JWST Discovers the First Known 'Steam Planet'

This exoplanet’s atmosphere is going full steam ahead.

A planet beyond our solar system called GJ 9827d has an atmosphere composed almost completely of hot water molecules, astronomers report in the Astrophysical Journal Letters October 4.

“We’re using the term ‘steam world,’” says astronomer Ryan MacDonald at the University of Michigan in Ann Arbor.

GJ 9827d was discovered in 2017 orbiting a star about 100 light-years from Earth. At about twice Earth’s size and three times Earth’s mass, it’s a type of planet called a sub-Neptune (SN: 8/8/22). Worlds like this are the most common in the galaxy, although our solar system doesn’t have any.

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But just knowing the planet’s size and mass isn’t enough to deduce what it’s made of. To probe exoplanet skies, astronomers analyze starlight filtering through the planet’s atmosphere as it passes in front of its parent star (SN: 6/7/24).

MacDonald and colleagues used the James Webb Space Telescope to observe two such passes, or transits, of GJ 9827d in November 2023. The Hubble space telescope had made similar observations and saw signs of water molecules in the planet’s atmosphere, astronomers reported last year. But it wasn’t enough to tell if the atmosphere just had a little water in it, or if it was a whole water world.

Combining the two telescopes’ views made it unambiguously clear that the atmosphere was nearly all water. The planet’s temperature is about 340° Celsius, so all of that water should be vapor.

Such steam worlds “have been predicted, but this is the first observational evidence that they really exist,” Macdonald says. “I feel like a Star Trek explorer.”

There might not be a solid rocky surface beneath the planet’s steamy skies. Deep in the atmosphere, the pressure from all that water should get high enough to force the water molecules into weird and exotic forms of matter, like supercritical fluids or hot high-pressure ices, MacDonald says.

That makes GJ 9827d an unlikely place to find life. But studying its atmosphere is good practice for observing planets that might be habitable.

“It is the proof of principle that we can detect heavier atmospheres,” MacDonald says. “We’re on the right track to where we want to be, astrobiologically.”