Researchers using the James Webb Space Telescope may have detected an atmosphere surrounding 55 Cancri e, a rocky exoplanet located 41 light-years from Earth. This is the strongest evidence yet for the existence of a rocky world with an atmosphere outside our solar system.
55 Cancri e is one of five planets detected so far around a star resembling our Sun, in the constellation Cancer. With a diameter almost twice that of our planet and a density slightly greater than that of the Earth, this exoplanet is classified as a “super Earth”: smaller than our planet, smaller than Neptune, and with a composition similar to that of the rocky worlds of our own solar system.
The results of the observations were published in Nature.
According to one of the co-authors of the study, Brice-Olivier Demory, of the University of Bern, “55 Crancri e is one of the most enigmatic exoplanets. Despite long periods of observation, all carried out at around ten locations on the ground and in space over the last decade, the very nature of this world has continued to elude us. At least, until today, when pieces of the puzzle were finally put together using the James Webb Space Telescope.”
Unexpectedly, these observations demonstrate that it might be possible for a very hot, heavily irradiated rocky planet to maintain a gaseous atmosphere, and this also bodes well for the telescope’s capabilities in characterizing cooler rocky planets – and therefore potentially habitable – which orbit stars similar to our Sun.
“The James Webb Telescope really pushes the boundaries of detecting rocky exoplanets. It’s really a new kind of science,” said Renyu Hu of NASA’s Jet Propulsion Laboratory, who led the study.
A super hot super Earth
Although the composition of 55 Cancri e is similar to that of rocky worlds in our solar system, describing it as “rocky” might give the wrong impression. This exoplanet actually orbits so close to its star – a year there lasts barely 18 of our hours, compared to 365 days here – that its surface is necessarily molten, and risks having the appearance of a deep ocean of boiling magma.
With such a tight orbit, it is very likely that the planet is also “locked” by the gravity of the star, that is to say that it no longer rotates on itself, its diurnal side being constantly exposed to its sun, while its nocturnal side is plunged into eternal night.
“The planet is so hot that some of its molten rock could evaporate,” says Hu.
Although James Webb cannot photograph 55 Cancri e directly, the camera can measure subtle changes in the solar system’s light as the planet orbits its star.
The first indication that this exoplanet may have a substantial atmosphere came from temperature measurements based on its thermal emissions, or energy released in the form of infrared light. If the planet is covered in dark molten rock, with a thin layer of vaporized stone, or without an atmosphere altogether, the dayside should be at a temperature of around 2200 degrees Celsius.
“The data indicates a relatively low temperature of around 1,500 degrees Celsius,” Hu continued. “This is a strong indication that energy is distributed from the daytime side to the nighttime side, probably using an atmosphere rich in volatile components. »
While lava currents can transport some of the heat to the night side, this is not effective enough to explain this cooling effect. In fact, the researchers write, the daytime side seems “too cold” by several hundred degrees, even though the heat is distributed evenly across the planet. This is because some of the infrared light emitted by the surface is absorbed by the atmosphere, and therefore never reaches the telescope.
An atmosphere fueled by gases
The research team believes that the gases enveloping 55 Cancri e come from the planet’s interior; after all, it is believed, the main atmosphere would have long since disappeared due to the high temperatures and intense radiation coming from the star.
It would therefore be a secondary atmosphere which is continually renewed by the gas-rich magma ocean.
Scientists say that if 55 Cancri e is far too hot to be habitable, it could offer a unique opportunity to study the interactions between the atmosphere, the surface and the interior of rocky planets, and thus provide information on the beginnings of Earth, Venus and Mars, all of which would have been covered in magma in the past.
“Ultimately, we want to understand the conditions that allow a rocky planet to maintain a gas-rich atmosphere, that is to say a key ingredient for the existence of a habitable planet,” maintains Mr. Hu.
