Tag Archives: planet formation

Chaotic Young Star System Holds the Key to Planet Formation

One of the major problems which has hindered our understanding of planet formation has been the lack of direct measurements of the mass of planet-forming protoplanetary discs. Now, by successfully measuring the mass of a unique protoplanetary disc for the first time, astronomers have confirmed that gravitational instabilities play a key role in the formation of planets.

The team of astronomers, led by Teresa Paneque-Carreño, a PhD student at the University of Leiden and the European Southern Observatory (ESO), used gas velocity data collected using the Atacama Large Millimeter/submillimeter Array (ALMA) to make observations of the young star Elias 2-27 which is surrounded by a disc of gas and dust with some extraordinary features.

The star which is located just under 400 light-years from Earth in the constellation Ophiuchus has been a popular target for investigation by astronomers for at least five decades which paid off in 2016 with the discovery that the young star is surrounded by a disc of gas and dust. This marks the first time, however, that such a mass measurement has been made and gravitational instabilities have been confirmed.

Using gas velocity data, scientists observing Elias 2-27 were able to directly measure the mass of the young star’s protoplanetary disk and also trace dynamical perturbations in the star system. Visible in this paneled composite are the dust continuum 0.87mm emission data (blue), along with emissions from gases C18O (yellow) and 13CO (red). (ALMA (ESO/NAOJ/NRAO)/T. Paneque-Carreño (Universidad de Chile), B. Saxton (NRAO))

“How exactly planets form is one of the main questions in our field. However, there are some key mechanisms that we believe can accelerate the process of planet formation,” explains Paneque-Carreño. “We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable.

“Elias 2-27 is the first system that checks all of the boxes.”

Teresa Paneque-Carreño, University of Leiden

Paneque-Carreño is the first author of one of two papers detailing the team’s findings–which give astronomers the key to unlocking the mystery of planet formation– published in the latest edition of The Astrophysical Journal.

What makes Elias 2-27 the Ideal System for Cracking the Planet Formation Mystery?

Researchers have known for some time that protoplanetary discs of gas and dust surrounding young stars are locations of planet formation and we have certainly no shortage of studies of such structures. But, despite having this knowledge and a wealth of observational data, the exact process that leads to the birth of a planet has remained a puzzle.

This beautiful image, captured with ALMA shows the protoplanetary disc surrounding the young star Elias 2-27 which could be the key to solving the mystery of planet formation (B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO))

Fortunately, telltale evidence of gravitational instabilities around Elias 2-27 made it the ideal star for astronomers in order to conduct a thorough investigation of planet formation.

“We discovered in 2016 that the Elias 2-27 disk had a different structure from other already studied systems, something not observed in a protoplanetary disk before: two large-scale spiral arms,” remarks principal investigator Laura Pérez, Assistant Professor at the Universidad de Chile. “Gravitational instabilities were a strong possibility, but the origin of these structures remained a mystery and we needed further observations.”

It was Pérez who suggested that ALMA–a series of 66 radio telescopes located in the Atacama Desert of northern Chile–should be trained on the spiral of gas and dust surrounding this young star.

It was this further study that revealed, not only does Elias 2-27 possess a protoplanetary disc with signs of gravitational instabilities within it, it also has something unique for such a structure: spiral arms.

Elias 2-27: A Unique and Chaotic Young Star System

The presence of spiral arms in the protoplanetary disc is believed to be the result of perturbations caused by density waves throughout the gas and dust that comprise it.

It is the first star-forming disc discovered with such features. But, to Paneque-Carreño it signals the presence of something else within the disc, chaos. This chaotic nature also gives rise to another characteristic never seen in a disc such as this.

“There may still be new material from the surrounding molecular cloud falling onto the disc, which makes everything more chaotic,” says the graduate of the Universidad de Chile. “The Elias 2-27 star system is highly asymmetric in the gas structure. This was completely unexpected, and it is the first time we’ve observed such vertical asymmetry in a protoplanetary disc.”

Elias 2-27 is a young star located just 378 light-years from Earth. The star is host to a massive protoplanetary disk of gas and dust, one of the key elements to planet formation. In this graphic illustration, dust is distributed along a spiral-shaped morphology first discovered in Elias 2-27 in 2016. The larger dust grains are found along the spiral arms while the smaller dust grains are distributed all around the protoplanetary disk. Asymmetric inflows of gas were also detected during the study, indicating that there may still be material infalling into the disk. Scientists believe that Elias 2-27 may eventually evolve into a planetary system, with gravitational instabilities causing the formation of giant planets. Because this process takes millions of years to occur, scientists can only observe the beginning stages. (B. Saxton NRAO/AUI/NSF)

It is the double-punch of this vertical asymmetry and large-scale perturbations giving rise to a spiral structure that Cassandra Hall, Assistant Professor of Computational Astrophysics, University of Georgia, believes has major implications for our theories of planet formation.

