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]

The JWST General Observer Programs in Cycle 1 include the following approved programs:

GO 1743 (12.7 hrs) -- Constraining the Atmosphere of the Terrestrial Exoplanet Gl486b
GO 1952 (15.5 hrs) -- Determining the Atmospheric Composition of the Super-Earth 55 Cancri e
GO 1981 (75.6 hrs) -- Tell Me How I’m Supposed To Breathe With No Air: Measuring the Prevalence and Diversity of M-Dwarf Planet Atmospheres
GO 2304 (17.9 hrs) -- Hot Take on a Cool World: Does Trappist-1c Have an Atmosphere?
GO 2420 (25.1 hrs) -- Probing the Terrestrial Planet TRAPPIST-1c for the Presence of an Atmosphere
GO 2512 (142.6 hrs) -- Seeing the Forest and the Trees: Unveiling Small Planet Atmospheres with a Population-Level Framework
GO 2589 (53.7 hrs) -- Atmospheric reconnaissance of the TRAPPIST-1 planets

Attached File(s)

 1743.pdf ( 24.08K ) Number of downloads: 4849
 2420.pdf ( 30.33K ) Number of downloads: 3411
 2304.pdf ( 26.98K ) Number of downloads: 3386
 1952.pdf ( 22.62K ) Number of downloads: 3657
 1981.pdf ( 75.07K ) Number of downloads: 3515
 2589.pdf ( 49.27K ) Number of downloads: 3615
 2512.pdf ( 114.16K ) Number of downloads: 3767

 

[JRehling]

This is tremendous. Many – perhaps most – of the Cycle 1 campaigns will revolutionize what we know about their respective subjects.

Some basic background information: Cycle 1 is one year, and TRAPPIST-1 can only be observed for ~100 days each year, around September 1. This means that those observations will be made almost as soon as possible once the nominal science mission begins. There is no proprietary period for one of the TRAPPIST-1 campaigns, so we seem to have a good chance of having basic atmospheric compositional data for several T1 planets by this time next year. To be succinct, there are no guarantees of any particular science return, even given nominal operations, due to all of the unknowns.

One other exoplanet campaign will be an observation of Alpha Centauri A looking for planets there.

[Mongo]

Here are the specific observations of the red dwarf terrestrial planets targeted in the above JWST programs (a total of 50 transits):

55 Cancri e:
1 transit, NIRCam/F444W (CO 1952)
1 transit, MIRI/LRS (CO 1952)

GJ1132b:
2 transits, NIRSpec/G395 (CO 1981)

GJ341b:
3 transits, NIRCam/F444W (CO 1981)

GJ4102b:
3 transits, NIRSpec/G395 (CO 1981)

GL486b:
2 transits, MIRI/LRS (CO 1743)

TRAPPIST-1b:
2 transits, NIRISS/SOSS (CO 2590)

TRAPPIST-1c:
4 transits, NIRSpec/S1600A1 (CO 2420)
2 transits, NIRISS/SOSS (CO 2590)

TRAPPIST-1e:
2 transits, MIRI/F1500W (CO 2304)

TRAPPIST-1g:
2 transits, NIRSpec/BOTS (CO 2590)

TRAPPIST-1h:
3 transits, NIRSpec/PRISM (CO 1981)
2 transits, NIRSpec/BOTS (CO 2590)

WOLF 347b:
2 transits, NIRSpec/G395 (CO 1981)

TOI 134.01 (L168-9b):
3 transits, NIRSpec/BOTS (CO 2512)

TOI 175.01 (L98-59b):
2 transits, NIRSpec/BOTS (CO 2512)

TOI 260.01 (GJ1008b):
2 transits, NIRSpec/BOTS (CO 2512)

TOI 402.01 (HD15337b):
1 transit, NIRSpec/BOTS (CO 2512)

TOI 402.02 (HD15337c):
2 transits, NIRSpec/BOTS (CO 2512)

TOI 455.01 (GJ3193b):
1 transit, NIRSpec/BOTS (CO 2512)

TOI 562.01 (GL357b):
1 transit, NIRSpec/BOTS (CO 2512)

TOI 776.01 (LP961-53b):
2 transits, NIRSpec/BOTS (CO 2512)

TOI 776.02 (LP961-53c):
2 transits, NIRSpec/BOTS (CO 2512)

TOI 836.01 (HIP73427b):
1 transit, NIRSpec/BOTS (CO 2512)

