The finding represents a ‘huge step’ toward finding habitable planets outside our solar system

Nearly 100 light years away, there’s a planet unlike any we’ve seen before. The planet, known as GJ 9827 d, is roughly twice the size of Earth and has an atmosphere composed almost entirely of water vapor, according to a new study.

“This is the first time we’re ever seeing something like this,” said Eshan Raul, who analyzed the James Webb Space Telescope, or JWST, data of GJ 9827 d as an undergraduate student at the University of Michigan. “To be clear, this planet isn’t hospitable to at least the types of life that we’re familiar with on Earth. The planet appears to be made mostly of hot water vapor, making it something we’re calling a ‘steam world.'”

Astronomers have theorized that such planets exist, but this study represents the first observational confirmation of a steam world. And, although GJ 9827 d can’t support life as we know it, discovering its unique atmosphere opens new possibilities for studying other small planets and their potential to host life.

The study, led by Caroline Piaulet-Ghorayeb at the University of Montréal’s Trottier Institute for Research on Exoplanets, used a technique called transmission spectroscopy to measure the atmospheric composition of GJ 9827 d. Transmission spectroscopy measures how much starlight is absorbed by a planet’s atmosphere at different wavelengths, or colors, of light.

To date, almost all exoplanets with measured atmospheres are mainly made of the lightest elements—hydrogen and helium—like the gas giants Jupiter and Saturn in our solar system, Piaulet-Ghorayeb said. That is, they’re markedly different from Earth’s life-enabling environment.

“GJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules like the terrestrial planets of the solar system,” Piaulet-Ghorayeb said. “This is a huge step.”

Once identified, these are the types of planets where scientists can look for life in the future, she added.

It’s that larger search for life that drew—and continues to draw—many researchers into the field, said U-M astrophysicist and NASA Sagan Fellow Ryan MacDonald. Now with JWST, science has an unparalleled tool for making discoveries.

“Even with JWST’s early observations in 2022, researchers were discovering new insights into the atmospheres of distant gas giants,” MacDonald said.

But studying atmospheres made of light gases like hydrogen is far less technically challenging than probing atmospheres with heavier molecules, like the water vapor seen on GJ 9827 d.

“Now we’re finally pushing down into what these mysterious worlds with sizes between Earth and Neptune, for which we don’t have an example in our own solar system, are actually made of,” MacDonald said. “This is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.”

Teamwork makes the steam world dream work

GJ 9827 d wasn’t a total unknown before this study. It was first spotted by the Kepler space telescope in 2017. Then, earlier this year, the Hubble space telescope found hints of water vapor in the planet’s atmosphere.

But there’s a big difference between detecting hints of water vapor in an atmosphere and asserting that the atmosphere is unambiguously blanketed in it.

To make that assertion, the team behind the new study used new observations with the JWST’s Near-Infrared Imager and Slitless Spectrograph, or NIRISS. The team used JWST to capture the spectrum of light that traveled through GJ 9827 d’s atmosphere as it passed in front of, or transited, its star.

The U-M cohort on this project was one of two groups who worked independently to determine what atmospheric composition would result in the observed spectrum. At U-M, Raul performed the atmospheric analysis using software developed by MacDonald. The process, called atmospheric retrieval, saw Raul generate millions of model atmospheres before honing in on the steam world explanation.

That means that Raul was one of the first people to ever see direct evidence that steam worlds exist.

“It was a very surreal moment,” said Raul, who is now working toward his doctorate at the University of Wisconsin-Madison. “We were searching specifically for water worlds because it was hypothesized that they could exist. If these are real, it really makes you wonder what else could be out there.”

He invited readers to stay tuned as JWST has opened the doors to a deeper understanding of worlds beyond our own. GJ 9827 d may ultimately represent an example of one of the most common types of planets in our galaxy.

“Being able to work with the data at this point in my career from what’s literally the most powerful telescope that’s ever been made,” Raul said, “I believe it goes to show there’s never been a better time for young people to get into astronomy.”

MacDonald agreed.

“Every few months, there are new breakthroughs happening in exoplanet astronomy,” he said. “We’re still just scratching the surface with top-level questions, so the phase space for discovery is vast.”