A Lava Planet That Should Be Bare Rock Appears to Keep a Thick, Reflective Atmosphere

For 37 hours, the James Webb Space Telescope stared at a planet that, by most expectations, should have nothing left to look at. TOI-561 b is a rocky world about 280 light-years away, orbiting so close to its star that a full year lasts about 10.7 hours. Planets in that position are expected to be scorched bare, their atmospheres stripped away by radiation long ago. The new analysis suggests this one kept its air.

The planet is a little more than twice Earth's mass and about 1.4 times as wide. It is tidally locked, so one side always faces the star and the other always faces away. The day side is likely covered by an ocean of molten rock, which is why it is counted among the lava planets. It is also lighter for its size than most planets in tight orbits, which tend to match Earth-like rock. Its star is unusual as well: at least 10 billion years old, more than twice the age of the sun, and poorer in heavy elements.

The first hint came from a 2025 study led by Johanna Teske, whose team measured heat from the day side each time the planet passed behind its star. It was too cool for bare rock, and too cool even for an atmosphere made only of rock vapor. The new work, led by Samuel Boucher of the Université de Montréal with Lisa Dang of the University of Waterloo as project lead, analyzed the full 37-hour stretch from the same Webb program. In that time the planet passed behind its star four times, and the long watch included the night side. It also let the team separate the planet's signal from the slow flickering of hot gas rising and sinking on the star's surface.

The numbers are stark. If the planet were dark, bare rock with no air to move heat around, the day side should reach about 5,300°F (3,200 K). Webb measured about 3,200°F (2,050 K). The night side came in at about 1,500°F (1,100 K), warm for a side that never sees its star, so some heat is being carried over. But not enough to explain the gap. The team estimates the rest is reflection: the planet appears to bounce back about 74% of incoming starlight, close to Venus. Lab tests show molten rock reflects very little, so something above the surface must be doing the work.

To identify it, the researchers compared Webb's readings with climate simulations. Heat transport alone could not match the data. The best fits had a thick atmosphere, with surface pressure between one and 10 times Earth's, holding something highly reflective. The leading candidates are mineral clouds formed from rock evaporating off the molten surface: silicon dioxide, the mineral in quartz and most sand, and magnesium silicate. In the models they gather near the boundary between day and night, where the air cools enough for them to condense, and they act like sunscreen.

What the air is mostly made of remains unknown. It could be thick carbon dioxide, like a far hotter Venus, or rich in sulfur dioxide, the gas from volcanoes on Jupiter's moon Io. Another lava planet, 55 Cancri e, is also thought to have a substantial atmosphere, probably carbon dioxide or carbon monoxide. Earlier work by Dang, Piette and colleagues pointed to gases from TOI-561 b's interior replenishing its air. Anjali Piette of the University of Birmingham, part of the team, said the finding challenges current theories, since such a planet would be expected to have lost its atmosphere billions of years ago. She asked whether it began with an unusually large gas supply or whether the molten ocean works as a planet-wide volcano constantly feeding the atmosphere.

There are caveats. The temperatures are best estimates: Webb observed in a narrow infrared band that misses the wavelengths where the planet emits most of its heat. The instrument's two detectors also did not agree perfectly once compared with the simulations. The cause could be the instrument, the star, or clouds or gases that look different at different wavelengths, and the team cannot yet say which.

Webb also recorded the planet crossing in front of its star during the same observation. That data will get its own paper and should help show how thick the atmosphere is and whether clouds are really there. Dang said the next question is whether the atmosphere resembles Venus or an extreme Io, and longer-wavelength observations could help. Carbon dioxide absorbs light at one of those wavelengths, so a full orbit watched with Webb's mid-infrared instrument could show whether the sky looks like Venus. The study appears in The Astronomical Journal.