“This could be a ‘smoking gun’ of gravitational instability, which may accelerate some of the earliest stages of planet formation,” says Hall, a co-author of one of the papers detailing these findings. “We first predicted this signature in 2020, and from a computational astrophysics point of view, it’s exciting to be right.”

This research has cracked the problem of measuring the mass of a protoplanetary disc, thus removing a significant barrier in our understanding of planet formation. This was possible in large part due to the high sensitivity of ALMA’s observing bands, particularly band 6 which covers light with a wavelength of 1.1 to 1.4 nanometres in combination with bands 3 and 7–which cover 2.6 – 3.6 nm and 0.8 -1.1 nm, respectively.

“Previous measurements of protoplanetary disc mass were indirect and based only on dust or rare isotopologues. With this new study, we are now sensitive to the entire mass of the disc,” says the second paper’s lead author Benedetta Veronesi, a postdoctoral researcher at École normale supérieure de Lyon. “This finding lays the foundation for the development of a method to measure disc mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation. “

“Knowing the amount of mass present in planet-forming discs allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form.”

Benedetta Veronesi, École normale supérieure de Lyon

More Planet Formation Mysteries to Solve

Even though this research has answered some of the questions surrounding the process of planet formation, like the best scientific discoveries, it has also given rise to new questions.

Whilst mysteries still remain surround the process of planet formation, equipped with the stunning observational power of ALMA researchers are up to the challenge (NRAO)

“While gravitational instabilities can now be confirmed to explain the spiral structures in the dust continuum surrounding the star, there is also an inner gap, or missing material in the disk, for which we do not have a clear explanation,” explains Paneque-Carreño.

Many of these questions are difficult to answer because of the vast difference between the timescales on which we live and those taken by the processes that birth planets.

“Studying how planets form is difficult because it takes millions of years to form planets. This is a very short time-scale for stars, which live thousands of millions of years, but a very long process for us,” said Paneque-Carreño. “What we can do is observe young stars, with disks of gas and dust around them, and try to explain why these disks of material look the way they do. It’s like looking at a crime scene and trying to guess what happened. “

Fortunately, researchers like Paneque-Carreño, Cassandra Hall, and Benedetta Veronesi are prepared to tackle this monumental challenge and solve planet formation’s remaining mysteries.

“Our observational analysis paired with future in-depth analysis of Elias 2-27 will allow us to characterize exactly how gravitational instabilities act in planet-forming discs and gain more insight into how planets are formed,” concludes Paneque-Carreño.

Comets have a heavy metal atmosphere

Using data collected by the Very Large Telescope (VLT) a team of astronomers has discovered iron and nickel in the atmosphere of around 20 different solar system comets–including some located far away from the Sun.

These findings will come as a surprise to astronomers because even though such heavy metals have been known to exist in solid form within comet interiors before, the vapour of such elements has only previously been associated with cometary atmospheres in hot environments.

This is the first time such vapour has been seen in the cooler atmospheres of comets that exist far from a star and could indicate some previously unknown mechanism or material on the surface of comets.

“It was a big surprise to detect iron and nickel atoms in the atmosphere of all the comets we have observed in the last two decades, about 20 of them, and even in ones far from the Sun in the cold space environment,” says Jean Manfroid, of the University of Liège, Belgium.

ESO/L. Calçada, SPECULOOS Team/E. Jehin, Manfroid et al.

This wasn’t the only surprise the team found, however. The Belgian astronomers–who have been studying comets with the VLT for 20 years–observed nickel and iron in the atmosphere of the comet in equal amounts.

Generally, iron is about ten times more abundant in the solar system than nickel, and comets are believed to be material left over from the formation of planetary bodies within the solar system. That means it’s something of a mystery why the comets the team observed should have such a relatively large abundance of nickel.

“Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time. In that sense, they’re like fossils for astronomers,” Emmanuel Jehin, also from the University of Liège. This discovery went under the radar for many years.”

Manfroid and Jehin are two of the authors of a paper published in the latest edition of the journal Nature documenting the team’s findings. And that isn’t the only research revealing metal in the atmosphere of such a body published in Nature this month.

The discovery is accompanied by the revelation that a separate team of researchers, this time located in Poland, has also found traces of nickel vapour in the atmosphere around the interstellar visitor 2l/Borisov.

This comet may sound familiar as it made headlines in 2019 when it became only the second object found within the solar system which originated from outside our planetary system.

