The (Potentially) Habitable Worlds of TRAPPIST-1

When the news about the seven planets of TRAPPIST-1 broke, I immediately wondered what Andrew LePage’s take on habitability would be. A physicist and writer with numerous online essays and a host of articles in magazines like Scientific American and Sky & Telescope, LePage is also a specialist in the processing and analysis of remote sensing data. He has put this background in data analytics to frequent use in his highly regarded ‘habitable planet reality checks,’ which can be found on his Drew ex Machina site. Having run a thorough analysis of the TRAPPIST-1 situation the other day, Drew now gives us the gist of his findings, which move at least several of the TRAPPIST-1 planets into a potentially interesting category indeed.

By Andrew LePage

Like so many other people interested in exoplanets, I made it a point to watch NASA’s press conference live on February 22. Based on the list of participants released by NASA a couple of days earlier, a number of people (myself included) suspected that this was going to be an announcement about new findings of the TRAPPIST-1 planetary system. Back in May of 2016, a team of scientists led by Michaël Gillon (University of Liège – Belgium) had announced the discovery of three Earth-size exoplanets orbiting TRAPPIST-1 – a very small red dwarf star known as an ultracool dwarf named after ESO’s ground-based TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope which had spotted the transits of these exoplanets during an observing campaign in 2015. This star and its system of transiting planets was a natural target for follow up observations by ground and space-based instruments.

As it turned out, NASA’s press conference did involve an announcement of the results of new observations of TRAPPIST-1. A total of 1,333 hours of new photometry including 518 hours of data from NASA’s Spitzer Space Telescope had been acquired since the original discovery paper about TRAPPIST-1 had been submitted by Gillon et al.. Most helpful of all was a virtually uninterrupted 20-day observation run by Spitzer from September 19 to October 10, 2016 which allowed a thorough evaluation of the system. In the end, Gillon et al. had identified the transits of a total of seven exoplanets orbiting TRAPPIST-1 – the largest number of exoplanets found so far orbiting a star. Most exciting of all was the claim that three of these Earth-size exoplanets were potentially habitable.

As my published work over the past couple of decades can testify, I am a long-time believer that the galaxy is filled with habitable planets (and moons!). However, I have also been quite skeptical of frequently dubious claims made by some in recent years that various new exoplanetary discoveries are potentially habitable. Back in May 2016 when Gillon et al. originally announced the discovery of the first three exoplanets found orbiting TRAPPIST-1, the ESO press release and other sources claimed that they were all potentially habitable. My published review of the data available at the time showed no support for this claim: two of the new exoplanets were much more likely to be slightly larger and hotter versions of Venus while the orbit of the third exoplanet was so poorly constrained that nothing meaningful could be said yet about its potential habitability. Naturally I was quite skeptical about this new claim being made by some of the same scientists about TRAPPIST-1. With a copy of the new discovery paper by Gillion et al. in hand along with other peer-reviewed papers on this system published in recent months, I performed a fresh review of the potential habitability of the exoplanets in this system.

Image: This diagram shows the changing brightness of TRAPPIST-1 over a period of 20 days in September and October 2016 as measured by NASA’s Spitzer Space Telescope and various ground instruments. The dips in brightness caused by transiting exoplanets are clearly seen. (ESO/M. Gillon et al.)

Definition of the Habitable Zone

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets is basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

One of the important criteria which we can use to determine if a planet is potentially habitable is the amount of energy it receives from its sun known as the effective stellar flux or S eff . According to the work by Ravi Kopparapu (Penn State) and his collaborators on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO 2 -rich atmosphere where the addition of any more of this greenhouse gas would not increase a planet’s surface temperature any further. For a star like TRAPPIST-1 with a surface temperature of 2559 K, this conservative outer limit for the HZ as defined by Kopparapu et al. (2013, 2014) has an S eff of 0.22 corresponding to a orbital semimajor axis of 0.048 AU. This S eff value for the outer limit of the HZ is lower than the 0.36 for a planet orbiting a more Sun-like star because ultracool dwarf stars emit so much of their energy in the infrared part of the spectrum where atmospheric absorption is important.

The inner limit of the HZ is conservatively defined by Kopparapu et al. (2013, 2014) by the runaway greenhouse limit where a planet’s temperature would soar even with no CO 2 present and lose all of its water in a geologically brief time in the process. For an Earth-size planet orbiting TRAPPIST-1, this happens at an S eff value of 0.91 which corresponds to a distance of 0.024 AU. Once again, this S eff value for the inner edge of the HZ is lower than the 1.11 for a Sun-like star because TRAPPIST-1 radiates so much of its energy in the infrared.

