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Modeling Climates at TRAPPIST-1

It’s a long name, but with the successful arrival of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander on Mars, we now go to work on the planet’s deep interior. With Centauri Dreams’ deep space perspective, my thoughts quickly turn to other stellar systems. We’ve all seen how hard it is to land on Mars, and have looked up into the night sky to find the ruddy pinprick that marks its naked eye presence. Given our Solar System’s scale, the task of getting humans to Mars looms as a major challenge.

Image: Who can resist the first clear photo from a Mars mission? Not me. Credit: NASA.

But suppose we were on a planet in the TRAPPIST-1 system. Here we have roughly Earth-sized planets packed into tight proximity around the parent red dwarf. TRAPPIST-1b is at 0.011 AU, while TRAPPIST-1c is at 0.015 AU. Even the most distant from the star, TRAPPIST-1h, orbits at 0.062 AU, so that these seven worlds are all closer to the host than Mercury in our system. TRAPPIST-1b and TRAPPIST-1c are no more than 1.6 times the distance between the Earth and the Moon apart.

With significant celestial targets this numerous and this obvious in the sky, would any civilization emerging in such a system not have a greater incentive to become spacefaring at an early stage in its development? Imagine another planet, perhaps with atmosphere and ecosystem of its own, looming larger than the Moon in our skies. And others not so much farther away.

Climate Among the Seven Worlds

Space telescopes have limited resources and they’re expensive to operate. Better, then, that we figure out as much as we can about potential objects of study before we even launch such tools as the James Webb Space Telescope, now expected to be sent aloft in 2021. Thus climate models of the TRAPPIST-1 planets are becoming something of a cottage industry.

Now we have a new paper out of the University of Washington that offers rigorous physical modeling both of the radiation environment and chemistry in the system. The models create spectral signatures for each of the possible gases in TRAPPIST-1 atmospheres.

“We are modeling unfamiliar atmospheres, not just assuming that the things we see in the solar system will look the same way around another star,” said Andrew Lincowski, UW doctoral student and lead author of a paper published Nov. 1 in the Astrophysical Journal. “We conducted this research to show what these different types of atmospheres could look like.”

Image: The small, cool M dwarf star TRAPPIST-1 and its seven worlds. New research from the University of Washington speculates on possible climates of these worlds and how they may have evolved. Credit: NASA.

It’s important to bear in mind that we can’t make G-class star assumptions about the planets orbiting this M-dwarf, an ultra-cool object not much larger than Jupiter in size and less than a tenth of the mass of the Sun. So when we talk about three of the TRAPPIST-1 planets being near or in the habitable zone where liquid water could exist at the surface, the statement acknowledges how little we know of conditions on any of these worlds.

We have to include the high degree of stellar activity we find on M-dwarf stars, which could disrupt the early atmosphere as well as destroying ozone that could protect life from UV radiation. In its early stages of development, such a star could put a rocky world with an ocean into a runaway greenhouse condition that might persist for hundreds of millions of years before, as the star gradually dims and enters the main sequence, the planet emerges into the habitable zone.

So we can’t expect terrestrial-class planets around these stars to go through the same development as planets around our G-class Sun. When next-generation searches of small rocky worlds finally occur, they will be our first spectrographic analyses of distant atmospheres looking not only for water vapor but a wide range of biosignature gases. Planetary evolution at M-dwarfs clearly needs to be understood to ensure accuracy.

What happens to planetary atmospheres around dim red stars like this one? Using the Hyak supercomputer system at the University of Washington, Lincowski and team modeled the TRAPPIST-1 planets drawing on the methods of terrestrial climate modeling and infusing into them photochemistry models that they believe provide as good a simulation of planetary conditions here as we have yet seen. The number of possibly ‘habitable’ worlds decreases to one. For it turns out that any or all of the TRAPPIST-1 planets could have stronger resemblance to Venus than to Earth, with whatever water once existed at the surface long departed.

“This may be possible if these planets had more water initially than Earth, Venus or Mars,” Lincowski adds. “If planet TRAPPIST-1 e did not lose all of its water during this phase, today it could be a water world, completely covered by a global ocean. In this case, it could have a climate similar to Earth.”

