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Probing TRAPPIST-1 Planetary Atmospheres

This week offers two interesting papers about the TRAPPIST-1 planets, one from Hubble data looking at the question of hydrogen in potential planetary atmospheres, the other drawing on data from the European Southern Observatory’s Paranal facility as well as the Spitzer and Kepler space-based instruments. We’ll look at the Hubble work this morning and move on to the second paper tomorrow. Both offer meaty stuff to dig into, for we’re beginning to characterize these seven planets, which form a unique laboratory for the study of red dwarf systems.

Published in Nature Astronomy, the Hubble results screen four of the TRAPPIST-1 planets — d, e, f and g — to study their potential atmospheres in the infrared, using Hubble’s Wide Field Camera 3 in data collected from December 2016 to January 2017. The data allow us to rule out a cloud-free hydrogen-rich atmosphere on three of these worlds, while TRAPPIST-1g needs further observation before a hydrogen atmosphere can be conclusively excluded.

Image: These spectra show the chemical makeup of the atmospheres of four Earth-size planets orbiting within or near the habitable zone of the nearby star TRAPPIST-1. The habitable zone is a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. To obtain the spectra, astronomers used the Hubble Space Telescope to collect light from TRAPPIST-1 that passed through the exoplanets’ atmospheres as the alien worlds crossed the face of the star. Credit: NASA, ESA, and Z. Levy (STScI).

Pay particular attention to the purple curves in the above image. These show the signature we would expect to see from gases like water and methane, which would be found if any of these planets had a hydrogen-dominated atmosphere like Neptune’s. The spectroscopic signature should be strong in the near-infrared. The Hubble results are indicated by the green crosses, clearly showing no evidence of such an extended atmosphere for TRAPPIST-1 d, f and e.

Julien de Wit (Massachusetts Institute of Technology), lead author on the paper, explains the significance of the finding:

“The presence of puffy, hydrogen-dominated atmospheres would have indicated that these planets are more likely gaseous worlds like Neptune. The lack of hydrogen in their atmospheres further supports theories about the planets being terrestrial in nature. This discovery is an important step towards determining if the planets might harbour liquid water on their surfaces, which could enable them to support living organisms.”

The work proceeded through transmission spectroscopy, in which some of the light of the star passes through the planetary atmosphere and leaves a distinctive trace in the star’s spectrum. The beauty of the TRAPPIST-1 system is that we have so many transits to work with. Moreover, all seven of the planets orbit their star much closer than Mercury is to the Sun, so we have transits occurring frequently, and the possibility of liquid water on some planetary surfaces. We’re also dealing with a planetary system that’s a relatively nearby 40 light years away.

Image: The graphic at the top shows a model spectrum containing the signatures of gases that the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation. The Hubble observations, however, revealed that the planets do not have hydrogen-dominated atmospheres. The flatter spectrum shown in the lower illustration indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmospheres. The researchers concluded that the atmospheres are composed of heavier elements residing at much lower altitudes than could be measured by the Hubble observations. Credit: NASA, ESA and Z. Levy (STScI).

If heavier gases like carbon dioxide, methane, water and oxygen are atmospheric constituents in this system, the James Webb Space Telescope may well be able to find them. What the Hubble work achieves is to take one possibility off the table before JWST goes to work, assuming the latter is successfully deployed in 2019.

The TRAPPIST-1 planets may have had hydrogen atmospheres when first formed, assuming they formed further away from the parent star and migrated into their present positions. The primordial hydrogen would then have been lost as the planets moved close to the star, allowing the formation of secondary atmospheres. Our own Solar System’s rocky planets evidently formed in hotter and drier regions much closer, on a relative basis, to the Sun.

Hannah Wakeford (STScI), one of the scientists involved with this work, adds:

“There are no analogs in our solar system for these planets. One of the things researchers are finding is that many of the more common exoplanets don’t have analogs in our solar system.”

The paper is de Wit et al., “Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1,” Nature Astronomy 5 February 2018 (abstract). Tomorrow we’ll look at work just published in Astronomy and Astrophysics on new constraints on the mass, density and composition of the seven planets around TRAPPIST-1.


{ 14 comments… add one }
  • Rob Flores February 6, 2018, 13:05

    Intriguing worlds. It would be a great coup if we knew the age of the primary, there being a great deal of difference between 3 billion years and 8 billion year old solar system. Amazingly for habitability at least, it seems to me the older the better. The Reason, these planets orbit so close to each other(5-10, Earth-moon Lunar distances, and these planets range 80-130 lunar volume sizes) that tidal heating is likely to be a large factor in crustal recycling, if you have very active core heat source you get a very unstable surface crust. If it is a younger solar system then the planet furthest out (H) becomes more habitable than the inner planets, it being devoid of a neighbor at least further out in that solar system(for now at least) moderating the tidal Heating some. Those solar storms though, bring industrial strength shielding if you visit.

  • Gary Wilson February 6, 2018, 13:41

    If you take the error bars into consideration several of those plots look suspiciously close to being flat, horizontal lines. Are we actually measuring any light going through the possible atmospheres of these planets? It looks very difficult to draw any conclusion so far. Or am I actually mis-reading the plots?

    • Nicky February 7, 2018, 0:57

      We might or might not. Because of the high error bars, it could be bare rock too. We can only exclude hydrogen-dominated atmosphere, others from tenuous atmosphere (basically bare rock) to thick Venus-like atmosphere are consistent.

  • Gary Wilson February 6, 2018, 15:59

    Trappist-1f may have some significant data points. I’m not sure about any of the others.

