JWST discovers the first known “steam world”

The atmosphere of this exoplanet is in full swing.

A planet outside our solar system called GJ 9827d has an atmosphere made up almost entirely of hot water molecules, astronomers report Astrophysical diary letters October 4th.

“We use the term ‘steam world,'” says astronomer Ryan MacDonald of 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 the size of Earth and three times the mass of Earth, it is a type of planet called sub-Neptune (SN: 8/8/22). Worlds like this are the most common in the galaxy, although there are none in our solar system.

But just knowing the size and mass of the planet is not enough to deduce what it is made of. To study the skies of exoplanets, astronomers analyze the starlight filtered through the planet’s atmosphere as it passes in front of its host star (SN: 06/07/24).

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

Combining the views from the two telescopes made it clear that the atmosphere was almost entirely water. The planet’s temperature is about 340° Celsius, so all water should be steam.

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 may be no solid rocky surface beneath the planet’s steamy sky. Deep in the atmosphere, the pressure of all the water should be high enough to force the water molecules into strange and exotic forms of matter, like supercritical fluids or high-pressure hot ice, MacDonald says.

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

“It’s proof of principle that we can detect heavier atmospheres,” says MacDonald. “Astrobiologically, we are on the right path to where we want to be.”