A paper detailing this second finding is also published in this month’s Nature.

Heavy Metal Rocks

Astronomers have known for some time that a variety of metals exist within the icy and rocky interiors of comets. There have even been suggestions that spent comets could be mined for precious or useful metals like gold, silver, platinum and iron.

This image features a comet located in the outer reaches of the Solar System: comet C/2016 R2 (PANSTARRS). (ESO/SPECULOOS Team/E. Jehin)

These solid metals within comets were not expected to be found as gases in the body’s atmosphere, though, unless that body is passing within close vicinity to a star.

It is the heat from these close brushes with stars like the Sun that causes solid metals within comets to ‘sublimate’–the process by which solid material changes directly into a gaseous state.

That means that distant comets in the cold environment of space away from the heat of the Sun shouldn’t have heavy metal atmospheres.

Yet, despite this, researchers have now found nickel and iron vapour in the atmospheres of comets up to 480 million kilometres from the Sun. A distance that is three astronomical units, or three times the distance between the Sun and the Earth.

In order to make this discovery, the team employed the technique of spectroscopy which reveals the signatures of specific chemical elements and the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on the VLT to assess the chemical composition of comets’ atmospheres.

The spectral lines of nickel and iron found by the team in comets’ atmospheres were extremely faint, which leads them to believe that the reason such elements have been missed in past is due to their tiny abundance. The team says that for every 100kg of water in the atmosphere of the comets they studied there is just one gram of iron and nickel respectively.

The Belgian astronomers believe that the equal amounts of iron and nickel together with the sublimation at low temperatures means there is something undiscovered at the surface of the comets they studied.

An artist’s impression of the completed ELT, which could play an important role in the investigation of cometary atmospheres. (ESO)

“Usually there is 10 times more iron than nickel, and in those comet atmospheres we found about the same quantity for both elements,” explains Damien Hutsemékers, also a member of the Belgian team from the University of Liège.”We came to the conclusion they might come from a special kind of material on the surface of the comet nucleus, sublimating at a rather low temperature and releasing iron and nickel in about the same proportions.”

The team intends to attempt to use new telescope technology such as the Mid-infrared ELT Imager and Spectrograph (METIS) on ESO’s upcoming Extremely Large Telescope (ELT)–currently under construction in the Atacama Desert region of Northern Chile– to discover what this material is.

The findings of this team are accompanied by the revelation that nickel vapour has also been discovered in the atmosphere of 2I/Borisov.

2I/Borisov: The Interstellar Intruder that keeps giving

The discovery that metal is also present in the atmosphere of the interstellar visitor 2I/Borisov was made by a team of astronomers in Poland. The team also used the VLT to catch a glimpse of the interstellar comet as it passed through the solar system.

The data collected with the VLT’s X-Shooter spectrograph revaled nickel vapour in the cold envlope surround 2I/Borisov.

ESO/L. Calçada/O. Hainaut, P. Guzik and M. Drahus

The discovery marks another surprise for astronomers, as again it details the discovery of sublimated heavy metals in a cold atmosphere.

“At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun,” says Piotr Guzik, the Jagiellonian University, Poland, a co-author on this second study. “It took numerous tests and checks before we could finally convince ourselves.”

This latter study shows that nickel was not uniquely present during the formation of our solar system, but as it can be seen in a comet from another planetary grouping, it may well be common in many such conglomerations.

 “All of a sudden we understood that gaseous nickel is present in cometary atmospheres in other corners of the Galaxy, Michał Drahus, also from the Jagiellonian University and another of the paper’s co-authors, says.

In unison, both these studies indicate that the comets of this solar system and the interstellar visitor 2I/Borisov share many similarities. Dahus adds: “Now imagine that our Solar System’s comets have their true analogues in other planetary systems — how cool is that?”

Jehin, meanwhile, believes these studies could inspire future research into cometary bodies and their atmospheres, and a re-examination of data already collected.

“Now people will search for those lines in their archival data from other telescopes,” the University of Liège researcher concludes. “We think this will also trigger new work on the subject.”

GW Orionis, a triple star system with a peculiar inner region. Unlike the flat planet-forming discs we see around many stars, GW Orionis features a warped disc, deformed by the movements of the three stars at its centre. This composite image shows both the ALMA and SPHERE observations of the disc. (ESO/Exeter/Kraus et al., ALMA (ESO/NAOJ/NRAO))

Warped gas disc torn apart by three stars directly observed for the first time

Astronomers have discovered a spectacular first in terms of star clusters and planet-forming discs of gas, a system–GW Orionis–with a warped disc with torn out inner rings. The team believes that the disc’s odd shape –which defies the common view of a flat plane orbiting planets and gas discs–was created when the misalignment of the three stars at the centre of the disc caused it to fracture into distinct rings.

