Modeling Early JWST Work on TRAPPIST-1

So much rides on the successful launch and deployment of the James Webb Space Telescope that I never want to take its capabilities for granted. But assuming that we do see JWST safely orbiting the L2 Lagrange point, the massive instrument will stay in alignment with Earth as it moves around the Sun. allowing its sunshield to protect it from sunlight and solar heating.

Thus deployed, JWST may be able to give us information more quickly than we had thought possible about the intriguing system at TRAPPIST-1. In fact, according to new work out of the University of Washington’s Virtual Planetary Laboratory, we might within a single year be able to detect the presence of atmospheres for all seven of the TRAPPIST-1 planets in 10 or fewer transits, if their atmospheres turn out to be cloud-free. Right now, we have no way of knowing whether any of these worlds have atmospheres at all. A thick, global cloud pattern like that of Venus would take longer, perhaps 30 transits, to detect, but is definitely in range.

“There is a big question in the field right now whether these planets even have atmospheres, especially the innermost planets,” says Jacob Lustig-Yaeger, a UW doctoral student who is lead author of the paper on this work. “Once we have confirmed that there are atmospheres, then what can we learn about each planet’s atmosphere — the molecules that make it up?”

Image: New research from UW astronomers models how telescopes such as the James Webb Space Telescope will be able to study the planets of the intriguing TRAPPIST-1 system. Credit: NASA.

Working with Lustig-Yaeger are UW’s Victoria Meadows, principal investigator for the Virtual Planetary Laboratory, and doctoral student Andrew Lincowski. The latter should be a familiar name if you’ve been following TRAPPIST-1 studies, because back in November of 2018 he was lead author on a paper on climate models for this fascinating system (see Modeling Climates at TRAPPIST-1).

We’ll now be hoping to follow up that work with early JWST data. Briefly, Lincowski and team pointed to the extremely hot and bright early history of TRAPPIST-1, a tiny M-dwarf 39 light years out with a radius not much bigger than Jupiter (although with considerably more mass — the star is about 9 percent the mass of the Sun). These early conditions could produce planetary evolution much like Venus, with evaporating oceans and dense, uninhabitable atmospheres. The Lincowski paper, though, did point to TRAPPIST-1 e as a potential ocean world.

These findings were in the context of a system among whose seven transiting worlds are three — e, f and g — that are positioned near or in the habitable zone, where liquid water might exist on the surface. Now we have Lustig-Yaeger and company modeling our early JWST capabilities. The paper finds that beyond the presence of an atmosphere, we may be able to draw further conclusions, particularly with regard to the evolution of what gas envelopes we find.

Although oxygen as a biosignature may not be detectable for the potentially habitable TRAPPIST-1 planets, oxygen as a remnant of pre-main-sequence water loss may be easily detected or ruled out… the 1.06 and 1.27 µm O 2 -O 2 CIA [collisionally-induced absorption] features are key discriminants of a planet that has an oxygen abundance greatly exceeding biogenic oxygen production on Earth and may therefore indicate a planet that has undergone vigorous water photolysis and subsequent loss during the protracted super-luminous pre-main-sequence phase faced by late M dwarfs,,,

Such features could be detected fairly quickly:

… in as few as 7-9, 15, 8, 49-67, 55-82, 79-100, and 62-89 transits of TRAPPIST-1b, c, d, e, f, g, and h, respectively, should they possess such an atmosphere. These quoted number of transits may be sufficient to rule out the existence of oxygen-dominated atmospheres in the TRAPPIST-1 system. Additional evidence of ocean loss could be provided by detection of isotope fractionation, which may also be possible in as few as 11 transits with JWST.

Moreover, the authors find that water detection could help to pare down various evolutionary scenarios on these worlds, particularly for TRAPPIST-1 b, c and d, assuming atmospheres high in oxygen content that have not been completely desiccated by the star’s early history. Thus we are probing planetary evolution, but assessments of habitability are going to be tricky, and it seems clear that we will need to turn such analysis over to future direct-imaging missions.

On balance, we are talking about getting useful results with a fairly low number of transits. JWST’s onboard Near-Infrared Spectrograph will use transmission spectroscopy — where the star’s light passes through a planet’s atmosphere to reveal its spectral ‘fingerprint’ — to detect the presence of an atmosphere via the absorption of CO 2 . Such analysis can likewise either detect or rule out oxygen-dominated atmospheres, while constraining the extent of water loss through measurements of H 2 O abundance. All of this provides fodder for other, still evolving observing strategies using the JWST instrument package that can begin the characterization of these compelling worlds.

The paper is Lustig-Yaeger et al., “The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST,” Astronomical Journal Vol. 158, No. 1 (21 June 2019). Abstract / preprint. The Lincowski paper referenced above is “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” Astrophysical Journal Vol. 867, No. 1 (1 November 2018). Abstract / Preprint.