Because of the tight orbits of these exoplanets and the constraints placed on their eccentricity, it is likely that they are synchronous rotators with the same side perpetually facing their sun. Detailed climate modeling over the last two decades now shows that synchronous rotation is probably not the impediment to habitability as it was once thought. In fact, it has been shown that slow or synchronous rotation can actually result in an increase of the S eff for the inner edge of the HZ. According to the recent work by Jun Yang (University of Chicago) and collaborators, the inner edge of the HZ for a slow rotator orbiting a star like TRAPPIST-1 would have an S eff of 1.44 corresponding to an orbital distance of just 0.019 AU.

Image: This diagram shows a comparison of the properties of the newly discovered planets of TRAPPIST-1 with the inner planets of our solar system. (NASA)

The Exoplanets of TRAPPIST-1

The first two exoplanets in this system, TRAPPIST-1b and c, have radii of 1.09 R E (or Earth radii) and 1.06 R E , respectively. While it was claimed back in May 2016 that these two exoplanets were potentially habitable, their S eff values of 4.3 and 2.3 are higher than the 1.9 value for Venus, which is most definitely not a habitable planet. With their Venus-like sizes, Venus-like rotation states and S eff values in excess of Venus’, these are most likely to be non-habitable, Venus-like worlds contrary to the original claims made in May 2016. Fortunately, Gillon et al. have now adopted the more conservative definition of the HZ of Kopparapu et al. (2014, 2014) so this dubious claim was not repeated in the new discovery paper.

As we move outward from the parent star of this system, things begin to become a bit more interesting. What is now designated TRAPPIST-1d has a radius of 0.77 R E which is intermediate between Earth and Mars in size and is therefore likely to be a rocky planet. With an S eff of 1.14, TRAPPIST-1d would seem to be comfortably inside the HZ for a slow rotator as defined by Yang et al.. However, as Gillon et al. mention in their new paper, more recent work by Kopparapu et al. (2016) has shown that Coriolis effects for synchronous rotators with short orbital periods will alter the global circulation pattern in a way which affects cloud formation on the dayside – clouds which help to reflect away much of the energy the planet receives from its sun moderating the surface temperature in the process. With an orbital period (and presumably a period of rotation) of just four days, TRAPPIST-1d is probably rotating too quickly to maintain sufficient cloud cover on its dayside to keep from experiencing a runaway greenhouse effect. While it is certainly worthy of continued detailed study, it would seem that the chances that TRAPPIST-1d is potentially habitable are not very promising and Gillon et al. do not categorize this new find of theirs in that way.

The situation with TRAPPIST-1e is substantially better and it has been identified in the new work by Gillon et al. as being potentially habitable. With an S eff of 0.66, this exoplanet is comfortably inside the conservatively defined HZ of TRAPPIST-1. With a radius of 0.91 R E , it is only slightly smaller than Earth and is not expected to be a volatile-rich mini-Neptune with poor prospects of being habitable. If it were not for the still unresolved issues associated with orbiting so close to an ultracool dwarf and how that affects the volatile inventories of such worlds, TRAPPIST-1e could be considered one of the best candidates currently known for being a potentially habitable exoplanet. Undoubtedly, detailed climate modeling of this exoplanet will help to determine the range of water and other volatile content values which could yield a habitable world much as is being done for our Earth-size neighbor, Proxima Centauri b, as well as the growing list of other potentially habitable red dwarf exoplanets.

The next planet out, TRAPPIST-1f, was also identified as being potentially habitable in the new work by Gillon et al.. Its S eff value of 0.38 is comparable to that of Mars but, since so much of the energy emitted by TRAPPIST-1 is in the infrared, it is still comfortably inside the conservatively defined HZ for this star. While the radius of 1.05 R E would suggest that TRAPPIST-1f is a rocky world like the Earth, other data hint otherwise and raises some possible problems.

Because of the packed nature of this planetary system with its orbits near resonance, it is expected that they would strongly interact with each other gravitationally producing variations in their transit timings. Gillon et al. performed an analysis of these transit timing variations (TTV) derived from all of their photometry data and found them to be on the order of tens of seconds to more than a half an hour – more than sufficient to estimate the masses of the inner six planets. Unfortunately, the uncertainties associated with current TTV-derived mass values are still rather large while the calculated densities (which can be used to help constrain the bulk compositions of these exoplanets) are even more uncertain still. What can be said is that all of these exoplanets are approximately Earth-mass (or M E ) objects. The calculated densities with their large uncertainties are also not inconsistent with a rocky composition… the one exception being TRAPPIST-1f.