TOI 836.02 (HIP73427c):
2 transits, NIRSpec/BOTS (CO 2512)

[Mongo]

There is also the following:

GTO 1201 (193.6 hrs) -- NIRISS Exploration of the Atmospheric Diversity of Transiting Exoplanets (NEAT)

ABSTRACT
We will use NIRISS SOSS to acquire transit and eclipse observations of a sample of 14 exoplanets that span the full available range of equilibrium temperatures (300-3000 K) and masses (1 MEarth-10 MJup) for planets amenable to atmospheric characterization. Our observations will measure the abundance of the molecules and aerosols present in the exoplanets’ atmosphere and determine the vertical temperature structure of the hottest targets. These results will allow us to address fundamental issues such as the formation process and formation location of these close-in planets, the presence and characteristics of particulate clouds, and non-equilibrium chemistry effects that might be at play in their atmosphere. Six of our targets are rocky and for these we intend to place some of the first constraints on the mean molecular weight – and hence bulk composition – of their atmospheres. In particular, we will observe multiple transits of the potentially habitable earth-like planets TRAPPIST-1 d & f, aiming to make the first detection of the atmosphere of a habitable planet. Finally, for two targets, WASP-121b and LTT 9779b, we will acquire observations continuously throughout a full orbital period to constrain their temperature-pressure profile as a function of longitude and study how heat is absorbed and redistributed in their atmosphere.

TRAPPIST-1d:
5 transits, NIRISS/SOSS (GTO 1201)

TRAPPIST-1f:
2 transits, NIRSpec/BOTS (GTO 1201)

GJ357b:
1 transit, NIRISS/SOSS (GTO1201)

L98-59b:
1 transit, NIRSpec/SOSS (GTO 1201)

And the following:

GTO 1331 (22.5 hrs) -- Transit Spectroscopy of TRAPPIST-1e

TRAPPIST-1e:
4 transits, NIRSpec/BOTS (GTO 1331)

And the following:

GTO 1224 (49.2 hrs) -- Transiting exoplanet characterization with JWST/NIRSPEC

L98-59b:
1 transit, NIRSpec/BOTS (GTO 1224)

[Mongo]

Combined transit list for the TRAPPIST-1 system:

TRAPPIST-1b:
5 transits, MIRI/F1500W (GTO 1177)
5 transits, MIRI/F1280W (GTO 1279)
2 transits, NIRISS/SOSS (CO 2590)

TRAPPIST-1c:
4 transits, NIRSpec/S1600A1 (CO 2420)
2 transits, NIRISS/SOSS (CO 2590)

TRAPPIST-1d:
5 transits, NIRISS/SOSS (GTO 1201)

TRAPPIST-1e:
4 transits, NIRSpec/BOTS (GTO 1331)
2 transits, MIRI/F1500W (CO 2304)

TRAPPIST-1f:
2 transits, NIRSpec/BOTS (GTO 1201)

TRAPPIST-1g:
2 transits, NIRSpec/BOTS (CO 2590)

TRAPPIST-1h:
3 transits, NIRSpec/PRISM (CO 1981)
2 transits, NIRSpec/BOTS (CO 2590)

[Decepticon]

What!?

Proxima Centauri is not on those lists!

[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.

[JRehling]

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!

[StargazeInWonder]

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!

[StargazeInWonder]

Another very important planet that will be observed by JWST is LHS 1140 b. It's the target of GO 2334:

https://www.stsci.edu/jwst/science-executio...on.html?id=2334

LHS 1140 b is rocky, very dense, receives about as much stellar radiation as Mars, and H2O has been detected in its atmosphere. Together with TRAPPIST-1 d and TRAPPIST-1 e, it's one of the most plausibly earthlike planets that JWST will observe. The big known difference from Earth is its high mass (6.5 ME). This is sure to be one of the most interesting observations in Cycle 1.

[Hungry4info]

The HST H2O detection is not particularly secure. See the source:

The Wide Field Camera 3 (WFC3) G141 grism data [...] shows tentative evidence of water. However, the signal-to-noise ratio, and thus the significance of the detection, is low and stellar contamination models can cause modulation over the spectral band probed. We attempt to correct for contamination using these models and find that, while many still lead to evidence for water, some could provide reasonable fits to the data without the need for molecular absorption although most of these cause also features in the visible ground-based data which are nonphysical