Hubble Reveals Possible Water-Vapor World

New Hubble Space Telescope observations of a planet 100 light-years from Earth have revealed water vapor in its upper atmosphere. But astronomers are still working out exactly how much water vapor there is — and what this world’s atmospheric composition means for others like it.

With a radius just about twice Earth’s, GJ 9827d lies right on the line dividing super-Earths (rocky worlds more massive than our own) from gas-enshrouded sub-Neptunes. Even though this type of in-between planet is the most common in the galaxy, we don’t have an example in our own solar system, so there’s a lot we don’t understand about these worlds.

Slightly larger planets are best explained as being Neptune-like, with hydrogen-helium atmospheres. But this world is close enough to its K-type star, whipping around it every six days, that it would have soon lost such a light atmosphere. Instead, the planet is a candidate “water world” — not the kind of ocean world inhabited by Kevin Costner, but rather one with a hot, steam-filled atmosphere.

The new transmission spectra, in which Hubble observed the spectrum of light passing through the planet’s upper atmosphere, supports the water world scenario. Using Hubble to observe the planet as it transited its host star 11 times between 2017 and 2020, a team led by  Pierre Alexis-Roy (University of Montreal, Canada) measured the size of the planet at near-infrared wavelengths from 1.1 to 1.7 microns. Around 1.4 microns, the transiting planet appears larger, indicating that a molecule in its upper atmosphere is absorbing the star’s light. And that molecule is water.

However, Roy and his colleagues couldn’t distinguish how much water there is. It could be that there’s just a trace of water high up in a Neptune-like atmosphere. But since such a light atmosphere would have long been stripped away, the more likely scenario is that the observations are picking up water vapor farther down in the atmosphere, where 60% of the molecules are water.

Regardless which scenario pans out, it’s still unclear whether the planet has a solid surface or not. In any case, the world is too hot for oceans on its surface.

“Our ‘water world’ scenario thus mostly describes an atmosphere where water is very abundant and perhaps is the most abundant molecular species of the atmosphere,” Roy explains. The research is published in the Astrophysical Journal.

Prajwal Niraula (MIT), who led the team that discovered GJ 9827d but wasn’t involved in the current study, notes that the team’s detection of water vapor is “weaker than we’d like it to be, as 3.39 sigma is right at the edge of becoming significant.” But he also notes that, given the featureless spectra of other small sub-Neptunes, even that hint of a feature is promising.

“Given the universality of water amongst the exoplanetary atmosphere, and the observed signal, this is a good start to warrant further investigation of this target,” he says.

Observations with the James Webb Space Telescope (JWST) will help, because more information at longer wavelengths can clarify how much water there is and even reveal other molecules in the air such as carbon dioxide. Indeed, Roy confirms, “our group is currently mid-way through an observation program of this very same target with JWST.”  Stay tuned!