ASU Scientists Contribute to Discovery of Unique "Steam-World" Exoplanet GJ 9827 d

A collaborative effort led by Ph.D. candidate Caroline Piaulet-Ghorayeb at the University of Montréal’s Trottier Institute for Research on Exoplanets (iREx), with key contributions from Arizona State University (ASU) researchers, has revealed groundbreaking insights into the atmosphere of exoplanet GJ 9827 d. Using data from the James Webb Space Telescope (JWST), the team discovered a water-rich atmosphere on the planet, marking a significant milestone in exoplanetary research.

Exploring New Atmospheric Frontiers
GJ 9827 d, located 98 light-years away and at roughly two times the size of the Earth, is the smallest planet known to date with a confirmed atmosphere. Previous studies had only succeeded in detecting atmospheres on much larger gas giants or mini-Neptunes, which are primarily hydrogen-based. This discovery marks the first detection of an exoplanet atmosphere where water vapor is abundant and hydrogen is not the dominant component.

The study was recently published in The Astrophysical Journal Letters. 

“This is an extraordinary breakthrough for smaller exoplanets,” said Luis Welbanks, 51 Pegasi b Fellow at the School of Earth and Space Exploration at ASU. “These observations of GJ 9827 d have moved closer to understanding atmospheres on planets similar in size to Earth, where the search for habitable conditions truly begins.”

Pioneering Observations with JWST
By analyzing light as it passed through the planet’s atmosphere, the team could distinguish between hydrogen-dominated atmospheres and heavier ones. GJ 9827 d, roughly twice the size of Earth, has an atmosphere rich in heavier molecules, particularly water vapor, signaling a major leap forward in atmospheric studies of small exoplanets.

“This is a huge step towards the goal of searching for atmospheres around smaller, terrestrial-like planets,” said Piaulet-Ghorayeb. “GJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules, just like the terrestrial planets of the Solar System, and the first confirmed example in a long time of a ‘steam world’ posited by the scientific community.”

New Horizons in the Search for Life
Although GJ 9827 d is not thought to be habitable due to its high temperatures, its water-rich atmosphere offers proof that such atmospheres can be detected on smaller planets, bringing researchers closer to identifying Earth-like environments beyond the Solar System.

“This is a much needed step in our understanding of planets, especially for those with sizes between Earth and Neptune for which we have no analogue here at home. This discovery sets the stage for future studies on planets where life could potentially exist,” said Welbanks. “It’s an exciting time for planetary science.”

Future JWST observations aim to further explore GJ 9827 d’s atmosphere, with ASU researchers poised to contribute to new groundbreaking discoveries exploring the composition of these steam worlds in the coming years.