JWST and Exoplanet Atmospheres Options

[JRehling]

In the next year, if all goes well, JWST will have begun collecting data on the composition of terrestrial exoplanet atmospheres. This is potentially one of the most exciting developments in the history of science, but it's not going to be easy; here is a very informative preview:

https://arxiv.org/abs/2101.04139

Perhaps the key point is that, with the given signal to noise ratios, it may be possible to derive spectra with remarkable fidelity and spectral resolution, but the weak signal in most or all possible cases means that the number of required observations, to build up the signal, will be prohibitive given the limited lifespan of JWST and the large number of systems that we'll want to observe. Rather than campaigns that produce detailed spectra of many candidate "earthlike" planets, we will see the observation time divided amongst many exoplanets and spectra with moderate detail – but likely enough to determine presence or non presence of key molecules. This still depends, of course, upon the exoplanets themselves, whose atmospheres, surfaces, and clouds may make the signal weaker or stronger in any particular case, and those are variables which we cannot possibly control or predict.

To add some sad detail to this, the paper calculates that for some desired measurements, the number of transits that would have to be observed would be over 100 or even 1000, and this is flatly impossible. If the JWST were devoted to the observation of just one particularly special exoplanet and we wished to ignore all other uses of the telescope, this threshold would still be impossible, and obviously, there is no lack of priority targets for the telescope.

Among some molecules of highest interest, the ease of detection will be, in descending ease, CH4, CO2, H2O, O2, and for the planets in the TRAPPIST-1 system, the number of required observations to provide a useful signal for O2 will be on the order of 40.

It seems likely that what we'll see is campaigns to obtain some spectral data for about 15-25 (that is my sense, not a definitive total) candidate "earthlike" planets over JWST's first three or so years, and then more sustained campaigns to follow up on those planets that look most promising after the initial surveys. Overall, the use of JWST for this type of observation will require a very strategic budgeting of the resource of observation time, giving us a little data about a lot of the candidates, and – hopefully – much better data on the few most promising cases. The end result will depend on details that we can only guess at now.

No matter what turns up from JWST, there will always be the opportunity and need for future instruments to extend the studies outward and examine the candidates a little farther. If JWST's "horizon" for this sort of science is a radius of X parsecs, then a future instrument with 4 times the light gathering would extend it to 2X parsecs, and a volume in space 8 times greater. JWST will be the beginning of a great exploration outwards that will never conclude so long as we can keep building bigger and better instruments, decade by decade.

[StargazeInWonder]

Further analysis of the TRAPPIST-1 b observations offer stronger constraints on any possible atmosphere, ruling out even a Mars-like atmosphere.

As far as surface conditions go, it appears that the closest solar system analogue to TRAPPIST-1 b, despite its larger-than-Earth size, is Mercury.

It will be interesting to see what Cycle 1 observations may have revealed about the other six planets in the system.

https://astrobiology.com/2023/05/constraini...bservation.html

[StargazeInWonder]

The scorecard for TRAPPIST-1 is now Vacuums 2, Atmospheres 0. (At least substantial atmospheres.)

JWST observations for TRAPPIST-1 c's dayside is consistent with being quite a bit like the Moon, though a thin atmosphere hasn't been ruled out.

https://www.nature.com/articles/s41586-023-06232-z

I'm not sure which planet in the system we'll learn about next – the observation programs and methodologies vary – but this is sure playing out in interesting fashion, like a serialized drama.

[StargazeInWonder]

This paper about LHS 475b almost certainly lends a clue as to why there have been JWST observations of several exoplanets that haven't yet led to publications summarizing those results: It's often going to be hard to reach specific conclusions, as the abstract explains, because of "the nature of the planet itself, rather than instrumental limits."

While the paper calls this planet "warm," it has an equilibrium temperature well above that of Mercury or Trappist-1b. This continues the streak that for terrestrial planets so hot, we have yet to confirm any atmosphere for any other them. In this case, however, no atmosphere has been ruled out.

https://www.nature.com/articles/s41550-023-02064-z

[Quetzalcoatl]

Bonjour,

Interesting detection of atmospheric gases of a sub-Neptune in habitable zone. With a potential presence of Dimethyl sulfide (DMS) to be confirmed...

https://www.nasa.gov/goddard/2023/webb-disc...phere-of-k2-18b

[Quetzalcoatl]

Bonjour,

Webb Detects Tiny Quartz Crystals in the Clouds of a Hot Gas Giant

https://www.nasa.gov/missions/webb/webb-det...-hot-gas-giant/

[StargazeInWonder]

There is the parable about someone looking for their lost keys under the light of streetlamps not because that is most likely where the keys were lost but because that's where light makes the search more feasible. In the current era of exoplanet characterization, planets that are large, hot, and/or close to their stars (and, of course, transiting) are not the only types of planet, and not the types we're most interested in, but they are the ones where JWST's capabilities make it possible to characterize them. It's fascinating, but also tantalizing as the candidate earthlike and habitable zone planets out there elude us for now.

I hope that, eg, Proxima b and the Tau Ceti planets lend themselves to characterization in the coming years, but it's clearly going to be a difficult challenge.

[Quetzalcoatl]

Bonjour,

NASA’s Webb Identifies Methane In an Exoplanet’s Atmosphere,

https://blogs.nasa.gov/webb/2023/11/22/nasa...ets-atmosphere/

[StargazeInWonder]

It's interesting that methane has been elusive in exoplanet atmospheres studied so far. I suppose that this is because most exoplanets with atmospheres studied so far have been hot ones, causing photodissociation to occur at a high rate. I think that we'll be seeing a lot more characterization of "warm" exoplanets soon.

[StargazeInWonder]

Another case of JWST finding that a hot terrestrial exoplanet shows no signs of a significant atmosphere. Probably not much of a surprise in this case, as the high density indicates not even a lot of silicates, much less volatiles.

https://astrobiology.com/2024/01/gj-367b-is...-sub-earth.html

[StargazeInWonder]

It's been hard to find atmospheres surrounding terrestrial exoplanets. This is (depending upon definitions) the first.

https://science.nasa.gov/missions/webb/nasa...ocky-exoplanet/