May 01, 2023
Astronomers have used the James Webb Space Telescope (JWST) to study a rocky exoplanet named GJ 486 b, which orbits a red dwarf star in just under 1.5 Earth days. Though too close to its star to be within the habitable zone, with a surface temperature of about 800 degrees Fahrenheit (430 degrees Celsius), the observations made using Webb’s Near-Infrared Spectrograph (NIRSpec) show hints of water vapor. This marks the first time a rocky exoplanet might have an atmosphere, if the vapor is associated with the planet, indicating the possibility of liquid water despite its scorching temperature and close proximity to its star.
GJ 486 b is about 30% larger than Earth and three times as massive, which means it is a rocky world with stronger gravity than Earth. The planet is expected to be tidally locked, with a permanent day side and a permanent night side. It was observed in two transits, each lasting about an hour, using three different methods to analyze the resulting data. The team ran computer models considering a number of different molecules, and concluded that the most likely source of the signal was water vapor.
If GJ 486 b has an atmosphere, then when it transits starlight would filter through those gases, imprinting fingerprints in the light that allow astronomers to decode its composition through a technique called transmission spectroscopy. However, the team cautions that the water vapor could be on the star itself – specifically, in cool starspots – and not from the planet at all.
“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,” said Sarah Moran of the University of Arizona in Tucson, lead author of the study.
Kevin Stevenson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, principal investigator on the program, added: “Water vapor 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.”
The water vapor detected could also be from starspots, which are very cool compared to the surrounding surface of the star. GJ 486 b’s host star is much cooler than the Sun, so even more water vapor would concentrate within its starspots. This would create a signal that mimics a planetary atmosphere. The team has not observed evidence of the planet crossing any starspots during the transits, but that does not mean that there aren't spots elsewhere on the star.
A water vapor atmosphere would be expected to gradually erode due to stellar heating and irradiation. As a result, 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 Webb program will use the Mid-Infrared Instrument (MIRI) 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 an atmosphere is present, the hottest part of the day side would be offset from the star, revealing a difference in temperature that MIRI can detect.