galaxy. Clearly its a tribute to our advances in science that we have found 4000 or so ‘needles’ thus far.

The astrometry technique, the first tool we examined in the exoplanet hunter’s toolkit isn’t particularly useful, but the second, the doppler technique has been a real boon. It kickstarted exoplanet-hunting as a viable scientific field in the ’90s and provided the majority of discoveries right up into the 2000s. Despite this, its the transit technique — photometry — the last piece of equipment that we turned over, that holds the most promise for the future.

It was a slow starter for sure, reaching maturity much later than the previous two methods mentioned. But, as the use of automated and space-based telescopes has become more prevalent, the ability to keep thousands of stars under constant observation is making the photometry technique the exoplanet-hunting tool that promises to push the boundaries of our understanding of planets elsewhere in the universe. 

As our catalogue of exoplanets expands, researchers also now begin to look beyond just spotting these other worlds. The CHEOPS telescope will launch this week (17/12/19) with its mission to spot exoplanets close-by that warrant further investigation. And it is the James Webb Space Telescope, launching in 2021, that will really delve into these selected planets.

The future of exoplanet hunting. Clockwise from top: The James Webb Space Telescope, the Extremely Large Telescope and the CHEOPS telescope.

Researchers will then use some of the methods I’ve listed above to examine the atmospheres of these planets, a ‘deep-dive’ that would have seemed like little more than a pipe-dream in 1995 when Michel Mayor and Didier Queloz spotted 51 Pegasi b. 

Exoplanet research, in many ways, represents one of the ultimate expressions of the drive to perform science. For its pioneers, the men and women that stocked our toolkit, it simply wasn’t enough to lie back staring at the stars, dreaming of other worlds. 

The rocket carrying CHEOPS splits depositing its cargo into a low-Earth orbit. (ESA)

New European exoplanet-hunting telescope launches into space

After an initial setback yesterday (17/12/19) due to a software error, the European Space Agency’s (ESA) CHaracterising ExOPlanets Satellite — or CHEOPS — telescope has finally launched from the European Spaceport in Kourou, French Guiana.

Blast off: CHEOPS begins its journey to space (NASA)

CHEOPS was aboard a Russian Soyuz-Fregat rocket which blasted off at 9:54 am European time. The Rocket will take approximately 145 minutes to place the CHEOPS unit into a rare pole to pole low-Earth orbit. 

The telescope hitched a ride with an Italian radar satellite, the rocket’s primary payload. 

CHEOPS being loaded aboard its method of transport (ESA)

CHEOPS is the result of a collaboration between 11 member countries within the ESA, with Switzerland taking the lead on the project. Two of the country’s leading Universities — the University of Geneva and the University Bern — worked together to equip CHEOPS with a state of the art photometer.

This powerful device will measure changes in the light emitted by nearby stars as planets pass by — or transit — them. This examination reveals many details about a planet’s characteristics, its diameter, and details of its atmosphere in particular. 

Another type of lift-off (ESA)

By combining a precise measurement of diameter with a measurement of mass, collected by an alternative method, researchers will then be able to determine a planet’s density. This, in turn, can lead to them deducing its composition and internal structure. 

CHEOPS was completed in a short time with an extremely limited budget of around 50-million Euros.

“CHEOPS is the first S-class mission for ESA, meaning it has a small budget and a short timeline to completion,” explains Kate Issak, an ESA/CHEOPS project researcher. “Because of this, it is necessary for CHEOPS to build on existing technology.”

CHEOPS: Informed by the past, informing the future

The project is acting as a kind of ‘middle-man’ between existing exoplanet knowledge and future investigations. It is directed to perform follow-up investigations on 400–500 ‘targets’ found by NASA planet-hunter Transiting Exoplanet Survey Satellite (Tess) and its predecessor, the Kepler observatory. Said targets will occupy a size-range of approximately Earth-Neptune.

Reaching new heights (ESA)

This mission then fits in with the launch of the James Webb Telescope in 2021 and further investigation methods such as the Extremely Large Telescope array in the Chilean desert, set to begin operations in 2026. It will do this by narrowing down its initial targets to a smaller set of ‘golden targets’. Thus, meaning its investigation should help researchers pinpoint exactly what planets in close proximity to Earth are worthy of follow-up investigation. 

“It’s very classic in astronomy that you use a small telescope ‘to identify’, and then a bigger telescope ‘to understand’ — and that’s exactly the kind of process we plan to do,” explains Didier Queloz, who acted as chair of the Cheops science team. “Cheops will now pre-select the very best of the best candidates to apply to extraordinary equipment like very big telescopes on the ground and JWST. This is the chain we will operate.”

