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Sign of water vapor from a scorching planet...or its star

MADRID, 1 May.

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Sign of water vapor from a scorching planet...or its star


Infrared observations with the James Webb Space Telescope of the rocky exoplanet GJ 486 b, too close to its star to be in the habitable zone and with a temperature of 430 degrees Celsius, have shown puzzling signs of water vapor.

If water vapor is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and proximity to its star. Water vapor has been seen on gaseous exoplanets before, but to date no atmosphere has been definitively detected around a rocky exoplanet. However, the team of astronomers who made the finding warns that the water vapor could be in the star itself, specifically in cold star spots, and not in the planet.

"We see a signal, and it's almost certainly due to water. But we still can't tell if that water is part of the planet's atmosphere, which means the planet has an atmosphere, or if we're just seeing a water signature." coming from the star," Sarah Moran of the University of Arizona in Tucson, lead author of the study, said in a statement.

"Water vapor in the atmosphere of a hot, rocky planet would represent a breakthrough for exoplanet science. But we have to be careful and make sure the star is not the culprit," added Kevin Stevenson of the Applied Physics Laboratory at Johns Hopkins University, Maryland, principal investigator for the program.

Just 26 light-years away in the constellation Virgo, GJ 486 b is about 30% larger than Earth and three times as massive, meaning it's a rocky world with stronger gravity than Earth. It revolves around a red dwarf star in just under 1.5 Earth days. It is expected to be tidally locked, with a permanent dayside and a permanent nightside.

GJ 486 b transits its star, crossing in front of the star from our point of view. If it does have an atmosphere, then when it transits, light from the star would filter through those gases, imprinting fingerprints on 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 analyze the resulting data. The results from all three are consistent in that they show a mostly flat spectrum with an intriguing rise in the shorter infrared wavelengths. The team ran computer models looking at several different molecules and concluded that the most likely source of the signal was water vapor.

While water vapor could potentially indicate the presence of an atmosphere in GJ 486 b, an equally plausible explanation is the star's water vapor. Surprisingly, even on our own Sun, water vapor can sometimes exist in sunspots because these spots are very cool compared to the star's surrounding surface. GJ 486 b's host star is much cooler than the Sun, so even more water vapor would be concentrated within its starspots. As a result, it could create a signal that mimics a planetary atmosphere.

"We didn't see evidence that the planet crossed any starspots during the transits. But that doesn't mean there aren't spots on other parts of the star. And that's exactly the physical scenario that would print this water signal into the data and could end up resembling a planetary atmosphere," explained Ryan MacDonald of the University of Michigan in Ann Arbor, one of the study co-authors.

A water vapor atmosphere would be expected to gradually erode due to heating and stellar irradiation. As a result, if an atmosphere is present, it would likely have to be constantly replenished by volcanoes that spew steam from the planet's interior. If the water is indeed in the planet's atmosphere, additional observations are needed to reduce the amount of water present.

Webb's future observations may shed more light on this system. A forthcoming Webb program will use the Mid-Infrared Instrument (MIRI) to observe the dayside of the planet. If the planet has no atmosphere, or only a thin atmosphere, the hottest part of the dayside is expected to be directly below the star. However, if the hottest spot is displaced, that would indicate an atmosphere that can circulate heat.

Ultimately, observations at shorter infrared wavelengths by another Webb instrument, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), will be needed to differentiate between planetary atmosphere and star-spot scenarios. "It's about putting together multiple instruments that will really determine whether or not this planet has an atmosphere," Stevenson said.