Clouds on a lava planet challenge what astronomers expected
New analysis of James Webb Space Telescope observations points to a reflective cloudy sky on TOI-561 b
A rocky lava planet once thought to be too extreme to have an atmosphere appears to be wrapped in one with a reflective layer of vaporized silicate mineral clouds, the same family of compounds that make up quartz and sand.
University of Waterloo Physics and Astronomy Professor Lisa Dang and collaborators used the James Webb Space Telescope (JWST) to observe the lava planet TOI-561 b for 37 hours, capturing nearly four complete trips of the planet around its star.
“We measured how the planet’s brightness and heat changed throughout its orbit,” says Dang. “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.”
Artist’s concept of the lava world TOI-561 b and its star, whose surface churns with hot, bubbling gas. Webb observations show the planet reflects most of its starlight, and models suggest bright silicate clouds above its magma ocean may be the reason. Image credit: CryoDragon Inc
The first study found the dayside was far cooler than 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 form on the planet’s dayside and act like sunscreen. By reflecting starlight and keeping the planet cooler, the clouds may help the planet hold onto an atmosphere that would otherwise be stripped away.
The results build on earlier evidence by Dang and collaborators that TOI-561 b possesses a thick atmosphere, likely replenished by gases released from its molten interior.
What that 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 sulfur dioxide, a gas released from Jupiter’s volcanic moon. The planet has an ocean of molten rock covering its surface, which could act as a planet-wide volcano that is constantly feeding its atmosphere.
“The next big question we will seek to answer is whether TOI-506 b’s atmosphere resembles Venus or an extreme 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.”
The paper’s first author, Samuel Boucher, began the project as an undergraduate research intern at the Trottier Institute for Research on Exoplanets, where Dang was a Banting Postdoctoral Researcher at the time. She continued supervising him after joining Waterloo as a professor, and the internship project grew into a published JWST study.
The paper, “A Highly Reflective Atmosphere on the Lava World TOI-561 b Revealed by JWST/NIRSpec Phase Curve,” was published in The Astronomical Journal.
