TRAPPIST-1b: JWST finds the planet may not have an atmosphere

The James Webb Telescope (JWST) has searched one of the well-known TRAPPIST-1 exoplanets for an atmosphere, and it didn’t find anything. Some models had predicted thick atmospheres for all of these worlds, but if they have no atmospheres at all it could lower the number of potentially habitable planets in the cosmos.

The TRAPPIST-1 system consists of a red dwarf star with at least seven planets orbiting it, four of which orbit in the habitable zone – the area around a star where temperatures allow for liquid water. Thomas Greene at NASA’s Ames Research Center in California and his colleagues used JWST to observe TRAPPIST-1b, the planet closest to the star.

They looked at the planet just before and after it passed behind the star, which is the optimal time to spot how heat from the star is affecting the planet. “People were predicting that these planets would have pretty thick atmospheres that would circulate the heat around the planet, so they wouldn’t be very bright,” says Greene. “So we took five observations because we thought we would have to stack them up to see anything, but when the data came back it was staring us right in the face.”

TRAPPIST-1b was far brighter than the researchers expected, indicating that the sunlight was hitting one side of the planet and not being absorbed by a thick atmosphere. The temperature of that side of the planet, called the day side, was about 230°C (446°F), about 100 degrees hotter than we would expect if there were an atmosphere distributing the heat around the globe.

The most likely explanation is that this planet’s atmosphere was lost soon after its formation, when the star was brightest, or that a powerful stellar flare stripped it away. “There’s a lot of energy getting dumped onto the planets, and that can do bad things to atmospheres,” says Greene. “There are probably important interactions between the star and the planets – it’ll be really interesting when we look at the results for the whole system.”

Over the next year, various groups of researchers plan to use JWST to observe each of the TRAPPIST-1 worlds. With so many of them, the system can serve as a laboratory to understand the precise conditions on a variety of exoplanets around red dwarf stars and whether they might be amenable to life, which is important because they are the most abundant type of stars in the universe and are expected to host the vast majority of exoplanets.