≡ Menu

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 Seff. 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 CO2-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 Seff of 0.22 corresponding to a orbital semimajor axis of 0.048 AU. This Seff 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 CO2 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 Seff value of 0.91 which corresponds to a distance of 0.024 AU. Once again, this Seff 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 Seff 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 Seff 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 RE (or Earth radii) and 1.06 RE, respectively. While it was claimed back in May 2016 that these two exoplanets were potentially habitable, their Seff 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 Seff 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 RE which is intermediate between Earth and Mars in size and is therefore likely to be a rocky planet. With an Seff 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 Seff of 0.66, this exoplanet is comfortably inside the conservatively defined HZ of TRAPPIST-1. With a radius of 0.91 RE, 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 Seff 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 RE 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 ME) 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 ME – 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 CO2-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 RE, 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 Seff 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.


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


Comments on this entry are closed.

  • Lord February 27, 2017, 10:55

    Would tidal heating be significant?

    • Andrew LePage February 27, 2017, 12:37

      Good question! With the orbital eccentricities of these exoplanets so poorly constrained by the current TTV analysis and with tidal heating being proportional to the square of eccentricity (all else being equal), only insanely high upper limits for tidal heating can be set at this time. We will have to wait for better values for eccentricity to be derived before this question can be meaningfully addressed.

      • Harry R Ray February 27, 2017, 19:41

        Even Planet h could be habitable if it meets ALL of the following requirements: ONE, its orbit is slightly eccentric. TWO: it is not covered in SO MUCH ICE that volcanoes CANNOT REACH THE SURFACE. THREE
        : it is massive enough that hydrogen released from the aforementioned volcanoes is retained for a signifigant amount of time, but not FOREVER. And finally; FOUR: It is SIGNIFIGANTLY MORE VOLCANIC THAN EARTH but not NEARLY as volcanic as Io. I base this on a new paper in The Astrophysical Journal, titled A Volcanic Hydrogen Habitable Zone, By Ramses M Ramirez and Lisa Kaltenegger, with the DOI: 3847/2041-8213.

    • Brett February 27, 2017, 14:22

      It would almost have to be. Seven planets in close proximity to both their central star and each other? Tides would probably be severe, although they’d be the worse on the two innermost planets (which as shown above, are almost certainly not habitable anyways).

  • Don Elder February 27, 2017, 12:30

    While these exoplanet discoveries are exciting what ruins it for me are the detailed “photos” and even on planet landscape “photos”. Seeing as how we are not even sure there was ever life on Mars, hearing a CNN anchor talking about life being “discovered” 39 light years away makes me want to throw a brick at the TV. Can’t we just stick to the actual facts we know and leave the wild speculation (reported as fact) to the Sci-Fi writers?

    • Patient Observer February 27, 2017, 20:56

      CNN has had issues in the past dealing with facts on earth. Why should facts regarding space be any different? I stopped watching CNN years ago and watch MSM news shows more for entertainment than for information.

      CNN and others news organizations have been using recycled or mislabeled photos and videos to spice up their stories. Based on such loose standards, they would not hesitate to use fanciful astronomical images to increase the impact of a story. Deceptive? Yes.

      This is not intended to be a political statement – just my personal observation regarding the level of factual accuracy for earth and non-earth events in news outlets such as CNN.

  • RobFlores February 27, 2017, 13:03

    For Trappist e. since their rotation is rather fast. (even If tide locked)
    could we expect a modest magnetic field from a liquid core?

    Mercury has a magnetic field about 1/150 of Earth. with 88 day orbital

    Trappist e, orbital period is about 6 days. This would imply
    tide locked rotation speed that should allow a liquid core to
    create a much stronger magnetic field than Mercury. At maximum
    compared to Earth’s rotation, (assuming similar core composition)
    this field could be as strong as 1/6 of Earths.

    Could a field close to this size help mitigate some of the flaring effects
    of the Red Dwarf.?

  • jonW February 27, 2017, 13:57

    Thank you Andrew! With about 650,000 miles between d and e’s orbits, and e and f only a little further apart, is there room at all for moon(s) around e? What limits could be put on stability of moon orbits in such a tight-packed system?