Queloz certainly has pedigree when it comes to exoplanets. The astrophysics professor was jointly awarded the 2019 Nobel Prize in Physics for the discovery of the first exoplanet orbiting a Sun-like star with Michel Mayor. 

The first task of the science team operating the satellite, based out of the University of Bern, will be to open the protective doors over the 30 cm aperture telescope — thus, allowing CHEOPS to take its first glimpse of the universe. 

The field of exoplanet research is heating up, and the Trappist-1 system is a prime target for future investigation (University of Bern)

Exoplanet researcher awarded for groundbreaking work

The field of exoplanet research is heating up, and the Trappist-1 system is a prime target for future investigation (University of Bern)
The field of exoplanet research is heating up, and the Trappist-1 system is a prime target for future investigation (University of Bern)

Exoplanet researcher Ignas Snellen — a professor in astronomy at the University of Leiden in the Netherlands — has collected the 2019 Hans Sigrist prize for his innovative work in the field of exoplanet research. The award of the prize to Snellen comes at the conclusion of a year which also marked the Nobel Committee’s recognition of the first observation of an exoplanet orbiting a Sun-like star, awarding its discoverers Michel Mayor and Didier Queloz the 2019 Nobel Prize in Physics.

The message from the scientific community seems to be clear, exoplanet research is a field to watch. With the launch of the CHEOPS satellite, later this month and the James Webb Space Telescope set to launch in 2021, applied science is finally catching up to aspirations held by astronomers for decades–the discovery of more and more diverse worlds outside our solar system.

In addition to this, the tantalizing possibility of catching a fleeting glimpse of a clue that we are not alone in the universe seems closer than ever to realization.

Exoplanet researcher Ignas Snellen accepts his award for groundbreaking research (Robert Lea)

But as Snellen explains, things could have been very different for him, falling into exoplanet research was something of a happy accident: “I was doing very different research, working with galaxies,” he says. “I didn’t really know where my research was going. That’s when I was asked to present a workshop on extrasolar planets.”

Despite the serendipity at play, Snellen says he recognised the potential growth for the young field almost immediately. “I thought ‘wow, what an amazing field!'”

And Snellen’s decision to pursue exoplanet research relates indirectly to Nobel prize winners Mayor and Queloz. “This was in 2001, so exoplanet research was still in its infancy, but the first transiting exoplanet had been discovered as well as a few others.”

Despite the fact that only a handful of exoplanets had been discovered when Snellen began his research in the field less than two decades ago, NASA’s catalog of extrasolar planets now numbers in excess of 4,000. Clearly, this is a stunning illustration of just how rapidly the field has advanced in this relatively short period of time.

Snellen’s work focuses on assessing the atmospheric composition of exoplanets. This possible to do when a planet passes in front of–or ‘transits’– its parent star. The planet’s atmosphere absorbs light from the star at certain wavelengths. As chemical elements absorb and emit light at certain frequencies, the resulting light spectrum forms a distinct ‘fingerprint’ by which they can be identified.

This method of transit spectroscopy holds great promise in terms of identifying potential ‘biomarkers’ such as molecular oxygen, water and carbon monoxide.

As the Hans Sigrist Prize is specifically designated to recognize scientists in the midst of their careers rather than acting as a ‘lifetime achievement’ type award, it is only fitting that its recipient should very clearly have their eyes on future goals. And, fortunately, the future is bright for exoplanet research.

The next generation of telescopes will probe exoplanets more thoroughly than ever before. Clockwise from top: The James Webb Telescope (ESA), The Extremely Large Telescope (ESO) and the CHEOPS telescope (ESA)
The next generation of telescopes will probe exoplanets more thoroughly than ever before. Clockwise from top: The James Webb Telescope (ESA), The Extremely Large Telescope (ESO) and the CHEOPS telescope (ESA)

The ESO’s CHEOPS telescope launches on 17th December with its mission to identify nearby, small rocky exoplanets. Snedden points out the mission’s role as an important first step in the exoplanet research, helping select targets for researchers to further investigate.

But the two projects that Snellen is most excited for and the launch of the James Webb Telescope in 2021 and the completion of the aptly named 39-meter diameter Extremely Large Telescope (ELT) scheduled for completion in 2025.

“At the moment we can only observe the atmospheres of large Jupiter-like gas giants,” he explains. “The James Webb will finally allow us to start examining the atmospheres of smaller, rocky, more Earth-like exoplanets.”

These planets will still differ quite a bit from Earth, elaborates Snellen, explaining that they will, for example, be much hotter than our planet. “The exciting thing is, we don’t yet know if these small rocky planets can actually hold an atmosphere,” Snellen says.

