It’s no surprise that the James Webb Space Telescope’s General Observers program should target TRAPPIST-1 with eight different efforts slated for Webb’s first year of scientific observations. Where else do we find a planetary system that is not only laden with seven planets, but also with orbits so aligned with the system’s ecliptic? Indeed, TRAPPIST-1’s worlds comprise the flattest planetary arrangement we know about, with orbital inclinations throughout less than 0.1 degrees. This is a system made for transits. Four of these worlds may allow temperatures that could support liquid water, should it exist in so exotic a locale.

Image: This diagram compares the orbits of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System. Credit: ESO/O. Furtak.

The parent star is an M8V red dwarf about 40 light years from the Sun. It would be intriguing indeed if we detected life here, especially given the star’s estimated age of well over 7 billion years. Any complex life would have had plenty of time to evolve into a technological phase, if this can be done in these conditions. But our first order of business is to find out whether these worlds have atmospheres. TRAPPIST-1 is a flare star, implying the possibility that any gaseous envelopes have long since been disrupted by such activity.

Thus the importance of the early work on TRAPPIST-1 b and c, the former examined by Webb’s Mid-Infrared Instrument (MIRI), with results presented in a paper in Nature. We learn here that the planet’s dayside temperature is in the range of 500 Kelvin, a remarkable find in itself given that this is the first time any form of light from a rocky exoplanet as small and cool as this has been detected. The planet’s infrared glow as it moved behind the star produced a striking result, explained by co-author Elsa Ducrot (French Alternative Energies and Atomic Energy Commission):

“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”

The TRAPPIST-1 work is moving relatively swiftly, for already we have the results of a second JWST program, this one executed by the Max Planck Institute for Astronomy and explained in another Nature paper, this one by lead author Sebastian Zieba. Here the target is TRAPPIST-1 c, which is roughly the size of Venus and which, moreover, receives about the same amount of stellar radiation. That might imply the kind of thick atmosphere we see at Venus, rich in carbon dioxide, but no such result is found. Let me quote Zieba:

“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”

Image: This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

What JWST is measuring is the 15-micron mid-infrared light emitted by the planet, using the world’s secondary eclipse, the same technique used in the TRAPPIST-1 b work. The MIRI instrument observed four secondary eclipses as the planet moved behind the star. The comparison of brightness between starlight only and the combined light of star and planet allowed the calculation of the amount of mid-infrared given off by the dayside of the planet. This is remarkable work: The decrease in brightness during the secondary eclipse amounts to 0.04 percent, and all of this working with a target 40 light years out.

Image: This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

I should also mention that the paper on TRAPPIST-1 b points out the similarity of its results to earlier observations of two other M-dwarf stars and their inner planets, LHS 3844 b and GJ 1252 b, where the recorded dayside temperatures showed that heat was not being redistributed through an atmosphere and that there was no absorption of carbon dioxide, as one would expect from an atmosphere like that of Venus.

Thus the need to move further away from the star, as in the TRAPPIST-1 c work, and now, it appears, further still, to cooler worlds more likely to retain their atmospheres. As I said, things are moving swiftly. In the coming year for Webb is a follow-up investigation on both TRAPPIST-1 b and c, in the hands of the system’s discoverer, Michaël Gillon (Université de Liège) and team. With a thick atmosphere ruled out at planet c, we need to learn whether the still cooler planets further out in this system have atmospheres of their own. If not, that would imply formation with little water in the early circumstellar disk.

The paper is Zieba et al., “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c,” Nature 19 June 2023 (full text). The paper on TRAPPIST-1 b is Greene et al., “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST,” Nature 618 (2023), 39-42 (abstract).