The JWST has found an exoplanet unlike any other. This unique world has an atmosphere almost entirely composed of water vapour. Astronomers have theorized about these types of planets, but this is the first observational confirmation.
The unique planet is GJ 9827 d. It’s about twice as large as Earth and three times as massive, and it orbits a K-type star about 100 light years away. The Kepler Space Telescope first discovered it during its K2 extension. In 2023, astronomers studied it with the Hubble Space Telescope. They detected hints of water vapour and described it as an ocean world.
“This is the first time we’re ever seeing something like this.”
However, the JWST results show that the atmosphere is almost completely comprised of water vapour.
The results are in new research published in The Astrophysical Journal Letters titled “JWST/NIRISS Reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d.” The lead author is Caroline Piaulet-Ghorayeb from the University of Montréal’s Trottier Institute for Research on Exoplanets.
Astronomers have wondered if steam planets can exist. Some thought that life could exist on them in the cooler, higher layers of their atmospheres. Others think it’s extremely unlikely. But there was no evidence to go on until now.
“This is the first time we’re ever seeing something like this,” said Eshan Raul, who analyzed the 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.'”
However, every exoplanet teaches us something. GJ 9827 d and its unique atmosphere will help scientists understand exoplanets better in general.
“If these are real, it really makes you wonder what else could be out there.”
The researchers used transmission spectroscopy to detect the exoplanet’s atmosphere. As the planet passes in front of its star, the atmosphere absorbs certain wavelengths of light in the starlight’s spectrum. Different chemicals absorb different wavelengths and reveal their presence.
The observations show that GJ 9827 d’s atmosphere is more than 31% water vapour by volume and has very high metal enrichment. The observations also show that no hydrogen or helium is escaping.
The exoplanet’s atmosphere may be strange, but in other ways, the planet itself is common. It’s a sub-Neptune, a planet larger than Earth but smaller than Neptune. Sub-Neptunes are the most common type of exoplanet we’ve found in the Milky Way.
This discovery is about more than sub-Neptunes and steam worlds. It’s about one of the key challenges in exoplanet atmospheres: the clouds-metallicity degeneracy.
When astronomers use transmission spectroscopy to examine and characterize an exoplanet’s atmosphere, high metallicity and clouds can produce the same signal. High metallicity can produce smaller spectral features, and clouds can also mute and flatten spectral features. Clouds can also mask the presence of molecular absorbers below the cloud deck. As a result, when scientists see a relatively flat spectrum or muted features, they struggle to determine if they’re seeing a metal-rich atmosphere with intrinsically small features or a low-metallicity atmosphere that’s partially obscured by clouds.
This research has broken the stalemate between clouds and metallicity.
Piaulet-Ghorayeb and her co-authors combined previous Hubble Space Telescope observations of GJ 9827 d with JWST observations. The JWST used its NIRISS (Near-Infrared Imager and Slitless Spectrograph) and SOSS (Single Object Slitless Spectroscopy) to analyze the exoplanet’s atmosphere during two transits. This provided enough wavelength coverage and precision to break the clouds-metallicity degeneracy. This is the first conclusive observation of a high-metallicity and water-rich atmosphere.
“This is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.”
This figure from the research shows GJ 9827 d’s two transits observed by the JWST. The broad wavelength coverage and the precision broke the clouds-metallicity degeneracy. Image Credit: Piaulet-Ghorayeb et al. 2024. Almost all the exoplanet atmospheres that have been characterized are mostly made of the lighter elements hydrogen and helium. These atmospheres are similar to Jupiter and Saturn in our Solar System. They’re nothing like Earth and its life-friendly atmosphere.
“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.”
Though GJ 9827 d isn’t habitable as far as our understanding of life goes, other exoplanets with similar metallicity are desirable targets in the search for life. Now that astronomers have broken the clouds-metallicity degeneracy, it changes our understanding of those planets and scientists’ ability to discern them. It’s all thanks to the JWST and its observing prowess.
Ryan MacDonald is a co-author of the new research and is a U-M astrophysicist and NASA Sagan Fellow. “Even with JWST’s early observations in 2022, researchers were discovering new insights into the atmospheres of distant gas giants,” MacDonald said, referring to the JWST’s spectroscopic characterizations of exoplanet atmospheres.
But those atmospheres were primarily composed of light gases, not heavier metals. These observations take us deeper into the atmospheres of sub-Neptunes. And though they’re the most common type of exoplanet in our galaxy, our Solar System is without one.
“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.”
The atmospheric steam didn’t jump out of the JWST observations. JWST produces an enormous amount of data, and to make sense of it, astronomers use modelling tools based on sampling algorithms and machine learning techniques. They typically employ several different models and work with all of the results to arrive at the most likely interpretation of the data.
The process of determining an atmosphere from data is called atmospheric retrieval. A 2023 paper presented a catalogue of 50 different atmospheric retrieval codes used by exoplanet scientists. The lead author of that paper is none other than Ryan MacDonald, a co-author of this new research. MacDonald wrote the software that analyzed and retrieved GJ 9827 d’s atmosphere, and co-author Raul used that software.
Raul generated millions of model atmospheres that matched the JWST observations before settling on the steam world model. In a sense, Raul was the first person to see proof 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.”