The team’s work models atmospheric conditions after extreme loss of volatiles early in planetary evolution, discriminating between oxygen- and carbon dioxide-dominated atmospheres and including interior outgassing as a contributor to the final composition. For depending on early conditions, water can be broken by ultraviolet light into its constituents. Hydrogen is thus released, which is light enough to escape the planet’s gravity. Oxygen dominates the thick atmosphere left behind, a remnant that has little to do with life. We have no analog to this kind of atmosphere in our own Solar System.

Differentiating among these worlds, the researchers’ modeling offers the insight that if one of these planets is likely to host life, it is TRAPPIST-1e, the world we’ll want to focus on in future astrobiological studies. TRAPPIST-1b appears hotter than Venus. Planets c and d, further out, still receive enough energy from the host to be Venus-like, with any atmosphere likely dense over an uninhabitable surface. As for planets f, g and h, the spread is wide. They could be frozen worlds or, depending on the amount of water at formation, Venus-like themselves.

The amount of water available to these worlds early in their formation is key here, and plays against the team’s calculations of ocean loss and oxygen accumulation for all seven of the TRAPPIST-1 planets. From the paper:

Our evolutionary modeling suggests that the current environmental states can include the hypothesized desiccated, post-ocean-runaway O2-dominated planets, with at least partial ocean loss persisting out to TRAPPIST-1 h. These O2-dominated atmospheres have unusual temperature structures, with low-altitude stratospheres and no tropospheres, which result in distinctive features in both transmission and emission, including strong collision-induced absorption from O2.

Thus we have a possible signature to look for in future observations. Or we could get atmospheres much more similar to Venus:

Alternatively, if early volatile outgassing (e.g. H2O, SO2, CO2) occurred, as was the case for Earth and Venus, Venus-like atmospheres are possible, and likely stable, throughout and beyond the habitable zone, so the maximum greenhouse limit may not apply for evolved M dwarf planets. If Venus-like, these planets could form sulfuric acid hazes, though we find that TRAPPIST-1 b would be too hot to condense H2SO4 aerosols.

What a prize TRAPPIST-1 has turned out to be. The authors call these worlds “…a natural laboratory to study planetary atmospheric evolution and the associated impact on habitability.”

Consider: We have seven transiting planets that range from well inside the putative habitable zone to well past its outer boundaries. The paper points out how useful this is in examining planet evolution as a function of distance from a star. Moreover, because of their orbital configuration, these planets make frequent transits and offer small star-to-planet ratios, which provide optimum values of signal to noise in the transit signature. We’ll see continued modeling of possible outcomes here as we gear up for next generation observations.

Co-author Victoria Meadows, principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory at the University of Washington, adds:

“The processes that shape the evolution of a terrestrial planet are critical to whether or not it can be habitable, as well as our ability to interpret possible signs of life, This paper suggests that we may soon be able to search for potentially detectable signs of these processes on alien worlds.”

The paper is Lincowski et al., “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” Astrophysical Journal Vol. 867, No. 1 (1 November 2018). Abstract / Preprint.

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{ 11 comments… add one }
  • Harry R Ray November 27, 2018, 13:14

    We should all consider an entirely NEW class of planet, the Post-Venus. Somewhere in their evolution, every planet in the TRAPPIST-1 system was a Venus analog, right down to being a slow rotator, instead of being tidally locked, but, TRAPPIST-1 is most likely 7.5 billion years old, so the question arises: How did things evolve from there? I agree with the authors that TRAPPIST-1b heated up PAST the Venus phase. It probably lost all of its atmosphere after it became tidally locked. TRAPPIST-1c is probably STILL a Venus analog, and probably still is NOT tidally locked. I am CERTAIN that TRAPPIST-1f,g, and h are ALSO past the Venus Phase, with their heavy carbon dioxide atmospheres frozen out on their tidally locked nightsides. That leaves TRAPPIST-1d and TRAPPIST-1e to consider. Due to a level of knowledge far below theirs, I will cede to their interpretation of Trappist-1e. However, for some reason, they seem to NOT take into effect, the very low mass of TRAPPIST-1d. The conventional wisdom is, a planet with only 30% Earth’s mass will, over 7.5 billion years, wind up with a Mars-like atmosphere, due to photoevaporation. However, the conventional wisdom assumes that the original atmosphere was Earth-like, and not Venus-like. Over that much time, a whole slew of scenarios may have occcured on TRAPPIST-1d. We will probably need a huge slot of supercomputer time to find out which is the right one for a planet with only 37% of Earth’s mass.