The researcher concludes by pointing out that the seven Earth-like planets of the Trappist-1 system are very likely the first targets for further investigation. An exciting prospect, given that at least three of these planets are believed to exist in that system’s ‘habitable zone’–an area where water can exist as a liquid, a key ingredient for life.

One of the most promising prospects for discovering liquid water in the Trappist-1 system is Trappist-1e, an exoplanet that is slightly denser than Earth. As liquid water requires a temperature that is not too hot and not too cold, and also a certain amount of pressure–the fact that Trappist-1e receives roughly the same amount of radiation from its star as Earth and its gravitational influence exerts a similar pressure, it seems a safe bet to predict liquid water will be found there.

Of course, the concept of discovering liquid water amongst the stars and drawing comparisons to Earth leads, inevitably to the question could any of these planets also host life?

Snellen urges caution, remarking that even if these biomarkers are found, it is still a long way from confirming the ‘Holy Grail’ of exoplanet research: the presence of extraterrestrial life.

“It would be a major clue,” Snellen points out. “But it’s too simple to say ‘OK we have molecular oxygen, this is a sign of life.’ As molecular oxygen is difficult to detect though, by the time we can identify it, we should also be able to see lots of other gases.”

The Hans Sigrist Prize was established in 1994 to recognise mid-career scientists who still have a significant time left in their careers to make further major contributions to their field. Thus far, two of the previous recipients have gone on to become Nobel Prize Laureates later in their careers.

In connection with the prize, Snellen will receive 100,000 CHF–around $100,000–to help further his research. Accepting his prize, Snellen first thanked the team of researchers that have supported him in his research over the past decade.

Hunting for exoplanets: past and future

Kepler-11 is a Sun-like star around which six planets orbit. At times, two or more planets pass in front of the star at once, as shown in this artist’s conception of a simultaneous transit of three planets observed by NASA’s Kepler spacecraft. Credit: NASA.

In 2018, NASA’s Kepler Space Telescope mission came to a predictable end after it ran out of fuel some 94 million miles from Earth. During its nine-year planet-hunting mission, Kepler discovered nearly 3,000 exoplanet candidates and more than 2,600 confirmed exoplanets in our galaxy, cementing the notion that our solar system isn’t all that special. So far, 55 exoplanets that are potentially habitable — meaning they orbit their stars at just the right distance that may allow water to flow on the surface — have been found, of which 20 are Terran or Earth-sized.

There are at least as many planets as there are stars in the galaxy

After studying thousands of exoplanets astronomers are now confident that:

  • Relatively small planets are common;
  • There are likely more planets than stars in the galaxy;
  • Planetary systems are flat like the solar system;
  • Planets and planetary systems are extremely diverse;
  • No exoplanets similar to Earth in size, distance, and type of star they orbit have been found.
Artist concept of Kepler in space. Image credit: NASA/JPL

“I think that exoplanets tell us about our place in the universe. That’s probably the main reason the discovery of the first exoplanets were awarded the Nobel Prize in Physics this year. We are seeing just how diverse planets are. Planets are more common than they were thought to be before the first exoplanets were found. The number of planets in our galaxy is at least as large as the number of stars. But while planets and planetary systems are so diverse, planets like Earth may be very, very rare,” Dr. Jack Lissauer, a scientist at the NASA Ames Research Center and co-investigator of the Kepler mission, told ZME Science at the World Science Forum in Budapest. The World Science Forum is a biannual international conference on global science policy, which is affiliated with UNESCO and ICSU.

An exoplanet is any planet orbiting a star other than the Sun. Just 24 years ago, we only knew of planets in our solar system — but not for lack of trying. Detecting exoplanets is very challenging because they’re much smaller and fainter than stars. Since exoplanets are not self-luminous, scientists had to think outside the box.

The most successful exoplanet detection method is transit photometry, which looks for periodic, repetitive dips in the visible light of stars caused by planets passing, or transiting, in front of them. Essentially, a transit is just a partial eclipse.

Credit: Imgur.

“It was getting all the precision to detect the small variations in the light of the stars for planetary transits. To be able to detect planets — actually the smallest one that we’ve found was about the size of Earth’s moon, it’s the smallest planet known,” Lissauer said.

This has led to some incredibly unexpected discoveries that exceeded even our wildest expectations. Planets such as Kepler-22b, which is a water world between the size of Earth and Neptune located more than 600 light-years away or Kepler-16b, which is part of a ‘Tatooine-like’ system 200 light-years away — it is home to the largest planet ever discovered orbiting two stars. Then there’s the exciting Kepler-11 system, home to no fewer than six planets and the fullest, most compact planetary system yet discovered beyond our own.