  • DJ Kaplan February 6, 2018, 17:08

    BTW, Falcon Heavy is UP THERE!

    • Paul Gilster February 6, 2018, 17:18

      The booster return was incredible to watch. What an achievement!

  • Geoffrey Hillend February 6, 2018, 17:47

    Gary Wilson: I am not the expert in spectroscopy but I suspect the plot shows the difference in the energy levels between atoms and molecules? The energy spacing in hydrogen and smaller atoms are farther apart than in the larger, heavier atoms and the spacing of quantum jumps or energy levels in molecules are even closer together in molecules than in the small and larger atoms which is why H20 and CH4 are not there since they have both contain hydrogen. The flatter spectrum indicate the quantum jumps are closer together, the energy spacing between them are closer which represent much less energy. This has to do with quantum mechanical idea that the more confined the system is, the further apart are the separation of it’s energy states of motion are so the smaller atoms like hydrogen are more confined so the energy levels must be spaced farther apart or wider than in the larger atoms. Ford, p. 109, 110. The Quantum World.

    I would like to see a more detailed explanation of the plot myself since I don’t understand why the hydrogen spectrum in the graph here is more bumpier and the heavier elements show a flatter spectrum.

  • Dave Moore February 6, 2018, 21:02

    I read the Barr, Dobos & Kiss’s paper about the composition of the planetary interiors. The actual densities of the Trappist planets are poorly determined, but if they are the average measurements, then they are of the sort of low density the Galilean moons have, indicating they were formed outside the snow line and migrated in. c & d are exceptions to this.
    Trappist b, the innermost one, interested me. It has an insolation temp of 400K, which would make it a hotter version of Venus. It’s composition is estimated to be rocky, overlayed with a thick layer of high pressure ice beneath a steam atmosphere.

    Being of similar size to Venus and close to its primary, it should, like Venus, over the billennia have lost a lot of Hydrogen, but unlike Venus, it wouldn’t have, at the bottom of its atmosphere, molten rock to absorb the oxygen, just ice. So a significant amount of Oxygen may well build up in its atmosphere. If we got a strong spectral signal of Oxygen on b, it would give us a clue into the planet’s internal composition and some idea of the other planets composition.

  • Michael C. Fidler February 7, 2018, 4:40

    The Near-Infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-Epoch Transit Spectra.

    “The seven approximately Earth-sized transiting planets in the TRAPPIST-1 system provide a unique opportunity to explore habitable zone and non-habitable zone small planets within the same system. Its habitable zone exoplanets — due to their favorable transit depths — are also worlds for which atmospheric transmission spectroscopy is within reach with the Hubble Space Telescope (HST) and with the James Webb Space Telescope (JWST). We present here an independent reduction and analysis of two HST Wide Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six planets (b through g). Utilizing our physically-motivated detector charge trap correction and a custom cosmic ray correction routine we confirm the general shape of the transmission spectra presented by deWit2016 for planets b and c. Our data reduction approach leads to a 25\% increase in the usable data and reduces the risk of confusing astrophysical brightness variations (e.g., flares) with instrumental systematics. No prominent absorption features are detected in any individual planet’s transmission spectra; by contrast, the combined spectra of the planets show a clear inverted water absorption feature. We show that this feature — along with the Spitzer transit depth measurements — are fully consistent with stellar contamination, as predicted by Rackham2017b. These spectra demonstrate how stellar contamination can overwhelm planetary absorption features in low-resolution exoplanet transit spectra obtained by HST and JWST and also highlight the challenges in combining multi-epoch observations for planets around rapidly rotating spotted stars.”


  • Harry R Ray February 7, 2018, 10:25

    One possibility NOT discussed in the paper is, that if planets b,c,e,and f were originally Kipping-Chen – like MINIMAL Neptunes, but then lost ALL of their Hydrogen, they may STILL have RESIDUAL atmospheres comprised MOSTLY OF HELIUM, with SURFACE PRESSURES ranging from HUNDREDS to THOUSANDS of times GREATER than the pressure at the surface of the Earth, resulting in exotic planets whose pressures at the TOP of their oceans(assumibly to be discussed in tomorrow’s post)similar to the pressure at the BOTTOM od the Challenger Deep. This would NOT preclude life, because living organisms exist at the bottom of the Challenger Deep, but at the surface, it is questionable(for hundreds) or doubtful(for thousands) whether TRAPPIST-1 is even VISIBLE in these scenarios. The only planet that ESCAPES both of these scenarios is TRAPPIST-1 d, for reasons that will be apparent tomorrow.

    • Harry R Ray February 12, 2018, 10:50

      Dr Ramirez: There apparently is a way to find out for sure if this scenario is correct. “A New Window into Escaping Exoplanet Atmospheres: 10830 A Line of Helium”. by Antonija Oklopcic, Christopher M Hirata.

      • Ramses Ramirez February 12, 2018, 18:15

        Hello Harry. I have not thought a whole lot about helium, but my impression is that as the second lightest element, it should escape rather easily, especially under hydrodynamic escape conditions. Smaller Earth-sized planets (like the TRAPPIST-1 worlds) so close to their star would have trouble holding on to helium for very long.

  • Geoffrey Hillend February 7, 2018, 17:52

    I don’t think that brightness and solar flares can affect the spectral signature of exoplanet atmospheres since these depend on differences in frequency for different atoms and molecules since each gas has a distinct frequency. Transit spectroscopy is the star light plus a planets reflected light minus the star light by itself gives the planets light or spectra. The spectral signature for solar flares is H alpha anyway.

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