TRAPPIST-1f has a TTV-derived mass of 0.68±0.18 M E – the most accurately known mass in this system so far. This yields a density that is 0.60±0.17 times that of Earth’s which is suggestive of a volatile-rich bulk composition. It could be that TRAPPIST-1f is a mini-Neptune with a deep hydrogen-rich atmosphere overlaying layers of high temperature/pressure phases of ice rendering it non-habitable. It might also be more of an ocean planet with a CO 2 -rich atmosphere a few times denser than the Earth’s capping a deep ocean of liquid water. Hubble observations might help to eliminate the former possibility by searching for hydrogen in an extended atmosphere although observations by JWST and other future instruments will be required to begin to explore the latter possibility.

But before too much is read into the apparent low density of this exoplanet, it should be remembered that TTV-derived masses are notorious for changing by rather large amounts as new data become available. NASA’s Kepler spacecraft is currently wrapping up Campaign 12 of its extended K2 mission where it observed a star field which includes TRAPPIST-1. With a virtually continuous photometric data set running from December 15, 2016 to March 4, 2017, it should be possible to calculate more accurate TTV-derived masses in the coming months.

It may turn out that the uncertainties in the mass and density of TRAPPIST-1f have been underestimated and it is actually a denser rocky world like the Earth. But even if the low density of TRAPPIST-1f is confirmed and it is unlikely to be potentially habitable, it nevertheless strongly suggests that small planets orbiting ultracool dwarfs can retain substantial amounts of their water and other volatiles contrary to some of the less optimistic predictions that have been made. This would markedly improve the habitability prospects of many red dwarf planets. For now, TRAPPIST-1f is a reasonable candidate for being potentially habitable – definitely better than TRAPPIST-1d but maybe not as good as e.

Image: This diagram shows the relative sizes of the orbits of the seven planets orbiting TRAPPIST-1. The shaded area shows the extent of the habitable zone (HZ) with alternative boundaries indicated by dashed lines. (ESO/M. Gillon et al.).

The last of their discoveries identified by Gillon et al. as being potentially habitable is TRAPPIST-1g. With a radius of 1.12 R E , it is unlikely to be a mini-Neptune but its currently ill-defined density as well as the fact that the smaller and closer TRAPPIST-1f may be volatile-rich makes it impossible to exclude the possibility. With a S eff of 0.26, TRAPPIST-1g is towards the outer edge but still comfortably inside the HZ for such a cool star. Once again, the claim made by Gillon et al. that this is a potentially habitable exoplanet it a reasonable one given what we currently know about this world. The final planet in this system, TRAPPIST-1h, still has an ill-defined orbit but it seems likely that it is outside of the HZ.

Summary

Contrary to my initial reservations, it does appear that the claim that the TRAPPIST-1 system contains three potentially habitable exoplanets has merit given what we currently know about them. There are obviously unresolved issues about how much of their original volatile inventories these exoplanets have managed to retain despite the higher luminosity of their parent star during its earliest history as well as its subsequent bouts of chromospheric activity like flares not to mention the relatively high flux of X-ray and extreme ultraviolet radiation that have already been observed. While losses of volatiles are expected, it is still not known with any certainty how this will ultimately affect the habitability of these and a growing list of similar red dwarf exoplanets. The fact that the initial TTV analysis of this system implies that TRAPPIST-1f has a volatile-rich bulk composition is a hopeful sign that exoplanets in the HZ of small red dwarfs can retain their volatiles, which improves the habitability prospects of such worlds.

Fortunately, TRAPPIST-1 with its seven transiting, Earth-size exoplanets is an ideal laboratory for exploring the question of how such worlds evolve and whether they can be habitable. New observations from NASA’s Hubble Space Telescope are already working their way through the peer-review process which may help constrain the properties of these exoplanets. We should also expect an analysis of the new Kepler data to provide more information in the next few months on the properties of these exoplanets especially better TTV-derived mass (and density) estimates. It is also possible that additional exoplanets will be found orbiting TRAPPIST-1, although it is unlikely that more will be found in the already tightly packed HZ. The commissioning of NASA’s James Webb Space Telescope and other instruments in the years to come also promises to shed much light on the properties of these exoplanets and their potential habitability. The excitement generated by these new finds is definitely well deserved.

A more detailed discussion of the history of TRAPPIST-1 observations, the properties of its exoplanets and their potential habitability can be found at “Habitable Planet Reality Check: The Seven Planets of TRAPPIST-1” (http://www.drewexmachina.com/2017/02/25/habitable-planet-reality-check-the-seven-planets-of-trappist-1/).

Selected References

Michaël Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star”, Nature, Vol. 533, pp. 221-224, May 12, 2016

Michaël Gillon et al., “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1”, Nature, Vol 542., pp. 456-460, February 23, 2017 (preprint of paper is available from the ESO at http://www.eso.org/public/archives/releases/sciencepapers/eso1706/eso1706a.pdf)

R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014