Dr. Jack Lissauer. Credit: NASA Ames Research Center.

“A year before that, or even eight months before that, no multi-planetary transiting system had been discovered. By the time of Kepler-11 discovery, there were two others. One had three planets, one had two planets, Kepler-11 had six. And we were able to derive the masses of the inner five by the perturbations they gave on one another so the transits were not periodic. It contains five of the lowest mass exoplanets at the time for which we had measurements of both their size and their mass, so we could have good estimates of what they’re made of, by getting their density,” Lissauer told ZME Science.

The car-sized telescope was launched primarily to detect small planets. For this purpose, it was designed to find out how many planets are out there not by observing the entire galaxy but by taking a sample in and near the habitable zone — the region at the right distance from the star so that the surface has liquid water.

Beyond Kepler

Although it has been decommissioned, Kepler’s legacy lives on. Scientists are still sifting through thousands of candidate exoplanets, a task which will keep them busy for many years to come. Kepler has also shaped future missions such as the Transiting Exoplanet Survey Satellite, or TESS.

Launched in April 2018, TESS is NASA’s latest planet hunter. Its mission is to survey planets orbiting 200,000 of some of the brightest stars close to Earth. Later, planets identified by TESS can be inspected for a closer look by the upcoming James Webb Telescope.

“I’m a co-investigator on TESS as well as Kepler and I think of Kepler as having done great science by detecting these very small planets — planets not hugely different from Earth in their properties, in some cases,” Lissauer said.

“TESS won’t detect planets as small and as long-period orbits as Kepler — it has much smaller cameras — but it detects planets around brighter stars. So, the purpose of TESS is really finding planets around very bright stars so there’s enough light from these stars that we can detect light passing through their thin atmosphere where they transit their stars. So, TESS is enabling us — with other instruments, especially the James Webb Telescope, which will be launched in two years, and some of the very large and extremely large telescopes on the ground — to study the composition of the atmospheres of mid-sized exoplanets. Not Earth analogs — that’s too difficult — but not these hot Jupiters. So, planets a big step closer to our own,” he added, explaining TESS’ major role in future planet-hunting efforts.

Besides TESS, there are exciting exoplanet-hunting missions. Europe’s CHaracterising ExOPlanets Satellite (CHEOPS) mission is destined to launch in December. Its mission is to that of a follow-up: it will be tasked with studying stars known to harbor planets, rather than surveying the sky in search of new ones.

By performing observations of multiple planetary transits, CHEOPS will be able to provide more precise measurements of a planet’s size, which can be combined with existing mass determinations to render accurate densities. Knowing these parameters, it is possible to determine the exoplanet’s composition and discriminate between Earth-like planets where life may blossom and other types of Earth-mass planets that challenge our current notions of habitability.

Artist impression of CHEOPS. Credit: ESA – C. Carreau.

In 2026, ESA will launch PLATO, which is short for the PLAnetary Transits and Oscillations of stars) mission. PLATO is designed to find and determine the properties of Earth-like planets that orbit the habitable zone around stars similar to the Sun. For the first time, PLATO will allow scientists to calculate accurately the properties of a large number of stars with planets, including their ages.

Meanwhile, ESA’s ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission is destined to study and characterize exoplanets, rather than discover them. ARIEL is scheduled to launch in 2028. The mission is designed to perform high-accuracy transit, eclipse, and phase-curve multiband observations of exoplanets. Scientists are confident that ARIEL will be able to provide a complex picture of the chemical nature of its targeted exoplanets, but also their host stars. This will allow researchers to investigate the nature of these exoplanets, how they formed, and how they evolved.

Kepler opened the gate for mankind’s exploration of the cosmos, and its successors are bound to offer even more surprises. There are billions — perhaps trillions — of stars in the Milky Way galaxy alone and, on average, each of those stars has at least one planet orbiting them. However, there really is no place like home.

“We only have Earth. It’s possible that there are other planets like Earth out there but even if they are very similar, they are very far away. We can’t do a migration. We can’t solve our problem as the Irish did in the late 1800s to solve the potato famine. We would be like Easter Island. If we don’t take care of this planet, we are toast! All the mass extinctions in the last 5 million years are coincident with the rise of CO2 in the atmosphere of our planet. We must stop this crazy behavior. We can’t just say something has to be done. We have to do things ourselves. We have to cut our carbon footprint,” Lissauer told the audience during an event at the 2019 World Science Forum, held between 20-24 November.