Webb Telescope Finds Lava Planet TOI-561 b Has a Reflective, Cloudy Atmosphere

The team watched TOI-561 b with Webb for 37 hours — three complete orbits. The planet's dayside proved far cooler than a bare rock should be. Two explanations were possible: the atmosphere reflects heat into space, or it transports heat to the nightside. The nightside, however, was not warm enough. Computer simulations of atmospheric circulation showed that heat redistribution alone cannot match the data. The researchers concluded that about three-quarters of the incoming heat must be reflected away, indicating a high albedo (reflectivity) of 0.6, attributable to clouds or haze.

Lisa Dang, project lead at the University of Waterloo, Ontario, explained: "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."

What are the clouds made of?

The reflective layer likely consists of mineral clouds. The paper identifies multiple possible cloud species forming on the dayside, including silicon dioxide (SiO2) and magnesium metasilicate (MgSiO3) — rock-forming compounds condensed high in the atmosphere. Think of the planet as wearing a cloudy, reflective sunscreen over an ocean of fire.

How does the atmosphere survive?

The results challenge atmospheric-escape theory, which holds that strongly irradiated super-Earths should have lost all volatile gases (light compounds such as water and carbon dioxide). The puzzle sharpens because TOI-561 b orbits a sun-like star, whereas most USP planets orbit red dwarfs — small stars notorious for radiation bursts that strip planetary atmospheres. TOI-561 b receives even more radiation than planets around typical red dwarfs, and it retains its air.

Dang 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."

The team offers several hypotheses, none yet confirmed:

• The planet may have begun with a much thicker primordial atmosphere, perhaps as a sub-Neptune, and the present air is a secondary atmosphere left after stellar radiation removed the original.
• The planet may have formed farther out and migrated inward while its young star was "calming down" and emitting less radiation, sparing the remaining atmosphere.
• Both scenarios together may apply.
• The planet may have "consumed" its original atmosphere by trapping the gases in its interior.

Co-author Anjali Piette, Assistant Professor in Astronomy at the University of Birmingham, raised another possibility: whether the molten-rock ocean "acts as a planet-wide volcano that is constantly feeding its atmosphere."

What comes next?

The next question, according to Dang, is whether TOI-561 b's atmosphere resembles Venus or an extreme version of Io, Jupiter's volcanic moon. Observing at longer wavelengths with Webb could help explain how a planet this hot has kept its atmosphere for billions of years. The answer will also guide astronomers studying other rocky super-Earths: how many of them hold atmospheres, and of what kind.