As well as being extraordinary in its own right, the astronomers believe that the warped disc could harbour exotic and strange exoplanets– not unlike Tatooine in Star Wars series– which formed within the inclined rings and are, for now, hidden from view.

“The idea that planets form in neatly-arranged, flat discs around young stars goes back to the 18th century and Kant and Laplace,” research team-leader Stefan Kraus, a professor of astrophysics at the University of Exeter in the UK, tells ZME Science. “Our images reveal an extreme case where the disc is not flat at all, but is warped and has a misaligned ring that has broken away from the disc.”

“‘Tatooine’ planets that orbit around 2 or 3 suns have already been envisioned by science fiction and some Tatooine exoplanets have already been found.  Here, we observe how such planets form and find that they can form on extreme, highly inclined orbits — in configurations that are completely different from the ‘neat’ arrangement observed in the Solar System.”

Stefan Kraus, professor of astrophysics, the University of Exeter
The left panel shows an artistic impression of the inner region of the GW Orionis disc, including the ring, which is based on the 3D shape reconstructed by the team. (ESO)

GW Orionis is Twisted

The team saw the warped shape of the system GW Orionis, which sits 1300 light-years from Earth in the constellation of Orion, in observations made by the Very Large Telescope (VLT) operated by European Southern Observatory (ESO), and the Atacama Large Millimeter/ submillimeter Array (ALMA) based in the Chilean desert. But, properly envisioning this shape and its cause meant studying the system for a staggering 11 years.

“The most important result from our study is that we can identify the cause for the misalignments and link it to the ’disc tearing’ effect that has been proposed by theorists 8 years ago, but has not been observed so far,” Kraus continues. “For this, it was essential to measure the orbital motion of the three stars that are in the centre of the system over their full 11-year orbital period. 

“We found that the three stars do not orbit in the same plane, but their orbits are misaligned with respect to each other and with respect to the disc.”

Stefan Kraus, professor of astrophysics, the University of Exeter
This animation allows the viewer to see the warped disc and the tilted ring of GW Orionis that was torn apart from it in spectacular detail. The animation is based on a computer model of the inner region of GW Orionis, provided by the team; they were able to reconstruct the 3D orbits of the stars and the 3D shape of the disc from the observational data.

“We have observed GW Orionis, a triple star system surrounded by a planet-forming disc, with several different telescopes including the VLT and ALMA. After observing the three stars for several years, our team was able to calculate the orbits very accurately,” team member Alison Young of the Universities of Exeter and Leicester tells ZME Science. “This data allowed us to build a detailed computer model of the system, which predicted that the disc would be bent and even torn to form a separate inner ring.”

“A couple of years later when we received the data back from the VLT and ALMA, the image of a disc bent and even torn to form a separate inner ring, were stunning.”

Alison Young of the Universities of Exeter and Leicester

A paper detailing their work is published in the journal Science.

ALMA, in which ESO is a partner, and the SPHERE instrument on ESO’s Very Large Telescope have imaged GW Orionis, a triple star system with a peculiar inner region. Unlike the flat planet-forming discs we see around many stars, GW Orionis features a warped disc, deformed by the movements of the three stars at its centre. This composite image shows both the ALMA and SPHERE observations of the disc. The ALMA image shows the disc’s ringed structure, with the innermost ring (part of which is visible as an oblong dot at the very centre of the image) separated from the rest of the disc. The SPHERE observations allowed astronomers to see for the first time the shadow of this innermost ring on the rest of the disc, which made it possible for them to reconstruct its warped shape. (ESO/Exeter/Kraus et al., ALMA (ESO/NAOJ/NRAO))
GW Orionis, a triple star system with a peculiar inner region. Unlike the flat planet-forming discs we see around many stars, GW Orionis features a warped disc, deformed by the movements of the three stars at its centre. This composite image shows both the ALMA and SPHERE observations of the disc. (ESO/Exeter/Kraus et al., ALMA (ESO/NAOJ/NRAO))

That Tears It! How GW Orionis got warped

The images of GW Orionis that the astronomers collected represent the first visualisation of disc-tearing ever captured by researchers. This tearing and the ‘warped’ effect it created marks this out as a planetary system exceptionally different from the solar system.

“The radial shadows in the VLT SPHERE image are clear evidence that the ring is tilted. To form a narrow shadow like this on the disc you need a fairly opaque ring of material that is at an angle to the disc surface blocking the starlight,” Young explains. “This result is consistent with some modelling done by members of the team which worked out the most likely orientations of the components of the system.”

