The largest planet orbiting TRAPPIST-1 appears to have no atmosphere

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A rocky planet orbiting a small star nearly 40 light-years from Earth is hot and has little atmosphere, new research suggests. This discovery raises questions about the possibility of the existence of an atmosphere in other orbits of the planetary system.

At the center of the system is a red dwarf named TRAPPIST-1; it hosts seven known planets with masses between 0.3 and 1.4 Earth masses, some of which may hold liquid water. The largest, TRAPPIST-1b, is the closest to its parent star and receives about four times the radiation that Earth receives from the Sun, said Thomas Green, an astrobiologist at NASA’s Ames Research Center in Moffett Field, California.

Like all the other planets in the system, TRAPPIST-1b is tidally locked, meaning one side of the planet always faces the star while the other faces away. Calculations show that if the stellar energy falling on TRAPPIST-1b were distributed around the planet—for example, by the atmosphere—and then reradiated uniformly in all directions, the planet’s surface temperature would be about 120° Celsius.

But daytime temperatures on the planet are actually about 230°C, Green and colleagues report online March 27 in Nature. This, in turn, suggests that there is little or no atmosphere to transfer heat from the perpetually sunlit side of the planet to the dark side, the team said.

To measure TRAPPIST-1b’s temperature, Green and his colleagues used the James Webb Space Telescope to observe the planet in the narrow infrared wavelength range five times in 2022. Because the observations were made just before and after the planet passed away from its parent star, astronomers were able to see the planet’s fully illuminated face, Green says.

The team’s results are “the first ‘deep’ look at this planet,” says Knicol Colon, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. “With every observation, we expect to learn something new,” she adds.

Astronomers have long speculated that planets around red dwarf stars might not be able to retain their atmospheres, mainly because the frequent and high-energy outbursts of such stars would destroy any gas mantle they may have had in their early years. However, there are some scenarios in which such flares could heat the planet’s surface and trigger volcanism, which in turn produces gases that can help form a new atmosphere.

“To be absolutely sure that this planet has no atmosphere, we need many more measurements,” says Michael Gillon, an astrophysicist at the University of Liege in Belgium, who was not involved in the new study. It is possible that when observed at a greater variety of wavelengths and at other angles, the planet may have signs of a gas shroud and, therefore, likely hints of volcanism.

In any case, says Laura Kriedberg, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, who was also not involved in the study, the new result “definitely prompts a closer look at the cooler planets in the system to see if the same is true and them”.

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