Astronomers have discovered that a rocky lava planet once thought to be too extreme to have any atmosphere appears to be wrapped in a reflective layer of clouds forming a protective 'sunscreen' around the planet.
An international research team, including University of Birmingham astrophysicist Dr Anjali Piette, published their findings in The Astronomical Journal . They used the James Webb Space Telescope (JWST) to observe a lava planet called TOI-561 b – some 280 light years from Earth – for 37 hours, capturing nearly four complete trips around the planet's star.
The team found evidence that suggests the planet has a substantial global atmosphere – albeit one which is not livable – which appears to reflect a significant amount of incoming starlight. Reflective silicate clouds may be responsible for cooling the planet and preventing the extreme temperatures expected for a bare-rock world. The atmosphere also transports some heat from the dayside to the nightside, cooling down the dayside.
What the planet's atmosphere is made of remains an open question. It could be a dense blanket of carbon dioxide, like a much hotter version of Venus, or it could be rich in sulphur dioxide, a gas released from a volcanic moon like Io.
Dr Anjali Piette, Assistant Professor in Astronomy at the University of Birmingham, said: "Our discovery challenges current theories. We would expect a heavily irradiated rocky planet like TOI-561 b to have lost its atmosphere billions of years ago.
"Instead, the evidence suggests it still has a significant atmosphere today. This raises major questions such as whether the planet started with an unusually large supply of gases or if the ocean of molten rock covering its surface, acts as a planet-wide volcano that is constantly feeding its atmosphere."
Initial observations found that the planet's dayside was far cooler than the team of astrophysicists expected for a bare, dark rock this close to its star. That missing heat could either be carried around to the night side or reflected away before the planet ever absorbed it.
By watching the full orbit, the team got the first look at the night side, which always faces away from the star, and found it isn't warm enough to account for the difference. Instead, TOI-561 b must reflect roughly three-quarters of the starlight that hits it, comparable to Venus.
To explain this brightness, the team compared the observations with a computer simulation of the planet's climate. These suggest that clouds made of silicate minerals, including silicon dioxide (SiO2) and magnesium silicate (MgSiO3), could exist on the planet's dayside and act like sunscreen.
Professor Lisa Dang, project lead at the University of Waterloo, Ontario, Canada, said: "We measured how the planet's brightness and heat changed throughout its orbit. This technique, known as a phase curve, revealed that the planet reflects a significant amount of light from its star, and that heat is being redistributed around the planet instead of only on the sunlit side. Together, these point to the presence of an atmosphere."
The results build on earlier evidence by Professor Lisa Dang, Dr Anjali Piette, and collaborators that TOI-561 b possesses a thick atmosphere, replenished by gases released from its molten interior.
Professor Dang added: "The next big question we will seek to answer is whether TOI-561 b's atmosphere resembles Venus or an extreme version of lo. Observing the planet at longer wavelengths with JWST could give us insights and help explain how a planet this hot has held on to an atmosphere for billions of years."