A 3D model of GW Orionis, (Kraus et al. 2020 Science 371)
A 3D model of GW Orionis, (Kraus et al. 2020 Science 371)

“This system is unusual because the orbits of the three stars are misaligned, unlike the planets in the solar system they do not orbit in the same plane, and these stars host a large disc that is also tilted relative to their orbits,” Young continues. “We see all sorts of intriguing structures now in images of protoplanetary discs but this is the first direct evidence of the disc tearing effect.”

The observations also gave the researchers an idea of the vast scale of the GW Orionis disc.

“The ring harbours about 30 Earth masses of dust, which is likely sufficient for planet formation to occur in the ring.  Any planets formed within the misaligned ring will orbit the star on highly oblique orbits and we predict that many planets on oblique, wide-separation orbits will be discovered in future planet imaging surveys.”

Stefan Kraus, professor of astrophysics, the University of Exeter

As well as being able to reconstruct the torn disc of GW Orionis from the ALMA data in conjunction with data collected from several other telescopes, the team has been able to piece together the process by which this tearing likely occurred. They conclude that it could be a result of those three, misaligned stars. Something that initially came as a surprise to the astronomers.

“One very intriguing aspect of GW Orionis is that the orbits of the stars are strongly misaligned with respect to each other, and they are also strongly titled with respect to the large-scale disc. This wasn’t clear at the time when we started the study and became only apparent after monitoring the orbit motion for the full 11-years orbital period.”

Stefan Kraus, professor of astrophysics, the University of Exeter
This computer simulation shows the evolution of the GW Orionis system. The scientists believe the disc around the three stars in the system was initially flat, much like the planet-forming disc we see around many stars. Their simulation shows that the misalignment in the orbits of the three stars caused the disc around them to break into distinct rings, which is exactly what they see in the observations of the system. (Exeter/Kraus et al.)

Alison Young explains that because the disc surrounds three stars and the orbits of those stars are misaligned with respect to each other, the gravitational pull on the disc is not the same all the way around. This means that the gas and dust orbiting in the disc around all three stars feels a different force at different positions in the disc. This is what tears the disc apart into separate rings.

“Our study shows that the strong distortions observed in the disc– such as the warp and torn-away ring–can be explained by the conflicting gravitational pull from the 3 stars.  The key aspect is that the orbits are strongly misaligned with the disc.

Stefan Kraus, professor of astrophysics, the University of Exeter

How Warped Rings and Multiple Suns Effect Exoplanets

One interesting consequence of the warping of this gas and dust is that fact that it will wrap rings of material around any planets forming within it. This tearing also has a marked effect on these exoplanets’ orbits. This leads to conditions that would make the exoplanets in the GW Orionis system significantly different from planets in our own solar system.

“The planets in our solar system all have more-or-less aligned orbits. Any planets that form in the warped disc or misaligned ring could have highly inclined orbits,” says Young. “Further out, the disc is flatter and any planets that form there are likely to orbit in a similar plane to the disc. Of course, any planets that form in the GW Orionis system will also have three suns!”

Kraus points out that planets with oblique orbits have been identified before–particularly in the case of ‘Hot Jupiters’–planets with a mass and size comparable to the solar system’s largest planet, but that orbit closer to their star and transit across its face.

“Hot Jupiters orbit their stars very close in, and it is clear that they have not formed on the oblique orbits were we observe them.  Instead, they must have been moved onto these orbits through migration processes,” Kraus says. “We haven’t found yet any long-period planets on oblique orbits–comparable to Earth or Jupiter. However, our research shows that such planets could form in the torn-apart rings around multiple systems. 

“Given that about half of all stars are found in multiple systems, there could be a huge population of such long-period planets with high obliquity.”

Stefan Kraus, professor of astrophysics, the University of Exeter
This artists impression shows the orbit of the planet in the triple-star system HD 131399. Two of the stars are close together and the third, brighter component is orbited by a gas giant planet named HD 131399Ab.

Existing under the glare of three suns would make the planets in the GW Orionis system similar in some ways to an exoplanet discovered by astronomers from the University of Arizona in 2016.

The young exoplanet HD 131399Ab, 340 light-years from Earth in the constellation Centaurus, has a scorching hot temperature of around 580 C and exists in a state of constant daytime. It too has been compared to the planet of Tatooine from the Star Wars series. But Straus believes the planets in GW Orionis could be much cooler than this–or could alternate between cool and hot climates.

“Planets on such orbits could have stable atmospheric conditions, but would be ‘ice worlds’ with low temperatures on their surfaces,” Kraus says. “Planets that might have formed in the circumstellar/ circumbinary disc would experience extreme temperature variations, depending on where they are on their orbit. This should result in a strongly variable climate.”