    • Andrew LePage February 28, 2017, 11:06

      A quick calculation of the Hill radius for TRAPPIST-1e (one quick measure of the region where an object can stably orbit TRAPPIST-1e) yields a result of about 85,000 km (~53,000 mi), assuming the orbit and mass parameters that currently exist. Of course, the presence of the nearby planets will affect this to some degree but there does seem to be a large enough region for satellites (natural or otherwise) to orbit stably around this exoplanet. What is still unanswered is if a moon could form in the space available. Based on the examples provided by the satellite systems of our gas giants (reasonable analogs of the TRAPPIST-1 system), my gut tells me that the exoplanets of TRAPPIST-1 will not have moons. Fortunately, these transiting exoplanets offer an excellent opportunity to search thoroughly for exomoons by looking for additional transit signals and/or by analyzing TTV.

  • spaceman February 27, 2017, 14:07

    Thank you, Andrew, for the great essay assessing the habitability of this intriguing system. It sounds like we are barely scratching the surface in terms of being able to reach conclusions about habitability– though this discovery seems like a huge step in that direction! Just curious, what are views on the frequency of habitable worlds and the Fermi paradox?

  • Marshall Eubanks February 27, 2017, 14:51

    Tidal heating requires variation in orbital distance, inclination or rotation angle, and damps down as orbits become circularized, and both rotations and orbits become resonantly locked.

  • Coacervate February 27, 2017, 16:57

    Thank you for this analysis. I anticipate so much from this system and so many more yet to be discovered in our “backyard”.

    I have always wondered why we seem to accept the search for life as a natural thing to do. To me it seems as though we are hard wired to search for life, gods, meaning. Perhaps not in that order?

    Should we ask why? Oops, that’s one for the Priests isn’t it? I mean to ask what is the survival advantage? How does it come to be that a drive to “seek out strange new worlds and life and civilizations” confers survival advantage on a tribe of curious monkeys, if indeed it does?

    This comment is not a criticism by any measure. Rather, it seems to me, our reactions to these recent developments, from Europa/Enceladus to Tabby’s star offer insights that bear exposition as well.

    Maybe this topic is best left to the artist/philosophers? As Paul Simon put it, “So beautiful or so what”.

    • Xcalibur February 28, 2017, 23:26

      I have always wondered why we seem to accept the search for life as a natural thing to do. To me it seems as though we are hard wired to search for life, gods, meaning. Perhaps not in that order?

      I think life is naturally interested in other forms of life. I could elaborate much further, but that’s the general principle.

  • Geoffrey Hillend February 27, 2017, 18:53

    Since exoplanet studies are still in the early stage, we are limited to what can be learned by transit spectroscopy which is limited to the infra red? According to this article, ultra violet spectroscopy can tell us how strong or weak is exoplanets magnetic field, but the technology is not refined enough at the present time. https://cor.gsfc.nasa.gov/RFI2012/docs/29.Cook.pdf Also, direct imaging is needed and cannot be accomplished without a chronograph or star shade.

    Consequently, at some point in the near future, we might be able to tell if a large Moon is needed for an exoplanet to have a strong magnetic field. Infra red and visible light spectroscopy can also give us the chemical molecules and elements in an exoplanets atmosphere which will tell us allot about the condition or health of a planetary atmosphere crucial to life or to see it there is too much Co2.

  • ljk February 27, 2017, 19:51


    Article Updated: 27 Feb , 2017

    by Matt Williams

    Full article here:


    To quote:

    “Assuming that the putative inhabitants of this solar system can use a transmitting antenna as large as the 500 meter FAST radio telescope in China to beam their messages our way, then the Allen Array could have found a signal if the aliens use a transmitter with 100 kilowatts of power or more,” said Shostak. “This is only about ten times as energetic as the radar down at your local airport.”

    • Marshall Eubanks February 28, 2017, 17:43

      “Assuming that the putative inhabitants of this solar system can use a transmitting antenna as large as the 500 meter FAST radio telescope in China to beam their messages our way, then the Allen Array could have found a signal if the aliens use a transmitter with 100 kilowatts of power or more,” said Shostak. “This is only about ten times as energetic as the radar down at your local airport.”

      That is still rather far off from a likely eavesdropping scenario, given that one of our airport radars is more likely to have a 5 meter antenna, lowering the SNR by a factor of 10^4.

