James Webb finds strong evidence that rocky planet 55 Cancri e has an atmosphere

After an atmosphere was previously found around dozens of gas giants outside our solar system, there is now – finally – fairly convincing evidence for the existence of an atmosphere around a rocky exoplanet: 55 Cancri e.

This was announced by an international team of researchers, including scientists from Leiden Observatory and SRON. Their findings appear in the magazine this week Nature.

About 55 Cancri e
The research article revolves around 55 Cancri e. This planet is located about 41 light-years away from Earth and orbits – together with four other planets – around the sun-like star Cancri. 55 Cancri e is almost twice the size of Earth, but only slightly more dense than our planet. This means that the planet can be counted among the super-Earths. These are planets that are larger than Earth, but smaller than Neptune and are likely similar in composition to the rocky planets in our own solar system.

Close orbit
55 Cancri e is close to its parent star, which means it is very heated and most likely has a molten surface. It’s best to imagine that surface as a bubbling magma ocean. This close orbit probably also ensures that the planet has a day and a night side, with the day side always pointing towards the mother star and the night side always facing away from the mother star.

An atmosphere?
55 Cancri e was already discovered in 2011. And since then, researchers have also been wondering whether this exoplanet might have an atmosphere. That is not self-evident and certainly not when we look at the circumstances in which 55 Cancri e finds itself; these do not immediately seem very beneficial for maintaining an atmosphere. Because, as mentioned, 55 Cancri e is very close to its parent star – about 25 times closer than Mercury is to the sun. As a result, the planet not only has a very high temperature, but it is also constantly bombarded with radiation and solar wind from its parent star.

James Webb
But new measurements from the James Webb Telescope now strongly suggest that those conditions cannot prevent 55 Cancri e from possessing an atmosphere. Researchers drew that conclusion after using James Webb to measure infrared light ranging from 4 to 12 microns coming from 55 Cancri e as the planet orbited its parent star. The researchers were particularly interested in the brightness while the planet was behind the parent star – in fact, they only measured the brightness of the parent star. They then also measured the brightness when the planet was next to the parent star – or the brightness of the parent star and planet together. By then subtracting the brightness measured when 55 Cancri e was behind the parent star from the brightness measured when 55 Cancri e was next to the parent star, the researchers were able to determine the amount of infrared light at different wavelengths coming from the dayside of the planet, calculate.

Heat energy
The researchers found the first indication that 55 Cancri e had an atmosphere when they examined temperature measurements based on heat energy – which is given off in the form of infrared light. These revealed that the dayside had a temperature of about 1540 degrees Celsius. That is a high temperature, but not as high as the researchers would have expected if 55 Cancri e did not have a (significant) atmosphere. In that scenario, according to their calculations, the dayside would have to reach a temperature of 2200 degrees Celsius. The fact that the temperature is lower on the day side indicates the presence of an atmosphere in which the energy from the (hot) day side is distributed to the (cooler) night side. It is probably an atmosphere rich in volatile substances.

Carbon monoxide or carbon dioxide
Its existence is further supported by other measurements by Webb. “We see evidence of a dip in the spectrum between 4 and 5 microns,” says researcher Aaron Bello-Arufe. “Less of this light reaches the telescope. This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, which absorbs this light.”

Space telescope Spitzer previously also examined 55 Cancri e and specifically the question of whether this exoplanet had an atmosphere. The observations hinted at the existence of a significant atmosphere, made up of volatile substances. But the Spitzer data could also be translated in another way. And in that alternative scenario, 55 Cancri e was simply ‘bare’, except for a thin layer of evaporated rock – rich in elements such as silicon, iron, aluminum and calcium. Given the circumstances in which 55 Cancri e finds itself, that was certainly not an unthinkable scenario; the planet is so hot that some of the molten rock on the surface is expected to evaporate. Webb’s new measurements are a lot clearer in that respect. The observed dip in the spectrum between 4 and 5 microns would not be seen on a planet without an atmosphere or an atmosphere consisting only of evaporated rock. In other words, everything indicates that 55 Cancri e has a real atmosphere.

It is suspected that 55 Cancri e was not born with this atmosphere, but more or less created it himself. This is because gases – possibly carbon monoxide and carbon dioxide – bubble up from the interior of the planet. “Lava can store large amounts of water and carbon dioxide,” says researcher Christiaan van Buchem. “We therefore think that the lava ocean can serve as a reservoir and continuously supply the atmosphere with gas.”