Evrim Yazgin
Cosmos science journalist
NASA’s James Webb Space Telescope (JWST) has detected the best evidence to date for the existence of an atmosphere around a rocky exoplanet.
The discovery could aid the continued search for habitable planets outside our solar system.
“55 Cancri e” is a hot rocky planet orbiting a star 41 light-years from Earth. Its orbit is just 2.25 million km from its host star – so close that the planet’s surface is likely molten. The planet is twice as big as Earth, making it a “super-Earth.”
Previous studies have hinted at the presence of an atmosphere on 55 Cancri e rich in volatiles – gases like nitrogen, oxygen and carbon dioxide. But astronomers could not rule out that the planet’s proximity to its star means that its atmosphere was stripped away long ago, except for a shroud of vapourised dust from the evaporation of the molten rock.
JWST verified the planet has an atmosphere in research published in Nature.
Despite not being able to capture a direct image of 55 Cancri e , JWST’s NIRCam and MIRI instruments are able to detect subtle changes in the light as the planet orbits the star.
The first sign that the planet could have an atmosphere came when astronomers realised the planet is actually much cooler than expected.
Earlier estimates suggest 55 Cancri e’s dayside should be about 2,200°C – nearly half the surface temperature of our Sun. Instead, the MIRI data shows the temperature is “only” about 1,500°C.
“This is a very strong indication that energy is being distributed from the dayside to the nightside, most likely by a volatile-rich atmosphere,” says lead author Renyu Hu from NASA’s Jet Propulsion Laboratory (JPL).
The spectroscopic technique is based on seeing which wavelengths of light are absorbed as the planet passes in front of the star. These wavelengths can be ascribed to certain atoms or molecules.
“We see evidence of a dip in the spectrum between 4 and 5 microns – less of this light is reaching the telescope,” explains co-author Aaron Bello-Arufe, also from NASA JPL. “This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, which absorb these wavelengths of light.”
This atmosphere, however, probably hasn’t been there for long, but is bubbling up from the planet’s interior.
“The primary atmosphere would be long gone because of the high temperature and intense radiation from the star,” says Bello-Arufe. “This would be a secondary atmosphere that is continuously replenished by the magma ocean. Magma is not just crystals and liquid rock; there’s a lot of dissolved gas in it, too.”
