Signs that a rocky exoplanet could have an atmosphere detected by JWST

Scientists working with the James Webb Space Telescope say new data potentially shows water vapour around a rocky exoplanet – a first if confirmed.

However, the water signature may also be coming from the star itself, so additional observations are needed.

Water vapour has been seen on gaseous exoplanets before, but to date no atmosphere has been detected around a rocky exoplanet – defined as those with sizes less than or equal to 1.4x Earth’s radius.

This [could] have important consequences for our understanding of the atmospheres and habitability of rocky exoplanets orbiting red dwarfs. Dr James Kirk

The new result, announced today by an international team including an Imperial College London researcher, is accepted in The Astrophysical Journal Letters.

The most common stars in the universe are red dwarf stars, which means that rocky exoplanets similar to Earth are most likely to be found orbiting such a star. Red dwarf stars are cool, so a planet has to hug it in a tight orbit to stay warm enough to potentially host liquid water and be habitable.

Such stars are also active, particularly when they are young, releasing ultraviolet and X-ray radiation that could destroy planetary atmospheres. As a result, one important open question in astronomy is whether a rocky planet could maintain, or re-establish, an atmosphere in such a harsh environment.

To help answer that question, astronomers used NASA’s James Webb Space Telescope to study a rocky exoplanet known as GJ 486 b. It is too close to its star to be within the habitable zone, with a surface temperature of about 430°C, but the new observations show hints of water vapour.

Understanding habitability

If the water vapour is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and close proximity to its star. However, the team cautions that the water vapour could be on the star itself – specifically, cool ‘starspots’ – and not from the planet at all.

Dr James Kirk, from the Department of Physics at Imperial, helped analyse the data. He said: “Previous searches for atmospheres of rocky exoplanets around red dwarfs have been inconclusive. Our study of GJ 486b demonstrates that there is water in the system, either in the planet’s atmosphere or the stellar atmosphere.

"If follow-up observations confirm that the water is in the planet’s atmosphere, this would have important consequences for our understanding of the atmospheres and habitability of rocky exoplanets orbiting red dwarfs.”

Co-lead author of the study Dr Sarah Moran, from the University of Arizona in Tucson, added: “We see a signal and it’s almost certainly due to water. But we can't tell yet if that water is part of the planet's atmosphere, meaning the planet has an atmosphere, or if we’re just seeing a water signature coming from the star.”

Co-lead author Dr Kevin Stevenson, from the Johns Hopkins University Applied Physics Laboratory, added: “Water vapour in an atmosphere on a hot rocky planet would represent a major breakthrough for exoplanet science. But we must be careful and make sure that the star is not the culprit.”

Analysing transits

GJ 486 b is about 30% larger than the Earth and three times as massive, which means it is a rocky world with stronger gravity than Earth. It orbits a red dwarf star once every two Earth days. It is expected to be tidally locked, with a permanent day side and a permanent night side.

GJ 486 b transits its star, crossing in front of the star from our point of view. If it has an atmosphere, then when it transits starlight would filter through those gasses, imprinting fingerprints in the light that allow astronomers to decode its composition through a technique called transmission spectroscopy.

The team observed two transits, each lasting about an hour. They then used three different methods to analyse the resulting data, which were consistent. The team ran computer models considering a number of different molecules, and concluded that the most likely source of the signal was water vapour.

While the water vapour could potentially indicate the presence of an atmosphere on GJ 486 b, an equally plausible explanation is water vapour on the star. The planet’s host star is cool enough that water vapour can exist in its photosphere. Since starspots (like sunspots on our Sun) are cooler than the surrounding area, the water vapour would concentrate there. As a result, it could create a signal that mimics a planetary atmosphere.

Co-author Dr Ryan MacDonald, from the University of Michigan in Ann Arbor, said: “We didn't observe evidence of the planet crossing any starspots during the transits. But that doesn't mean that there aren't spots elsewhere on the star. And that's exactly the physical scenario that would imprint this water signal into the data and could wind up looking like a planetary atmosphere.”

Seeking confirmation

If an atmosphere is present, it would likely have to be constantly replenished by volcanoes ejecting steam from the planet’s interior. If the water is indeed in the planet’s atmosphere, additional observations are needed to narrow down how much water is present.

Future Webb observations may shed more light on this system. An upcoming program will use the Mid-Infrared Instrument to observe the planet’s day side. If the planet has no atmosphere, or only a thin atmosphere, then the hottest part of the day side is expected to be directly under the star. However, if the hottest point is shifted, that would indicate an atmosphere that can circulate heat.

Ultimately, observations by another JWST instrument – the Near-Infrared Imager and Slitless Spectrograph – at shorter, bluer wavelengths towards the visible part of the spectrum will be needed to differentiate between the planetary atmosphere and starspot scenarios.

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Top image: Artist's concept of the rocky exoplanet GJ 486 b.
Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

'High Tide or Rip-Tide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ486b from JWST Observations' is accepted in The Astrophysical Journal Letters.

Based on a press release by NASA.