Further Questions and Future Investigations: Delving Deeper into GW Orionis

Questions still remain about the GW Orionis system especially in light of research from another team who investigated the system with the ALMA telescope. This work-published in The Astrophysical Journal Letters earlier this year– suggests that our understanding of how the disc became warped is missing a vital component. “We think that the presence of a planet between these rings is needed to explain why the disc tore apart,” says Jiaqing Bi of the University of Victoria, Canada, lead author of a paper.

Speaking to ZME Science exclusively, Kraus addresses this earlier research: “This alternative scenario, where a yet-unseen planet located between the inner and middle ring might be the cause for the unusual disc shape, is more speculative, as such as planet has not been found yet,” the astrophysicist says. “Also, the paper’s authors had less information on the 3-dimensional shape of the disk as their ALMA observations had 6x lower solution and they did not have scattered-light images showing the shadows. Plus, they did not know the full orbits.”

Young continues by adding one future question regarding GW Orionis she would like to see answered also concerns the mechanism that caused the warping of the as and dust planet-forming disc.

“An important question we need to look at is how these systems came to be misaligned in the first place. Was the disc formed with the stars, did the material forming the disc arrive later, or did the system get disrupted at some point?”

Alison Young of the Universities of Exeter and Leicester

“Think of a star as a spinning top tilted at an angle,” the researcher suggests. “We want to find out how tilted the stars are so we can check whether a star’s tilt–or ‘spin axis’– matches the tilt of its disc, or if the stars in a binary or triple system have the same or different tilts.”

Some members of the team that made this discovery are currently developing a technique for measuring the spin axis of stars which could massively aid the understanding of how these systems formed.

An Upcoming survey conducted by the ALMA telescope array could help shed light on the motion of gas and dust in planet-forming discs such as that found in the GW Orionis system. (NSF/NRAO)
An Upcoming survey conducted by the ALMA telescope array could help shed light on the motion of gas and dust in planet-forming discs such as that found in the GW Orionis system. (NSF/NRAO)

Remembering that whilst this is not the first system discovered with such a warped disc, it is the first with a directly observed torn disc. This means the key to answering lingering questions likely lies in the direct observation of more systems that share features with GW Orionis.

“There are a few planet-forming discs that show some evidence of warping but for these, it is unclear what is causing the effect or there is an alternative scenario that can explain the observations, that has not been ruled out yet,” adds Young. “This is the first time that disc tearing has been directly observed and the only system so far for which we can link the structure with the physical mechanism behind it.”

Young suggests that the results of a larger survey performed by the ALMA array could provide clearer information about the motion of gas in planet-forming discs and their chemical composition, thus helping the team gather more information about the GW Orionis disc.

“We would like to obtain high-resolution observations of molecular emission from GW Orionis to shed more light on the motion of the gas in the disc and perhaps reveal any planets that are forming,” she explains. “Of course, we also are keen to understand if there are differences in how planets might form in warped discs compared to flat discs around a single star and we will be working on new computer models to look at this, using what we have learned from our observations.”

ALMA and SPHERE view of GW Orionis (side-by-side)
The ALMA image (left) shows the disc’s ringed structure, with the innermost ring separated from the rest of the disc. The SPHERE observations (right) allowed astronomers to see for the first time the shadow of this innermost ring on the rest of the disc, which made it possible for them to reconstruct its warped shape.

Young explains the importance of the GW Orionis images the team captured, whilst focusing on one image that for her, brought home the significance of the investigation in which she played a part.

“I find the SPHERE image [above left] in particular amazing because we can really see the disc is a 3-dimensional structure with a surface covered in bumps and shadows. We are looking at what could eventually become an unusual type of planetary system in the very process of forming.”

Alison Young of the Universities of Exeter and Leicester

For Stefan Kraus, the beauty of investigating a system such as GW Orionis is the wonder to imagining what it is like to stand on the surface of such a world and stare up into sky. Kraus concludes: “Half of the sky would be covered by a massive disc warp that is being illuminated by the 3 stars, intercepted by narrow shadows that are cast by the misaligned disc ring.”

“I find it fascinating to imagine how the sky would look like from any planet in such a system — one would see not only the 3 stars dancing around each other at different speeds but also a massive dust ring extending over the whole firmament.”