      • ljk March 1, 2017, 0:30

        I do not care for when the media declares that some group has done SETI, especially for brief periods, and they shout “No Aliens Found!” when a signal of artificial alien origin is not detected, usually in the radio realm though with an increase in the optical realm too.

        It is a further of a lack of basic understanding about SETI from the public and media. It also gives a sense of panic and then relief that an alien signal might actually be detected, but if you only look briefly then you probably won’t find an ETI signal but you can always claim you tried. In one band of the electromagnetic spectrum.

  • ljk February 27, 2017, 20:19

    Volcanic hydrogen spurs chances of finding exoplanet life

    FEBRUARY 27, 2017


    ITHACA, N.Y. – Hunting for habitable exoplanets now may be easier: Cornell University astronomers report that hydrogen pouring from volcanic sources on planets throughout the universe could improve the chances of locating life in the cosmos.

    Planets located great distances from stars freeze over. “On frozen planets, any potential life would be buried under layers of ice, which would make it really hard to spot with telescopes,” said lead author Ramses Ramirez, research associate at Cornell’s Carl Sagan Institute. “But if the surface is warm enough – thanks to volcanic hydrogen and atmospheric warming – you could have life on the surface, generating a slew of detectable signatures.”

    Combining the greenhouse warming effect from hydrogen, water and carbon dioxide on planets sprinkled throughout the cosmos, distant stars could expand their habitable zones by 30 to 60 percent, according to this new research.

    “Where we thought you would only find icy wastelands, planets can be nice and warm – as long as volcanoes are in view,” said Lisa Kaltenegger, Cornell professor of astronomy and director of the Carl Sagan Institute.
    Their research, “A Volcanic Hydrogen Habitable Zone,” published today in The Astrophysical Journal Letters.

    Full article here:


    To quote:

    With this latest research, Ramirez and Kaltenegger have possibly added to that number by showing that habitats can be found, even those once thought too cold, as long as volcanoes spew enough hydrogen. Such a volcanic hydrogen habitable zone might just make the Trappist-1 system contain four habitable zone planets, instead of three. “Although uncertainties with the orbit of the outermost Trappist-1 planet ‘h’ means that we’ll have to wait and see on that one,” said Kaltenegger.

  • ljk February 27, 2017, 20:22

    First evidence of rocky planet formation in Tatooine system

    27 February 2017

    Evidence of planetary debris surrounding a double sun, ‘Tatooine-like’ system has been found for the first time by a UCL-led team of researchers.

    Full article here:


    To quote:

    Published today in Nature Astronomy and funded by the Science and Technology Facilities Council and the European Research Council, the study reports on the remains of shattered asteroids orbiting a double sun consisting of a white dwarf and a brown dwarf roughly 1000 light-years away in a system called SDSS 1557.

    The discovery is remarkable because the debris appears to be rocky and suggests that terrestrial planets like Tatooine – Luke Skywalker’s home world in Star Wars – might exist in the system. To date, all exoplanets discovered in orbit around double stars are gas giants, similar to Jupiter, and are thought to form in the icy regions of their systems.

    Not a big fan of using the Star Wars reference, since it will automatically imply life on such worlds without any real evidence. There are better and more accurate ways to relate this news to the public.

  • ljk February 28, 2017, 10:51

    James Webb Space Telescope To Search for Second Earth Among Weird Habitable Worlds in the TRAPPIST-1 Solar System

    February 27, 2017


    To quote:

    Using the JWST, scientists will be able to study the atmospheric signatures of the TRAPPIST-1 planets and determine whether they possess Earth-like components such as water, oxygen, and methane, for example, to weigh the chance that extraterrestrial life inhabits another world in the solar system.

    “If it’s got liquid water, it’s got water in its atmosphere, it doesn’t have something super poisonous in its atmosphere, that’s a place that could support life, and that would be tremendously exciting,” Smith says. “We won’t be able to detect that life with JWST, but I think that would be a tremendous discovery in and of itself to say, ‘that place could have life.”

