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.

[Hungry4info]

Proxima Cen b is not a transiting planet.

[JRehling]

Reading between the lines (or in the proposals), there's a generally underlying strategy to the Cycle 1 exoplanet programs: These are efforts to make revolutionary observations, yes, but they're also exploratory, gauging the capabilities of JWST to make these sorts of observations, which is, one must emphasize, still very unknown until one makes the effort.

So many of these Cycle-1 programs are, naturally, aimed at making observations where a signal is more likely. The value of those will not only be to learn about one given world/system, but to establish how strong the signal is in those cases, and thereby judge which targets will be worthy of future observation time, and how much observation time will be needed.

The literature on JWST's ability to characterize terrestrial exoplanet atmospheres discusses situations where hundreds of transits might be needed to achieve certain detections. The Cycle-1 programs only involve about 2 to 5 transits per target. This is an exploration of the whole system – the stars, the planets, the atmospheres, and the JWST itself – and like the early questions in a game of Twenty Questions, will set up the more specific and perhaps time intensive campaigns to be made later.

I have no doubt that if JWST allocated a large fraction of its time to Proxima Centauri that it would collect data that scientists would love to have. But if that signal were weak or inconclusive, that might be a very poor return on the heavy investment. These Cycle-1 observations are going to make us much more knowledgable about which targets will be worthwhile for a lot (or, as the case may be, none) of the future Cycles' time.

[Mongo]

More planned TRAPPIST-1 observations:

GTO 1177 (75.0 hrs) -- MIRI observations of transiting exoplanets

TRAPPIST-1b:
5 transits, MIRI/F1500W (GTO 1177)

The TRAPPIST-1 observations and spectroscopic observations of WASP-107b are being done in collaboration with the European MIRI GTO team (Wright PI).

GTO 1279 (26.3 hrs) -- Thermal emission from Trappist-1 b

TRAPPIST-1b:
5 transits, MIRI/F1280W (GTO 1279)

The program is conducted in coordination with a similar program from Tom Greene; the difference between the two programs being just the use of a different MIRI filter (15.0 microns versus 12.8 microns).

This program is considered as a first step towards future ambitious programs, requiring tens of eclipses to characterize spectroscopically the
atmosphere of Earth mass temperature exoplanets.

The earlier post listing approved transit observations of the TRAPPIST-1 system has been updated.

[Mongo]

For context, the various programs listed above total almost a month of JWST time. Add in the many other exoplanet programs not listed above, and exoplanet observations constitute a very large fraction of the total planned observation time in the first year.