Stefan Kraus, professor of astrophysics, the University of Exeter
This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)

Planets could orbit Supermassive Black Holes

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)
This is an artist’s impression of planets orbiting a supermassive black hole. (Kagoshima University)

The idea of stars orbiting the supermassive black holes that researchers believe lurk at the centre of most galaxies has been long established as a matter of fact in science. In ‘active galactic nuclei’ or AGNs, these black holes are surrounded by haloes of gas and dust in a violent churning environment. Such clouds of gas and dust have the potential to birth not only stars but planets as well. Yet, the question of whether planets can also orbit these spacetime events has yet to be established. 

Enter Keiichi Wada, a professor at Kagoshima University, and Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan. These scientists from the distinct fields of active galactic nuclei research and planet formation research respectively have calculated that as a result of gas disc growth, an entirely new class of planets may form around supermassive black holes. 

“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” Wada points out. 

In their research published in the Astrophysical Journal, the duo of theoreticians propose that protoplanetary discs that surround young stars may not be the only potential site for planet formation. The researchers instead focused calculations and mathematical models on the denser dust discs found around supermassive black holes in AGNs, thus arriving at a surprising conclusion. 

“Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed [at a distance of] around 10 light-years from a black hole,” says Eiichiro Kokubo. 

“Around black holes, there might exist planetary systems of astonishing scale.”

One of the hindrances to the formation of planets in such discs of dust has previously been the amount of energy generated in AGNs, Researchers had believed that this energy output would prevent the coagulation of ‘fluffy ice dust’ that can help the growth of dust grains that can lead to planet formation in protoplanetary discs.

But, what Wada and Kokubo discovered was that the huge density of dust discs around supermassive black holes in AGNs —potentially containing as much as a hundred thousand times the mass of the Sun worth of dust, which is a billion times more massive than a typical protoplanetary disc — helps protect the outer layers from bombardment from high-energy radiation such as gamma rays. 

 A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)
A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)

This helps form a low-temperature region similar to that found in protoplanetary discs, and thus, in turn, increases the likelihood of fluffy deposits building.

The process would lead to the formation of planets within a period of several hundred million years, according to the pair, and also result in much denser and more populated collections of planets. 

Unfortunately, the limits of current methods of identifying exoplanets would make identifying planets around a supermassive black hole challenging to say the least. 

“ Doppler spectroscopy, transit photometry, gravitational micro-lensing, or direct imaging are hopeless,” warn the duo in their paper. They go on to suggest that a method called photometry with an x-ray interferometer located in space could be a possible solution — if a way of distinguishing the effect caused by such planets from the natural variability of the AGN can be developed. 

For now, researchers will have to look to mathematical models alone to theorise about the potential for planets in orbit around black holes. 

Original research: https://arxiv.org/pdf/1909.06748.pdf

Artist’s impression of the interior of a hot, molten rocky planet.

Molten exoplanets may explain the formation of Earth-like worlds

Artist’s impression of the interior of a hot, molten rocky planet.
Artist’s impression of the interior of a hot, rocky, molten exoplanet. (© University of Bern, illustration: Thibaut Roger)

Researchers from the University of Bern have discovered that the Earth would be approximately 5% larger if it were hot and molten rather than rocky and solid. Pinpointing the difference between rocky exoplanets and their hot, molten counterparts is vital for the search for Earth-like exoplanets orbiting stars outside the solar system. 

The fact that rocky exoplanets that are approximately Earth-sized are small in comparison to other planets, makes them notoriously difficult for astronomers to spot and characterise. Identification of a rocky exoplanet around a bright, Sun-like star will likely not be plausible until the launch of the PLATO mission in 2026. Thankfully, spotting Earth-size planets around cooler and smaller stars such as the red dwarfs Trappist-1 or Proxima b is currently possible. 

But, searching for molten exoplanets could help astronomers probe the darkness of space — and identify Earth-sized rocky-exoplanets around stars like our own. 

“A rocky planet that is hot, molten, and possibly harbouring a large, outgassed atmosphere ticks all the boxes,” says Dan Bower, an astrophysicist at the Center for Space and Habitability (CSH) of the University of Bern. “Such a planet could be more easily seen by telescopes due to strong outgoing radiation than its solid counterpart.”

Learning more about these hot, molten worlds could also teach astronomers and astrophysicists more about how planets such as our’s form. This is because rocky planets such as the Earth are built from ‘leftovers of leftovers’ — material not utilised in either the formation of stars or giant planets. 

“Everything that doesn’t make its way into the central star or a giant planet has the potential to end up forming a much smaller terrestrial planet,” says Bower: “We have reason to believe that processes occurring during the baby years of a planet’s life are fundamental in determining its life path.”

