The GJ 9827 system about 100 lightyears away in the constellation of Pisces contains three known super-Earth planets orbiting a nearby K dwarf star.
With a mass almost twice that of Earth, GJ 9827 d is the largest of the triplets and also has the widest orbit.
The innermost two companion worlds are very dense and so are thought to be predominantly rocky planets.
But GJ 9827 d has a lower density, meaning it probably contains a significant amount of ‘volatiles’ like water, and possibly a thick atmosphere.
Understanding GJ 9827 d's atmosphere
GJ 9827 d orbits relatively closely to its sun and has a calculated temperature of some 350°C (662°F).
This implies that its atmosphere – contrary to most similar-sized exoplanets – can’t be dominated by hydrogen, as its high temperature would mean that such light gases would quickly escape.
Previous observations of the planet have only lent support to this expectation by failing to detect any hint of hydrogen or helium streaming away from the planet.
The problem is that just knowing the density alone of a planet like this doesn’t allow astronomers to work out what its atmosphere is like.
Though the bulk has escaped, some of the hydrogen might still be retained as a thin envelope of hydrogen around the planet.
Or it could have a thicker atmosphere of heavier gas molecules mixed in with some hydrogen.
To understand exactly what’s going on, what’s needed is someone spectroscopically analysing the atmosphere to try to directly detect the gas molecules that make it up.
Observations
Though clouds can really hamper such observations of worlds like this one, because they block views of the lower atmosphere, observations by the Hubble Space Telescope in 2023 indicated that molecular features can be detected for this planet.
The much better capabilities of the James Webb Space Telescope (JWST) ought to be able to reveal GJ 9827 d’s atmospheric composition.
During her PhD in the department of physics at the University of Montreal, Caroline Piaulet-Ghorayeb attempted exactly that.
She used the near-infrared spectrometers on JWST to observe GJ 9827 d for two transits across its sun, the starlight filtering through its atmosphere
Piaulet-Ghorayeb and her team saw the clear spectral fingerprint of water molecules in the planet’s atmosphere.
In fact, Piaulet-Ghorayeb calculates that water vapour makes up at least 30% of the atmosphere. GJ 9827 d is a volatile-rich ‘steam world’.
The planet is hot enough that this water wouldn’t condense into liquid droplets in the atmosphere or on the surface.
Instead, at higher pressures the water would be in a supercritical state, where it mixes well with hydrogen gas.
And despite the high temperature of the planet, this thick atmosphere made of heavier molecules would be stable.
This is consistent with the non-detections of escaping hydrogen or helium.
Piaulet-Ghorayeb and her colleagues say that further planned JWST observations of the planet ought to be able to detect other constituents of the GJ 9827 d atmosphere, such as carbon monoxide or carbon dioxide, and possible sulphur dioxide.
And this will help researchers understand how this planet formed with such a volatile-rich atmosphere.
Lewis Dartnellwas reading JWST/NIRISS reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d by Caroline Piaulet-Ghorayeb et al. Read it online at: arxiv.org/abs/2410.03527.
This article appeared in the January 2025 issue of BBC Sky at Night Magazine.
