Scientists suspect this scorched alien planet with a sunlike star has air

NASA’s James Webb Space Telescope has found the strongest evidence yet that a small, rocky planet outside our solar system has air, even though it orbits precariously close to its star.

The exoplanet, called TOI-561 b, is less than 1.5 times as wide as Earth and circles its star in less than 11 hours. That's so close — less than 1 million miles away from it in space — one side always faces the star. Having that permanent daylight likely causes the hemisphere to get hot enough to melt rock, bathing the world in a hellish lava ocean.

But what surprised scientists is the alien planet’s lightness. 

"What really sets this planet apart is its anomalously low density," said Johanna Teske, a Carnegie Science Earth and Planets Laboratory researcher and the study’s lead author, in a statement. "It’s not a super-puff, but it is less dense than you would expect if it had an Earth-like composition."

If TOI-561 b has an atmosphere, it would mean even rocky exoplanets exposed to the harshest conditions can have air, providing new insights for scientists on the hunt for habitable worlds elsewhere in the galaxy.

At first, the team wondered whether TOI-561 b, a world 280 light-years away, formed with unusual materials. The exoplanet's host is a G-type star like the sun and only slightly smaller, but it's very old with relatively little iron. That may mean the planet has similar chemistry to that of worlds born early in the universe. But that alone could not explain the measurements.

The researchers then tested another idea: What if the planet has a thick atmosphere? To do this, Webb measured how much heat the planet gives off by tracking changes in infrared light as the planet passed behind its star. This method lets scientists estimate surface temperature.

If the planet were a bald rock with no air to circulate heat, its dayside should reach about 4,900 degrees Fahrenheit. Instead, Webb measured a much cooler temperature of about 3,200 degrees — still cooking, but far lower than expected. The research team's findings appear in the The Astrophysical Journal Letters.

"We really need a thick volatile-rich atmosphere to explain all the observations," said Anjali Piette, a coauthor from the University of Birmingham in the United Kingdom, in a statement.

An atmosphere could cool the planet by driving strong winds that move heat to the dark side. Gases such as water vapor could also block some heat from escaping, making the planet appear cooler to the telescope. Bright clouds made of rock-like material may reflect some starlight as well.

The team plans to keep studying the planet to map temperatures across its surface and learn what its atmosphere is made of. Scientists don't yet know how a small planet could hold onto its atmosphere under such intense radiation. One idea some have proposed is that gases may constantly cycle between the lava ocean and the air hanging over it. 

"This planet must be much, much more volatile-rich than Earth," said Tim Lichtenberg, another coauthor. "It’s really like a wet lava ball."