  • Ramses Ramirez February 28, 2017, 13:11

    Interesting analysis of the potential habitability of the TRAPPIST-1 planets based on their current locations in the habitable zone (HZ). However, it is also important to consider the temporal evolution of the HZ. I and others have pointed out that M-dwarf planets that are in the HZ today would not have been during pre-main-sequence, when the stars would have been much, much brighter. Such planets could have been in a runaway greenhouse state for over a hundred (if not hundreds) million years, removing all traces of surface water and turning them into dessicated husks. We already have evidence of what such a process may do to planets in our own solar system- Venus. If that’s not enough, these planets would have been exposed to enormously large amounts of radiation that dwarf anything they are receiving now. Perhaps these planets could have replenished their volatiles later on, or they could have migrated from a farther location into the HZ at later times when the star had dimmed, but it is hard to make any determinations of the potential habitability of these planets (especially M-dwarf planets) based on their current HZ locations.

  • Jens Knudsen February 28, 2017, 16:58

    Hm.. Hope we get one of these Sun gravitational lens missions underway one of these days. Or something else that makes it possible to investigate this system much more extensively. It deserves attention. And probably many more that we don’t know of yet.

  • Marshall Eubanks February 28, 2017, 17:45

    Thinking about this overnight, I know think that this system is likely to have non-zero forced eccentricities (just as the Galilean satellites do) and thus some serious tidal heating (just as the Galilean satellites do).

  • ljk February 28, 2017, 21:56

    Far-away planet systems are shaped like the Solar System

    Wednesday, March 1, 2017 — Researchers at The Australian National University (ANU) have found that far-away planet systems are shaped like the Solar System, with multiple planets aligning with the host star on a flat plain, in a discovery that could increase the chance of finding alien life.

    Co-researcher Associate Professor Charley Lineweaver said NASA’s discovery of the seven-planet system being on a flat plain supported this research, which challenges the usual assumption that planet systems are flared like bellbottoms.

    “Other planet systems in the Universe seem to be much like our Solar System,” said Dr Lineweaver from the ANU Research School of Astronomy and Astrophysics (RSAA).

    “The more we find out about these planet systems the more it seems the Solar System is unexceptional.”

    Full article here:


    To quote:

    The lead author of the research paper, being published in The Monthly Notices of the Royal Astronomical Society, is RSAA PhD student Tim Bovaird: http://arxiv.org/abs/1702.08126

    “The wealth of the Kepler planet data allows for the first time detailed studies of planet systems outside the Solar System. We are now able to ask and answer questions like, how common are planet systems like our own?” Mr Bovaird said.

    Simulations of these planet systems had previously only matched the observed data by assuming a Kepler Dichotomy, an assumption that there are two types of star: one type with only one planet, and another type with multiple planets.

    “Simulations with flared planet systems were slightly easier to perform and that is what researchers had assumed,” Mr Bovaird said.

  • Xcalibur February 28, 2017, 23:51

    This was an excellent and sober review of the TRAPPIST situation.

    To go off on a tangent, you mentioned that slow rotation rate leads to dayside cloud cover, thereby increasing albedo and reducing the greenhouse effect. I recently learned that Venus may have once been habitable due to this same effect — until the h20 was split apart, the hydrogen lost to space, and a feedback loop caused by the resulting excess of co2.

    This is promising for theoretical terraforming of Venus. Not only would we not have to take on the very difficult task of speeding up its rotation, but it would be better not to do so. The main concern would be bombarding it with enough hydrogen + iron aerosol to convert the atmosphere, cooling the planet, and introducing oxygen. From there, perhaps we could find some way of strengthening the magnetosphere to prevent loss of hydrogen to space.

    Again, this is a tangent, but I felt the need to bring it up due to the possible parallels between our solar system and TRAPPIST — in which there are multiple terrestrial planets in different parts of the habitable zone.

    • Harry R Ray March 1, 2017, 10:34

      It all depends on what you mean by “slow”. A few years ago, TRAPPIST-1d would be considered in the conservative habitable zone(it still IS in the optimistic habitable zone)because of what you mentioned above. However, now it is believed that corialis forces are still strong enough to PREVENT cloud formation even with a “slow”(by Earth and Mars standards)rotation rate of four days.

