TRAPPIST-1 b: measuring the temperature of a rocky exoplanet 40 light-years away with the James Webb Space Telescope

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

El sistema planetario TRAPPIST-1 es uno de los más fascinantes que conocemos. Cuenta con siete exoplanetas —denominados b, c, d, e, f, g y h, en este caso en orden de lejanía de su estrella— que se descubrieron en 2017 y causaron sensación al saberse que al menos cuatro de ellos —d, e, f y g— están en la zona habitable de su estrella. Al ser un sistema tremendamente compacto —no es mucho mayor que la órbita de Calisto alrededor de Júpiter— que se halla alrededor de una pequeña enana roja, es un candidato ideal para ser estudiado por la nueva generación de telescopios en busca de indicios de atmósfera y biomarcadores a través de espectroscopía de transmisión. Sobre todo en el infrarrojo, la región del espectro observada por el telescopio espacial James Webb (JWST). Y dicho y hecho: el JWST ha logrado medir la temperatura del planeta más interno y caliente, TRAPPIST-1 b. Las malas noticias es que parece no tener atmósfera.

How do we know? Well, because the European instrument MIRI of the JWST has measured the temperature of the day side of TRAPPIST-1 b when observing in the infrared medium and it seems that it is around 500 K (227 ºC). That is, incompatible with life, although colder than the day side of Mercury. To do this, the telescope disappeared from the system during five secondary eclipses of TRAPPIST-1 b, which is when the planet passes behind the star (the primary eclipse is when it passes in front). MIRI measured the intensity of the infrared radiation emitted by the star and the illuminated side of the planet together, and then only that emitted by the star throughout the five eclipses. By subtracting both intensities, we can calculate the temperature of the day side 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 capable of detecting changes 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.

And what does this have to do with the presence or not 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 be different, but the 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 are TRAPPIST-1 c or TRAPPIST-1 h—so its temperature had to be 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 could not be excluded. Now yes (of course, yes it could have a very thin atmosphere).

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, had been measured above all). Naturally, what everyone is waiting for is the JWST analysis of potentially habitable planets (TRAPPIST-1 d, e, f and g) to find out if they have atmospheres and look for biomarkers on them. But it won’t be easy. Depending on the density, composition, and presence of clouds, it can take anywhere from a couple of transits to more than 30 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 more than a hundred transits. In return, the detection of carbon dioxide or methane may require much less, on the order of ten. The number of transits will also depend on the distance to the star. For example, to detect water in the atmosphere of TRAPPIST-1e will require about 50 transits, assuming that there are no clouds, because if there are, it could be more than a thousand. On this same planet, it would take about 300 transits to detect oxygen.