  • Thomas Goodey November 27, 2018, 15:57

    “Imagine another planet, perhaps with atmosphere and ecosystem of its own, looming larger than the Moon in our skies. And others not so much farther away.”

    See ‘Get Out Of My Sky’ by James Blish.

    • ljk November 28, 2018, 10:22

      Wouldn’t such close worlds cause major tides and geological shifts, a.k.a. earthquakes? Would they contribute to making life possible there or keep anything beyond microbes from evolving due to so much disturbance? Would they also affect each other’s weather?

  • Robin Datta November 27, 2018, 18:27

    Biological life with any semblance to ours on those planets would have started with whatever resources were left after the stellar blowtorch transitioned to tranquillity, and with other constraints such as tidal locking. A wider consideration, including xenobiology may offer useful and perhaps even fruitful insights.

  • Laz November 27, 2018, 22:41

    If the TRAPPIST worlds are a bust in terms of habitability, what can we humans in our trademark arrogance do to “fix” them?
    Does the compact size of the system allow for easier transfer of material from one world to another?
    Might somebody have already started this “trade” and what would it look like to our telescopes?

  • Michael November 28, 2018, 8:31

    Although the luminousity of RD’s are high and long during the contraction phase the frequency of the light is very important. It is the frequency of the light that determines how an intercepted atmosphere reacts. If say the wavelength is less than required to break down water the water will hang around much longer even though it is much hotter.

  • Derek P Hendricks November 28, 2018, 12:34

    Trappist1d is no Venus . The balance between atmospheric stripping , and volatile replenishment , has ensured that the planet maintains a sub-earth atmosphere ( with oxygen ) . It is desertified on the day side , and arctic on the night side .
    “E” is a tide-locked Earth analogue .
    “F” is an Antarctica/Hoth type world .
    “G” is chlorine-soaked , super-atmosphere water-world .
    “H” is a giant Europa/Ganymede hybrid.
    Remember , most systems tend to moderate with time . These worlds have settled into middle-age , don’t let abstract numbers mislead you too much .
    D.H.

  • Andrew LePage November 28, 2018, 12:39

    I’ve been very interested in this system since its discovery – it’s a natural laboratory for studying the evolution of Earth-size exoplanets orbiting in and around the HZ of red dwarfs. Paul’s discussion about the comparative closeness of these exoplanets to each other reminded me a of a piece I wrote earlier this year about the view of these planets from the middle planet in this system, TRAPPIST-1e. The piece, which includes simulated images of the exoplanets and predictions of their appearance in the sky, can be found here:

    https://www.drewexmachina.com/2018/04/18/alien-skies-the-view-from-trappist-1e/

  • Alex Tolley November 28, 2018, 12:42

    It has been suggested that we should think of many rocky worlds inside teh HZ as Venus-like, rather than Earth-like. In this case, the long, more luminous pre-main sequence phase seems to justify this for M-Dwarfs, just as Ramirez has suggested in a recent paper reviewing habitability (see Extending the Habitable Zone.

    However, if the period of a waterworld slowly evolving to a Venus analog is long, could life evolve and then migrate to the lithosphere to survive the loss of surface habitability? As long as the surface remains cool enough, and the rocks wet enough, this may be a biosphere that is even common, although we would probably not be able to detect such life (even methanogen released CH4 might be undetectable in a dense, O2 atmosphere due to its short lifetime).

    For those interested in subsurface life, it is a loss that the Mars Insight lander is the first to drill well below Mars’ surface where microbes may still reside, yet has no instruments to detect such life. Maybe a later mission will test drill samples.

  • James Duckett November 29, 2018, 1:06

    While I understand the attention being paid to RDs in the context of exoplanets, exobiology, etc., I have to admit I was much more excited by the recent news concerning the possibility of having discovered a Solar “sibling”; seeing as how we know life exists on at least on one planetary body (if not more) in the Solar System, I’ve always thought that investing time and energy in investigating the closest solar analogs/twins would go a long way in establishing just how “unique” our own system is, by virtue of eliminating the consideration of those factors affected by a parent star’s age, metallicity, luminosity, etc. Hopefully, I’ll still be around to appreciate whatever the data from the upcoming generation of instruments tells us about the significance of calling a G2V system “home”.

  • AlexT November 29, 2018, 5:49

    The Short summary of this article:
    “We know well, that we know nothig about those planets.”
    What is probability that those calculations reflects at least 1% of real state?
    Before we can make some models that somehow connected to reality , we need to collect significantly more observations.

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