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.

[Mongo]

Had tried to upload this listing of Trappist-1 observations with JWST as an Excel file, but was not allowed. Here is a screenshot:

[StargazeInWonder]

JWST gets first glimpse of 7-planet system with potentially habitable worlds:
https://www.nature.com/articles/d41586-022-04452-3

There's hardly any news here, but this serves to summarize what we knew, and the status of a sliver of the many TRAPPIST-1 observations that take place in Cycle 1.

Extended, hydrogen-rich atmospheres had already been tentatively ruled out by HST observations for some of the planets.

Perhaps the most substantive news in the article is in the indirect reference to timeframes in which we might expect preliminary analyses to be published. "Within the next year we’ll have a family portrait."

[StargazeInWonder]

Looking at the transiting terrestrial exoplanets that are, and aren't, being targeted by JWST in Cycle 1 has led to a somewhat obvious dynamic that hadn't fully clicked for me before: The candidate "earthlike" planets that are being observed are those which orbit unusually small red dwarfs. For obvious reasons, the signal to noise ratio correlates with the apparent size of the planet's atmosphere divided by the apparent diameter of the star. As a result, Kepler 186 and TOI 700, which have radii of 0.52 and 0.42 times that of the Sun are not promising targets, whereas planets orbiting TRAPPIST-1, LP 890-9, and LHS 1140, with radii of 0.12 to 0.21 that of the Sun, are targets.

That is a rather unfortunately broad restriction, and highlights how transit observations are not going to be promising for many terrestrial exoplanets, and in particular, perhaps not for any that orbit sunlike stars. The situation is made worse for observing terrestrial planets orbiting sunlike stars with ground-based telescopes, as the transits occur infrequently, and are likely to occur in part or entirely during daylight hours at any given telescope site.

As some studies have suggested that planets in the "habitable zone" of small red dwarfs might systematically lose their atmospheres because of the harsh environment near the star, this entire mode of observation may be dead on arrival for observing any earthlike (or even venus-like or mars-like) atmospheres. Of course, we will find out soon for the three stars mentioned earlier if this means of study has any promise or not.

If the observation of transiting candidate "earthlike" planets does indeed turn out to be futile, the next promising kind of study would be direct imaging studies of the spectra of resolvable planets orbiting nearby sunlike stars, like Alpha Centauri, Tau Ceti, and Epsilon Eridani (should such planets exist). And then we'll be waiting for ELT and GMT to come online about 5-10 years from now.

[HSchirmer]

Looking at the transiting terrestrial exoplanets that are, and aren't, being targeted by JWST in Cycle 1 has led to a somewhat obvious dynamic that hadn't fully clicked for me before: The candidate "earthlike" planets that are being observed are those which orbit unusually small red dwarfs.

First thought: interesting observation, IIRC it also is selective for exoplanets with VERY short orbital periods- selection to observe multiple transits during the primary mission.

Second thought (tangent) "Dwarves"
There's a great 'mostly true' tale about JRR Tolkien submitting "The Hobbit" to his publisher, and getting the manuscript back with lots of red-line changes by the young junior editor who had been assigned to Tolkien's book. Tolkien's use of 'Dwarves' and 'Elves' was redlined and noted as 'Dwarfs' and 'Elfs' "cf OED 1920" (editor shorthand for "compare with Oxford English Dictionary, 1920 edition")

JRR met with the young editor and quietly stated the plurals are 'Dwarves' not Dwarfs, and 'Elves' not Elfs.
The junior editor reached over, took out the 1920 Oxford English Dictionary and flipped to 'Dwarfs'; whereupon JRR took the dictionary, opened it to the first page, grabbed an ink pen, and started writing.
The junior editor was flabbergasted, "What are you DOING?"
JRR, "I'm autographing a copy of my dictionary for my new friend; you see I was the EDITOR of the 1920 Oxford English Dictionary. I made a mistake about Dwarfs and Elfs, it really should be Dwarves and Elves. Just don't tell anybody..."

[StargazeInWonder]

IIRC it also is selective for exoplanets with VERY short orbital periods- selection to observe multiple transits during the primary mission.

Among planets that are of approximately earthlike temperatures, these are in effect the same selection. A main sequence star with small radius is always a very cool red dwarf, and so its "habitable zone" is very close and the orbital period very short.

There are also observations being made of very hot planets, in which case the orbital period may be short even if the star is larger.

It is a bit of bad luck in the case of TRAPPIST-1 that the star may only be observed for a short portion of the year, in order to maintain the protocol of keeping the sunshield in position. During those times, TRAPPIST-1 is getting a lot of JWST's time, since it can't be observed the rest of the year.