  • Dmitry Novoseltsev March 1, 2017, 5:31

    Let’s try to solve the problem of “reverse”. We are looking for a planet comfortable for life as we know it, i.e., with a higher amount of oxygen in the atmosphere and low level radiation on the surface. With such proximity to an active red dwarf last means a very strong magnetic field that will protect the surface and also the atmosphere from blowing the solar wind is orders of magnitude greater than on Earth. Then the world must be the most powerful auroras. The characteristic red and green luminescence in polar lights is the illumination of the atomic oxygen. Then in the atmospheres of planets in transit typical spectra of this atomic oxygen has to be clearly visible. So?

    • Jens Knudsen March 4, 2017, 6:00

      Hm – is it possible to create, artificially, a magnetosphere, around a planet that needs one (‘Venus’)?

      • Michael March 4, 2017, 16:10

        Yes, if we had floating cities in polar regions around Venus we could, we can generate very large magnetic fields using the energy of the Sun.

      • Dmitry Novoseltsev March 8, 2017, 23:19
        • Dmitry Novoseltsev March 8, 2017, 23:25

          But specifically for Venus I would rather have electrostatic shields – large solar sail type as Dyson Dots, only electrically charged. They also would reduce the solar heating, creating shaded areas. Energy for power sources of electric charge (the electron gun) might be generated due to the lighting and heating of the reverse (facing the Sun) side of the sail. But for Mars or planets TRAPPIST-1 a more effective magnetic shield.

  • ljk March 1, 2017, 10:51

    The Prospects For Life On TRAPPIST-1 Keep Getting Better

    Maddie Stone

    March 1, 2017, 4:45 pm

    Less than a week ago, the citizens of Earth were introduced (technically, re-introduced) to a star system 39 light years away hosting seven Earth-sized exoplanets, three of which lie squarely in the habitable zone. As if that wasn’t exciting enough, researchers are now suggesting that a fourth of the TRAPPIST-1 planets might be habitable, too — if we stretch our imaginations a bit.

    New models out of Cornell University suggest that the so-called habitable zone, where rocky planets can support liquid water and perhaps life, might be larger than previously assumed, if we consider volcanic hydrogen (H2) as a potential climate-warming greenhouse gas. Not only could planets whose atmospheres are warmed by H2 (instead of CO2) remain habitable in more distant orbits, certain fingerprints of life might be more readily visible in their atmospheres.

    If the study’s findings hold up, then TRAPPIST-1h — the outermost of the recently-discovered TRAPPIST planets, currently imagined to be an ice world — might be capable of supporting oceans. “The TRAPPIST planets are really cool,” Ramses Ramirez, lead author on the new study published today in The Astrophysical Journal Letters, told Gizmodo. “Certainly three are in the traditional habitable zone, but with hydrogen, a fourth planet might be in it.”

    Full article here:


    To quote:

    Julien de Wit, an MIT exoplanet researcher and co-author on TRAPPIST-1 study that made international headlines last week, was very excited at the possibility.

    “Such a study is perfectly timely as it further highlights the needs for observational constraints to refine our understanding of habitability,” de Wit told Gizmodo in an email. “It is really exciting to know that we are at the verge of revisiting our understanding of/perspective on these concepts underlying fundamental questions such as ‘are we alone?'”

    Ramirez agrees. “There has been too much of a focus, not just on CO2 and water, but on Earth-centric [concepts in general],” he said. “It’s naive to think other [habitable] planets would be just like the Earth — we just don’t know.”

    “Maybe life on other planets is very similar to Earth, but maybe not. And if not, we should be open to other possibilities.”

  • Harry R Ray March 1, 2017, 11:05

    Sorry, I meant “Mayne”, not “Moyne”

  • ljk March 1, 2017, 22:06


    Published: 1 Mar , 2017

    by Evan Gough

    NASA’s announcement last week of 7 new exoplanets is still causing great excitement. Any time you discover 7 “Earth-like” planets around a distant star, with 3 of them “potentially” in the habitable zone, it’s a big deal. But now that we’re over some of our initial excitement, let’s look at some of the questions that need to be answered before we can all get excited again.

    Full article here:


  • Andrei March 3, 2017, 13:29

    Thank you Andrew LePage for the very nice post going trough the facts here.
    Since the planets are thought to have moved inward there’s indeed a fair chance they have kept both a significant atmosphere and water up until the time they arrived in their current orbits.
    Now to my single question. As these planets orbit so closely, is it not also possible that the rotation will end up being bound to nearby passing planets instead of to the central star?

