Tag Archives: planet habitability

An artist’s view of the TRAPPIST-1 system. The TRAPPIST-1 star is home to the largest batch of roughly Earth-size planets ever found outside our solar system. An international study involving researchers from the Universities of Bern, Geneva and Zurich now shows that the exoplanets have remarkably similar densities, which provides clues about their composition (NASA/JPL-Caltech)

The Trappist-1 exoplanets could be worlds of water or rust

The star system of Trappist-1 is home to the largest group of Earth-like planets ever discovered elsewhere in the Universe by astronomers. This means that investigating these seven rocky worlds gives us a good idea of how common exoplanets with similar compositions to our own are in the Milky Way and the wider cosmos.

New research has revealed that these planets, which orbit the Trappist-1 star 40 light-years away from Earth, all have remarkably similar compositions and densities. Yet, all are less dense than Earth.

The results could indicate these are worlds with much more water than is found on earth, or possibly even that these planets are composed almost entirely of rust.

An artist’s view of the TRAPPIST-1 system. The TRAPPIST-1 star is home to the largest batch of roughly Earth-size planets ever found outside our solar system. An international study involving researchers from the Universities of Bern, Geneva and Zurich now shows that the exoplanets have remarkably similar densities, which provides clues about their composition (NASA/JPL-Caltech)
An artist’s view of the TRAPPIST-1 system. The TRAPPIST-1 star is home to the largest batch of roughly Earth-size planets ever found outside our solar system. An international study involving researchers from the Universities of Bern, Geneva and Zurich now shows that the exoplanets have remarkably similar densities, which provides clues about their composition (NASA/JPL-Caltech)


This similarity between the exoplanets makes the Trappist-1 system significantly different from our own solar system which consists of planets with radically compositions and densities. Trappist-1’s worlds are dense, meaning they are like the rocky planets in our solar system, Earth, Mars, Venus, and Mercury. The system seems to be lacking larger gas dominated planets like Jupiter, Saturn, Uranus and Neptune.

The finding, documented in a paper published in the Planetary science Journal shows how the system, first discovered in 2016, offers insight into the wide variety of planetary systems that could fill the Universe.

This chart shows, on the top row, artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth. On the bottom row, the same numbers are displayed for the bodies of our inner solar system: Mercury, Venus, Earth and Mars. (NASA/ JPL - Caltech)
This chart shows, on the top row, artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth. On the bottom row, the same numbers are displayed for the bodies of our inner solar system: Mercury, Venus, Earth and Mars. (NASA/ JPL – Caltech)

“The new observations allowed us to use transit data from a much longer time span than was available to us for the 2018 calculations,” explains Simon Grimm of the University of Bern, who as well as being involved in the current study, was part of a 2018 team that provided the most accurate calculation of the masses of the seven planets thus far. “With the new data, we were able to refine the mass and density determinations of all seven planets.

“It turned out that the derived densities of the planets are even more similar than we had previously expected.”

Seven Exoplanets with Similar Densities

The similarity in densities observed in the Trappist-1 exoplanets seen by the astronomers hailing from the Universities of Bern, Geneva and Zurich, indicates that there is a good chance they are composed of the same materials at similar ratios.

These are the same materials that we believe form most terrestrial planets, iron, oxygen, magnesium, and silicon. But there is a significant difference between our terrestrial world and the Trappist-1 exoplanets.

Comparison of TRAPPIST-1 to the Solar System: A planet's density is determined by its composition, but also by its size: Gravity compresses the material a planet is made of, increasing the planet's density. Uncompressed density adjusts for the effect of gravity, and can reveal how the composition of various planets compare. (NASA/JPL-Caltech 4)
Comparison of TRAPPIST-1 to the Solar System: A planet’s density is determined by its composition, but also by its size: Gravity compresses the material a planet is made of, increasing the planet’s density. Uncompressed density adjusts for the effect of gravity, and can reveal how the composition of various planets compare. (NASA/JPL-Caltech )

The planets in the Trappist-1 system appear to be about 8% less dense than the Earth. This suggests that the materials that comprise them exist in different ratios than they do throughout our home planet.

This difference in density could be the result of several different factors.

One of the possibilities being investigated by the team is that the surfaces of the Trappist-1 exoplanets could be covered with water, reducing their overall density. Combining the planetary interior models with the planetary atmosphere models, the team was able to evaluate the water content of the seven TRAPPIST-1 planets with what Martin Turbet, an astrophysicist at the University of Geneva and co-author of the study describes as “a precision literally unprecedented for this category of planets.”

For the Trappist-1 system’s four outermost planets, should water account for the difference in density, the team has estimated that water would account for 5% of their overall masses. This is considerably more than the 0.1% of Earth’s total mass made up of water.

Rust Nevers Sleeps for the Trappist-1 Planets

Another possibility that could explain why the Trappist-1 exoplanets have lower densities than Earth is the fact that they could be composed of less iron than our planet is–21% rather than the 32% found in the Earth.

It’s also possible that the iron within the seven exoplanets could be bonded with oxygen forming iron oxides–commonly known as rust. This additional oxygen would reduce the planet’s densities.

Shown here are three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar compositions (NASA/ JPL - Caltech)
Shown here are three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar composition (NASA/ JPL – Caltech)

Iron oxides give Mars its rust-red colour, but they are pretty much confined to its surface. Its core is comprised of non-oxidized iron like the solar system’s other terrestrial planets. If iron-oxide accounts for the seven exoplanet’s lower densities it implies that these worlds are rusty throughout and lacking solid non-oxidized iron cores.

“The lower density might be caused by a combination of the two scenarios – less iron overall than and some oxidized iron,” explains Eric Angol, an astrophysicist at the University of Washington and lead author of the new study. “They might contain less iron than Earth and some oxidized iron like Mars.”

Angol also points out that the Trappist-1 planets are likely to have a low-water content, an idea supported by previous research. “Our internal and atmospheric structure models show that the three inner planets of the TRAPPIST-1 system are likely to be waterless and that the four outer planets have no more than a few per cent water, possibly in liquid form, on their surfaces,” says Turbet.

This seems to favour the theory that the lower density of the Trappist-1 planets is a result of one or both of the iron scenarios suggested by the researchers.

There’s Still a Lot to Learn from Trappist-1


Since its discovery in 2o16, the Trappist system has been the subject of a wealth of observations made by both space and ground-based telescopes alike. Before it was decommissioned at the start of January 2020 the team used the Spitzer Space Telescope to collect their data. This telescope, operated by NASA’s Jet Propulsion Laboratory, alone has clocked in more than 1,000 hours of targeted observations of the exoplanets.

This new study demonstrated the importance of studying systems such as Trappist-1 for extended periods of time.

Caroline Dorn, an astrophysicist at the University of Zurich also highlights the fact that studying systems like this could answer questions about the habitability of exoplanets and the possibility of life elsewhere in the Universe.

“The TRAPPIST-1 system is fascinating because around this one star we can learn about the diversity of rocky planets within a single system,” concludes Dorn. “And we can actually learn more about an individual planet by studying its neighbours as well, so this system is perfect for that.”

“The night sky is full of planets, and it’s only been within the last 30 years that we’ve been able to start unravelling their mysteries, also for determining the habitability of these planets.”

Original Research

Agol. E., Dorn. C., Grimm. S. L., et al, ‘Refining the transit timing and photometric analysis of TRAPPIST-1: Masses, radii, densities, dynamics, and ephemerides,’ Planetary Science Journal, [https://arxiv.org/abs/2010.01074]

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.