The TRAPPIST-1 planetary system is one of the most fascinating we know of. There are seven exoplanets —named b, c, d, e, f, geh, in this case in order of distance from their star— that were discovered in 2017 and caused a sensation when it became known that at least four of them —d, e, fyg— are in their star’s habitable zone. Being a tremendously compact system – not much larger than Callisto’s orbit around Jupiter – orbiting a small red dwarf, it is an ideal candidate to be studied by the new generation of telescopes in search of atmospheric signals and biomarkers by transmission spectroscopy. . Especially in the infrared, the region of the spectrum observed by the James Webb Space Telescope (JWST). And said and done: JWST managed to measure the temperature of the innermost and hottest planet, TRAPPIST-1 b. The bad news is that it seems to lack atmosphere.

Artist’s impression of the planet TRAPPIST-1 b (NASA, ESA, CSA, J. Olmsted (STScI).

How do we know? Well, because JWST’s European MIRI instrument measured the dayside temperature of TRAPPIST-1 b when looking in the mid-infrared and it appears to be around 500 K (227 °C). That is, incompatible with life, although colder than the dayside of Mercury. To do this, the telescope observed the system during TRAPPIST-1 b’s five secondary eclipses, which is when the planet passes behind the star (the primary eclipse is when it passes in front). MIRI measured the intensity of infrared light emitted by the star and the illuminated side of the planet together, and then just that emitted by the star during the five eclipses. By subtracting both intensities, we can calculate the dayside temperature of the exoplanet. The task is not easy because the difference in brightness of the system during a secondary eclipse is only 0.1% (the star is about a thousand times brighter than the planet), but MIRI is able to detect variations of up to 0.027% . MIRI made the observations in the 13.5 to 16.6 micron range, although they were also repeated with a 12.8 micron filter to confirm the results.

TRAPPIST-1 b is closest to its star of the seven planets (NASA/JPL-Caltech).
Mean light curve of the five minor eclipses of TRAPPIST-1 b observed by MIRI (NASA, ESA, CSA, J. Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA ) , Pierre-Olivier Lagage (CEA)).

And what does this have to do with the presence or absence of an atmosphere? The reason is that if TRAPPIST-1 b had a relatively dense atmosphere around it, its temperature would be lower, as there is heat transport between the day side and the night side. Depending on the density and composition of the atmosphere and whether TRAPPIST-1 b undergoes tidal coupling or not, the temperature will differ, but models agree that it should be around 100ºC lower than detected. It is important to remember that we already knew that TRAPPIST-1 b was not in the habitable zone—and neither was TRAPPIST-1 and TRAPPIST-1 h—so its temperature must have been high. But we weren’t sure if there was an atmosphere around it or not. Observations with Hubble and Spitzer only ruled out the presence of a “bloated atmosphere”, but the presence of a more compact and dense atmosphere cannot be excluded. Now yes (of course, yes it could have a very rarefied atmosphere).

TRAPPIST-1 b temperature compared with Earth, Mercury or a TRAPPIST-1 b with atmosphere (NASA, ESA, CSA, J. Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)).
TRAPPIST-1 system (Gillon et al.).
TRAPPIST-1 system. Planets d, e, f and g are in the habitable zone (grey zone) (Gillon et al.).

This negative and, at the same time, expected result may not be very spectacular, but let’s not forget that we are measuring the temperature of a rocky planet located 39 light years away. In fact, it is the smallest and coldest exoplanet whose temperature we have been able to measure (until now, the temperature of hot Jupiters, very large and very hot exoplanets, has been measured above all else). Naturally, what everyone is waiting for is JWST analysis of potentially habitable planets (TRAPPIST-1 d, e, f and g) to find out if they have atmospheres and look for biomarkers in them. But it won’t be easy. Depending on the density, composition and presence of clouds, it can take anywhere from a few to over 30 transits to find out if there is an atmosphere around these worlds. The detection of possible biomarkers will be even more complex. For example, it is estimated that detecting oxygen (if any) in the atmosphere of some of these planets will require over a hundred transits. In contrast, detecting carbon dioxide or methane can require much less, on the order of ten. The number of transits will also depend on the distance from the star. For example, to detect water in the atmosphere of TRAPPIST-1e it will take about 50 transits, assuming there are no clouds, because if there are, there could be more than a thousand. On this same planet, it would take about 300 transits to detect oxygen.

Necessary transits with the JWST NIRSpec instrument to know if any of the 7 TRAPPIST-1 planets have an atmosphere based on their characteristics (Lustig-Yaeger et al.).
Properties of the TRAPPIST-1 system (Lustig-Yaeger et al.).

Undoubtedly, detecting possible atmospheres or biomarkers on TRAPPIST-1’s planets will be a long and tedious process, but we have already taken the first step. And, as we said, we must leave with the incredible achievement of having managed to measure the temperature of a rocky Earth-sized world 39 light-years away.