    • Andrew LePage March 4, 2017, 10:25

      The answer to your single question is yes, it is POSSIBLE that the rotation of some of these planets will be somehow bound to their neighbors but I think that it is highly improbable. The tidal forces created by the parent star, TRAPPIST-1, are two to three orders of magnitude larger than the tidal forces created during a close pass of a neighbor (with the tidal forces on average being weaker still compared to the parent star). Depending on the details of the orbits and the shapes of these exoplanets, it is much more likely that the periodic gravitational interactions between these planets MIGHT be able to perturb one or more of these synchronous rotators into a super-synchronous rotation state like Mercury which rotates three times for every two orbits and, given the near resonances which exist, the exoplanets MIGHT present approximately the same face towards a neighbor every orbit or two. But that is a coincidence of the ratios of the orbital/rotation periods and not the result of tidal effects directly forcing the same face towards a neighbor during a close encounter. We will need to learn a lot more about the dynamics of this system before those sorts of predictions can be made.

      • Andrei March 4, 2017, 11:48

        Thank you Andrew for you nice reply.
        I did in deed have Mercury as an example in the back of my head when writing my question.
        But I see you are more likely to be correct there, the alternative I suggested will only happen if the planets have changed orbits by migrating closer to the star and not just one but several conditions in that process need to be right.
        So possible but far from the first thing to expect.

  • Marshall Eubanks March 4, 2017, 12:23

    What this _will_ cause is librations of the Trappist planets. Suppose, for example, they have continents, and thus some sort of mass anomaly as a function of longitude. Then, for a single planet close in to the star in a 1:1 spin orbit resonance, the biggest mass anomaly will tend to point at the star (be at the sub-stellar point). (Note that for a planet such as the Earth, with light granite continents floating on heavier basaltic rocks, gravity highs and topographic highs tend to be anticorrelated, so this does not mean that the local equivalent of Mt. Everest would be at the sub-stellar point.)

    Now, for a sinlge planet, everything would settle down over time. But, for multiple planets, these gravity highs would be tugged back and forth – such motions are called _librations_ (technically, forced librations). Such motions (a swinging back and for of the body about a uniform synchronous rotation) for Europa are at the km level; detecting such motions is a major goal for the Europa Clipper mission. Europa does not have much topography and has no atmosphere; I could easily imagine the Trappist-1 planets as having much larger topography and also much larger librations (and, if they have substantial topography, possibly large atmosphere-libration interactions as well). How you would detect any of this from 40 light years away is a good question, but I wouldn’t rule it out.

  • ljk March 4, 2017, 23:10



    March 3, 2017

    The seven Earth-sized planets discovered last month orbiting the star TRAPPIST-1 will be ideal targets for the James Webb Space Telescope (JWST), scheduled for launch next year, to probe in a search for signs of life.

    Viewed as the Hubble Space Telescope’s scientific successor, JWST, a joint project of NASA, the European Space Agency (ESA), and the Canadian Space Agency, will observe in the infrared and use spectroscopy to identify the chemical contents of exoplanets’ atmospheres.

    Spectroscopy separates light into individual wavelengths. Every chemical has its own unique wavelength signature, so the technique is capable of identifying individual atmospheric components.

    This means JWST will be able to search the atmospheres of all seven TRAPPIST-1 planets – assuming all have atmospheres – for chemicals produced by biological processes, known as chemical biomarkers.

    Full article here:


    Nice size comparison chart here:


    • ljk March 8, 2017, 19:35

      A detailed look at JWST from Universe Today:


      To quote:

      One big question around the TRAPPIST system is “Do the planets have atmospheres?” The Webb can help us answer this.

      The NIRSpec instrument on JWST will be able to detect any atmospheres around the planets. Maybe more importantly, it will be able to investigate the atmospheres, and tell us about their composition. We will know if the atmospheres, if they exist, contain greenhouse gases. The Webb may also detect chemicals like ozone and methane, which are biosignatures and can tell us if life might be present on those planets.

      You could say that if the James Webb were able to detect atmospheres on the TRAPPIST 1 planets, and confirm the existence of biosignature chemicals there, it will have done its job already. Even if it stopped working after that. That’s probably far-fetched. But still, the possibility is there.