A Lava Planet Is Wearing Cloud Sunscreen, According to the Webb Telescope

Keypoints:

• The James Webb Space Telescope studied TOI-561 b, a lava world about 280 light-years away
• The planet reflects about 75% of its star's light
• Clouds made of silicate minerals may act like a giant sunscreen
• The planet orbits its star in just over 10 hours
• Scientists are puzzled that it still seems to hold an atmosphere

Imagine a planet so close to its star that a full year lasts about 10 hours and its surface is covered in molten rock. You would expect it to be a bare, scorched ball. But the James Webb Space Telescope has found signs that this lava planet is wrapped in a thick, reflective atmosphere.

The planet is called TOI-561 b and it sits about 280 light-years from Earth, in the constellation Sextans. It is a rocky world that is intensely heated by its star. Planets like this are supposed to lose their gas early, because stellar winds and radiation blast it away over billions of years.

The Webb team tracked the planet's glow as it circled its star. They used about 37 hours of observations, including four secondary eclipses and three transits, to build what is called a phase curve. That is a map of how the planet's brightness and heat change through its orbit. The results showed two surprising things. The planet reflects around 75% of the light that hits it, and heat is being carried from the sunny side to the night side instead of staying put. Both are signs of an atmosphere.

To explain the shine, the astronomers ran computer simulations. These suggest the dayside could have clouds made of silicate minerals, the same kind of material found in sand and rocks. These clouds would bounce starlight back into space like a giant sunscreen and also help spread heat around the planet.

That still leaves a big question. How has the atmosphere survived for billions of years? One idea is that the planet began with an unusually large supply of gas. Another is that the oceans of lava act like a planet wide volcano, constantly feeding fresh gas into the air. A thick layer of carbon dioxide, like on Venus but far hotter, is also possible.

The team hopes to look at the planet at longer wavelengths with Webb, which could reveal what its air is actually made of.