Astronomers Detect an Atmosphere on the Coolest ‘Lava World’ Yet

HD 3167 b is hot enough to melt rock, yet strangely cool for a planet hugging its star. Scientists think an atmosphere may explain why.

There are planets that are hot, and then there are planets where the ground itself may be molten. HD 3167 b belongs firmly in the second category.

The rocky exoplanet, located approximately 154 light-years away, circles its star once every Earth day. Its orbit is so tight that the side permanently facing the star is probably hot enough to sustain seas of molten rock. However, when astronomers recently measured the planet’s temperature, they found something unexpected: HD 3167 b was actually cooler than it should be. This is a sign the planet has an atmosphere.

“What’s so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere, because it’s bombarded by stellar wind and gets more high-energy photons from the star. But it seems that many of these lava worlds do,” said Edwin Kite, University of Chicago associate professor of geophysical sciences and co-author of the study.

“These planets are too hot for life, but by studying them, we can say something about the processes that matter for other rocky worlds.”

If confirmed, HD 3167 b would be the coolest lava world yet found with signs of an atmosphere, extending a curious pattern that has begun to emerge among some of the hottest rocky planets astronomers know.

Planets this close to their stars generally are not supposed to have an easy time hanging onto air. Strong stellar winds and high-energy radiation can gradually strip gases away. In essence, the closer a rocky planet sits to its star, the more severe that punishment becomes.

Yet astronomers are increasingly finding hints that some extremely hot rocky planets may retain atmospheres anyway. For a bare rocky world without an atmosphere, the side facing the star should become extremely hot because there is little way to move that heat elsewhere.

An atmosphere changes things because gases can transport heat toward the nightside, lowering the temperature on the blazing dayside. Clouds could also reflect some incoming starlight back into space, cooling the surface further.

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What kind of atmosphere?

This is where things get murky.

Researchers don’t yet know what the atmosphere is made of. One possibility is vaporized rock. HD 3167 b is so hot that minerals containing silicon and oxygen could evaporate from its molten surface and enter the atmosphere. But observations of other lava worlds have raised another possibility. Their atmospheres may contain more familiar gases such as carbon dioxide, carbon monoxide or even water vapor.

That would make these worlds much more interesting than simple balls of exposed magma.

HD 3167 b is thought to have a silicate-rich composition broadly resembling the minerals found in Earth’s mantle. Its surface may therefore offer researchers a natural laboratory for watching interactions between molten rock and an atmosphere under conditions impossible to reproduce on Earth.

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The team will need additional James Webb observations to determine what gases are present.

Four other lava worlds with evidence of atmospheres are even hotter. HD 3167 b now pushes the apparent boundary toward cooler temperatures and could help astronomers determine the temperature at which a scorched rocky planet can maintain an atmosphere.

Astronomers ultimately want to understand which rocky planets can keep atmospheres and which end up as barren rocks. Atmospheres are essential to climate, surface pressure and the long-term possibility of liquid water.

Most rocky planets observed around small stars so far appear to be bare. Lava worlds may offer an easier place to start working out the physics.

The study was published in The Astrophysical Journal Letters.