This drove Bower and a team of colleagues mostly from within the Planet S network to attempt to discover the observable characteristics of such a planet. The resulting study — published in the journal Astronomy and Astrophysics — shows that a molten Earth would have a radius 5% or so larger than the actual solid counterpart. They believe this disparity in size is a result of the differences in behaviour between solid and molten materials under the extreme conditions generated beneath the planet’s surface. 

As Bower explains: “In essence, a molten silicate occupies more volume than its equivalent solid, and this increases the size of the planet.”

Artist’s impression of the interior of a hot, rocky, molten exoplanet (with labels). (© University of Bern, illustration: Thibaut Roger)

This 5% difference in radii is something that can currently be measured, and future advances such as the space telescope CHEOPS — launching later this year — should make this even easier. 

In fact, the most recent collection of exoplanet data suggests that low-mass molten planets, sustained by intense starlight, may already be present in the exoplanet catalogue. Some of these planets may well then be similar to Earth in regards to the material from which they are formed — with the variation in size no more than the result of the different ratios of solid and molten rock. 

Bower explains: They do not necessarily need to be made of exotic light materials to explain the data.”

Even a completely molten planet would fail to explain the observation of the most extreme low-density planets, however. The research team suggest that these planets form as a result of molten planets releasing — or outgassing  — large atmospheres of gas originally trapped within interior magma. This would result in a decrease in the observed density of the exoplanet. 

Spotting such outgassed atmospheres of this nature should be a piece of cake for the James Webb Telescope if it is around a planet that orbits a cool red dwarf — especially should it be mostly comprised of water or carbon dioxide. 

The research and its future continuation have a broader and important context, points out Bower. Probing the history of our own planet, how it formed and how it evolved. 

“Clearly, we can never observe our own Earth in its history when it was also hot and molten. But interestingly, exoplanetary science is opening the door for observations of early Earth and early Venus analogues that could greatly impact our understanding of Earth and the Solar System planets,” the astrophysicist says. “Thinking about Earth in the context of exoplanets, and vice-versa offers new opportunities for understanding planets both within and beyond the Solar System.”

Original research: Dan J. Bower et al: Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations, Astronomy & Astrophysics. DOI: https://doi.org/10.1051/0004-6361/201935710

Birthplace of giant planets: Monash astrophysicists discover a baby planet sculpting a disc of gas and dust. Credit: ESO/ALMA.

‘Baby’ planet two to three times the size of Jupiter discovered

It may be an infant, but that doesn’t mean it’s small. Researchers have discovered a new ‘baby’ planet, at least twice the size of Jupiter, carving a path through a stellar nursery. 

Astrophysicists from Monash University have used the ALMA telescope in Chile to discover a ‘baby’ planet inside a protoplanetary disc. But despite being a youngster, this infant is still between two to three times the mass of Jupiter — the most massive planet in our solar system. 

Birthplace of giant planets: Monash astrophysicists discover a baby planet sculpting a disc of gas and dust. Credit: ESO/ALMA.

The giant ‘baby’ was found inside in the middle of a gap in the gas and dust that forms the planet-forming disc around the young star HD97048. The study — published in Nature Astronomy — is the first to provide an origin of these gaps in protoplanetary discs — also known as ‘stellar nurseries’ because they act as the birthplaces for planets— which have thus far puzzled astronomers. 

“The origin of these gaps has been the subject of much debate,” says the study’s lead author, Dr Christophe Pinte, an ARC Future Fellow at the Monash School of Physics and Astronomy. “Now we have the first direct evidence that a baby planet is responsible for carving one of these gaps in the disc of dust and gas swirling around the young star.”

The team discovered the new planet by mapping the flow of gas around HD97048 — a young star not yet on the main sequence, which sits in the constellation Chamaeleon located over 600 light-years from Earth. 

Observing the flow in this material, the team hunted for areas in which the flow was disturbed, in a similar way to disturbance a submerged rock would cause in a stream flowing over it. They were able to ascertain the planet’s size by recreating this ‘bump’ or ‘kink’ in the flow using computer models. 

Using the same method of locating ‘bumps’ in gas flow around young stars, the team previously discovered a similar new ‘baby’ planet around another young star roughly a year ago. Those findings were published in the Astrophysical Journal Letters.

That initial discovery — found in the stellar nursery around HD163296 360 light-years from Earth — was the first of its kind and provided a ‘missing link’ in scientists understanding of planet formation.

These two studies add to what is only a small collection of known ‘baby’ planets. 

“There is a lot of debate about whether baby planets are really responsible for causing these gaps,” says Associate Professor Daniel Price, the study’s co-author and Future Fellow at the school. “Our study establishes for the first time a firm link between baby planets and the gaps seen in discs around young stars.”

Original research: https://www.nature.com/articles/s41550-019-0852-6