Published in 'Astrophysical Journal Letters', the study was led by Caroline Piaulet-Ghorayeb, a Ph.D. candidate at the Universite de Montreal's Trottier Institute for Research on Exoplanets (IREx), along with an international team of collaborators.

Earlier in 2024, IREx researchers had detected water in GJ 9827 d's atmosphere using Hubble Space Telescope (HST) data, making it the smallest known exoplanet, at about twice the size of Earth, to have a confirmed atmosphere. These new observations with JWST add crucial detail, showing the presence of a water-rich atmosphere and raising intriguing possibilities about exoplanetary atmospheres.

A challenging target
Until now, detecting atmospheres around smaller, Earth-sized planets has been difficult. Most atmospheric studies focused on large gas giants or mini-Neptunes-planets with hydrogen-dominated atmospheres, much like Jupiter or Neptune in our Solar System. "For now, all the planets we've detected that have atmospheres are giant planets, or at best mini-Neptunes," said Piaulet-Ghorayeb, the lead author. "These planets have atmospheres made up mostly of hydrogen, making them more similar to gas giants in the Solar System than to terrestrial planets like Earth, which have atmospheres dominated by heavier elements."

A distinctive atmosphere
What sets GJ 9827 d apart is its unique atmosphere, which is rich in heavier molecules, including water vapor, rather than being dominated by hydrogen. By combining data from JWST's Near-Infrared Imager and Slitless Spectrograph (NIRISS) with previous HST data, Piaulet-Ghorayeb and her team demonstrated that the planet's atmosphere differs significantly from those of larger planets. "It's closer in molecular weight to the carbon dioxide or nitrogen-rich atmospheres that we are currently looking for on smaller rocky planets, where we would eventually look for life," she explained.

The team used transmission spectroscopy to analyze the light passing through the planet's atmosphere as it transited its star. By integrating JWST data with previous HST observations, they confirmed that the detected spectral features originate from the planet's atmosphere, not from contamination by the star.

Further analysis revealed two possible atmospheric types for GJ 9827 d: one with a cloudy, hydrogen-dominated atmosphere and trace water, and the other with a denser, water-rich atmosphere. Based on its proximity to its host star, GJ 9827 d likely has a superheated, dense atmosphere, possibly in a mixed state of gas and steam.

Uninhabitable but still significant
While GJ 9827 d is unlikely to support life due to its high surface temperature (about 350 degrees Celsius), the discovery represents a major step forward in the search for Earth-like exoplanets. "This is a huge step towards the goal of searching for atmospheres around smaller, terrestrial-like planets," added Piaulet-Ghorayeb. The discovery of a water-rich atmosphere on a planet like GJ 9827 d supports the possibility that other small, rocky exoplanets may also have such atmospheres, paving the way for future exploration.

The presence of such "steam worlds," planets with thick, water-vapor-rich atmospheres, has long been theorized by scientists. These planets, much like icy moons such as Europa or Ganymede, orbit close enough to their stars that water exists as steam rather than under an ice layer. Continued JWST observations of GJ 9827 d are planned to further investigate the components of its intriguing atmosphere.

Research Report:JWST/NIRISS Reveals the Water-rich "Steam World" Atmosphere of GJ 9827 d

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