The current understanding is uncertain: the Hubble may have detected a minor amount of water vapor within a predominantly hydrogen-rich atmosphere, or the planet might primarily possess a water-based atmosphere, a remnant after its original hydrogen/helium atmosphere was vaporized by stellar radiation.
Björn Benneke commented on the novelty of this discovery, saying, “Until now, we had not been able to directly detect the atmosphere of such a small planet. And we’re slowly getting in this regime now.” He further speculated on the transition of atmospheres in smaller planets, comparing them to Venus, which has a carbon dioxide-dominated atmosphere.
With surface temperatures as high as Venus, reaching around 800 degrees Fahrenheit, GJ 9827d would be a steamy and uninhabitable world if its atmosphere is primarily composed of water vapor.
The team is currently considering two possibilities regarding the planet’s atmospheric composition. One theory suggests that GJ 9827d retains a hydrogen-rich atmosphere with traces of water, classifying it as a mini-Neptune. Alternatively, it could be akin to a warmer version of Europa, Jupiter’s moon known for its subsurface ocean. Benneke theorizes, “The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapor on top of some smaller rocky body.”
If the planet has managed to hold onto a water-rich atmosphere, it likely originated farther from its host star where temperatures are colder and water exists as ice, later migrating closer and undergoing significant radiation exposure, causing hydrogen to heat and potentially escape. The other hypothesis posits that the planet formed near the hot star, with a minor amount of water in its atmosphere.
Hubble observed GJ 9827d during 11 transits over three years. These observations allowed astronomers to analyze the spectral signature of water molecules in the planet’s atmosphere. If the planet has clouds, they are low enough to not obstruct Hubble’s view, allowing the detection of water vapor above the clouds.
Thomas Greene, an astrophysicist at Nasa’s Ames Research Center, highlighted the importance of this discovery, saying, “Observing water is a gateway to finding other things.” He anticipates future studies by Nasa’s James Webb Space Telescope, which will offer more in-depth infrared observations and the potential to detect other vital molecules.
GJ 9827d orbits a red dwarf star, GJ 9827, in the Pisces constellation, discovered by Nasa’s Kepler Space Telescope in 2017. The exoplanet completes its orbit every 6.2 days and lies 97 light-years from Earth.
The Hubble Space Telescope, a collaborative project between Nasa and the European Space Agency, is managed by Nasa’s Goddard Space Flight Center in Maryland, with scientific operations conducted by the Space Telescope Science Institute in Baltimore.
Contributors to this discovery and report include Ray Villard from the Space Telescope Science Institute in Baltimore, Pierre-Alexis Roy and Björn Benneke from the Trottier Institute for Research on Exoplanets at the Université de Montréal.