James Webb’s infrared vision lets astronomers take an exoplanet’s temperature

The James Webb Space Telescope’s super-sensitive infrared vision has helped astronomers measure the temperature of a rocky exoplanet – TRAPPIST-1b – the first detection of light of any kind emitted by an exoplanet as small and relatively cool as terrestrial planets in Earth’s solar system.

That’s not to say TRAPPIST-1b might have an environment some future astronaut might enjoy. The dayside temperature is roughly 230 degrees Centigrade (450 Fahrenheit). The data suggest the planet does not have a discernible atmosphere.

An artist’s impression of TRAPPIST-1b and the cool red dwarf it orbits along with six others. Image: NASA

But the observation marks another significant step on the road to determining if planets orbiting small stars like TRAPPIST-1 can sustain an atmosphere capable of supporting life as it’s currently understood.

“These observations really take advantage of Webb’s mid-infrared capability,” said Thomas Greene, an astrophysicist at NASA’s Ames Research Center, lead author on a study published in the journal Nature. “No previous telescopes have had the sensitivity to measure such dim mid-infrared light.”

TRAPPIST-1 is a cool, red dwarf star 40 light years from Earth. In 2017, astronomers reported the discovery of seven rocky planets orbiting the M-class dwarf, all of them similar in size and mass to the inner terrestrial planets in Earth’s solar system. All of the TRAPPIST-1 worlds orbit closer to their sun than Mercury does to the Sun, but the star’s low temperature means they receive similar amounts of energy.

TRAPPIST-1b, the system’s innermost planet, orbits well inside the star’s habitable zone, about one hundred times closer to its star than Earth, and receives about four times the solar energy. The planet completes one orbit every 1.5 Earth days.

“There are ten times as many of these stars in the Milky Way as there are stars like the Sun, and they are twice as likely to have rocky planets as stars like the Sun,” said Greene. “But they are also very active – they are very bright when they’re young and they give off flares and X-rays that can wipe out an atmosphere.”

But co-author Elsa Ducrot said it’s easier to study terrestrial worlds around smaller, cooler stars.

“If we want to understand habitability around M stars, the TRAPPIST-1 system is a great laboratory,” she said. “These are the best targets we have for looking at the atmospheres of rocky planets.”