JWST and Exoplanet Atmospheres |
JWST and Exoplanet Atmospheres |
Sep 14 2021, 05:44 PM
Post
#1
|
|
Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
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. |
|
|
Jan 3 2022, 11:17 AM
Post
#2
|
|
Senior Member Group: Members Posts: 1441 Joined: 26-July 08 Member No.: 4270 |
Proxima Cen b is not a transiting planet.
-------------------- -- Hungry4info (Sirius_Alpha)
|
|
|
Jan 3 2022, 12:56 PM
Post
#3
|
|
Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
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. |
|
|
Jan 3 2022, 01:31 PM
Post
#4
|
|
Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
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. |
|
|
Jan 4 2022, 03:39 PM
Post
#5
|
|
Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
Thanks for these updates, Mongo.
A key point, similar to what I said in my last post here, from one of the program descriptions: 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. This Cycle-1 slate of programs is not at all aimed right at the bullseye of the planets and observations that we care most about: It's exploratory, looks at a lot of terrestrial-sized hot exoplanets that I'd never heard of and are not of intrinsically high interest, and doesn't in any case devote much very observation to any one planet. Instead, these programs focus on diversity of terrestrial exoplanets (range of temperatures and stars) and higher signal (larger and hotter ones). This will do a lot to calibrate the capabilities of JWST (and the nature of exoplanets atmospheres). The zero-proprietary period for many of these means that plans for future cycles can processed immediately… just getting out one landmark paper is not the point of all of this work. I'm sure that in Cycles 3-6, etc., we're going to see a lot more targeting of the most interesting cases, and then there'll be a better idea of how much observation time is required for the desired goals or, in some cases, if the hopes cannot be achieved and a target will just have to be ignored to spend the valuable resource time on other cases. For example, the most promising Kepler discovery, Kepler-452b, is deemed to have no chance of producing a useful signal, and will likely never be studied by JWST at all. It's not that the interest isn't there, but that the capability isn't. There may be some blockbuster discoveries in Cycle 1, but what they're really doing is setting up success in future cycles, which is wonderful. The biggest discoveries might be coming in 2025 or so, but we won't know until we know! |
|
|
Jan 25 2022, 10:25 PM
Post
#6
|
|
Member Group: Members Posts: 234 Joined: 14-January 22 Member No.: 9140 |
This is a fascinating topic. I looked over the proposals and a list of known terrestrial exoplanets and came up with the perhaps-surprising, perhaps-sad count that there are only about two exoplanets known now that are (1) transiting, (2) less than 5 Earth masses, (3) receive a level of radiative heating between that of Venus and Mars, (4) are believed to permit a favorable signal to noise ratio for JWST to be able to characterize the atmospheric composition. Those are TRAPPIST-1 d and TRAPPIST-1 e.
It could always turn out that more candidate objects of interest will be confirmed before Cycle 3, etc., and could increase the list a little or a lot. There are a lot of unconfirmed TESS candidates now. In addition, there are several nearby planets that fit this description but do not transit. It looks like Cycle 1 observations target few if any of those, but may be aimed at hotter terrestrial planets or larger cool planets which will establish how viable it is to characterize a planet's atmosphere without a transit. Those include Proxima b, Ross 128 b, Luyten's Star b, Teegarden's Star b, and Tau Ceti e. In principle, an almost transiting planet will allow us to subtract the light from the planet+star in one situation to the light from the star alone one half orbit later. The problem is, the star's output might vary over that span of time (for example, due to a sunspot), by much more than the signal from the planet. So set your expectations appropriately. JWST might characterize only as few as two candidate "earthlike" planets. Or many more! |
|
|
Lo-Fi Version | Time is now: 5th June 2024 - 04:02 AM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |