NASA’s Webb May Have Spotted Water Vapor Around a Rocky Exoplanet…or Maybe Not?

The faint glow of a red star in the distance illuminates the surface of a planet. The viewer sees just a warm crescent of the planet's surface.
An artist’s concept of the rocky exoplanet GJ 486 b, which orbits a red dwarf star that is only 26 light-years away in the constellation Virgo. Recent observations of GJ 486 b transiting in front of its star found potential signs of an atmosphere, but the results are still inconclusive. (Credit: NASA/ESA/CSA/Joseph Olmsted (STScI))

NASA’s James Webb Space Telescope has delivered images and details about the cosmos that have been stunning and even scientifically refining. But a new study documenting Webb’s recent observations of a rocky planet 26 light-years away are downright confounding, and have left researchers at the edge of a major discovery.

Dubbed GJ 486 b after the red dwarf (or M dwarf) star that it orbits just about every 1.5 Earth days, the rocky exoplanet discovered in 2021 is a super-Earth roughly 30% larger than Earth and three times more massive.

In their pursuit to characterize planetary atmospheres that, in turn, would help determine what planets could potentially support life, researchers pointed Webb’s Near Infrared Spectrograph (NIRSpec) instrument at GJ 486 b last December as it crossed in front of its star two times. Webb collected the frequencies of infrared light in a spectrum as the planet passed, a process known as transit spectroscopy, and with this technique the team could determine whether the planet has an atmosphere and what molecules are present.

Using three methods to analyze the data, the team — which includes astrophysicist Kevin Stevenson of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland — found the spectrum lacked the most common atmospheric molecules, including carbon dioxide, methane and hydrogen. The only close match was one familiar to Earth: water vapor.

At 800 degrees Fahrenheit (430 degrees Celsius), GJ 486 b is inhospitable to life, as we know it, no matter how much water vapor it might have, emphasized Sarah Moran, a planetary scientist at the University of Arizona in Tucson and lead author of the study, which has been accepted for publication in The Astrophysical Journal Letters. “It’s in no way habitable.”

But given the planet’s temperature, any water vapor would have to be part of an atmosphere, likely replenished by volcanoes ejecting steam from the planet’s interior. Such a circumstance would make GJ 486 b not only the first rocky exoplanet known to have an atmosphere but, with an atmosphere entirely made of water vapor, also one totally foreign to what we know.

“There is no comparable atmosphere in our own solar system,” Stevenson said. “We could be exploring a brave new world that would speak to the diversity of exoplanet atmospheres out there.”

Except there’s a chance Webb’s observations might not mean any of that. The water vapor seen might not come from GJ 486 b at all, the team reported. Instead, it might come from a counterintuitive place: the planet’s star.


A graphic showing an exoplanet crossing in front of a red star. To the left, a box zooms in on the planet and explains how water vapor may be in the atmosphere of this rocky exoplanet GH 486 b. Another box on the right zooms in on a star spot and explains how water vapor can form there. A spectrum of light observed by Webb on the bottom shows how determining where the water vapor is coming from can be done by observing the planet in visible light.
(Credit: NASA/ESA/CSA/Johns Hopkins APL/Joseph Olmsted (STScI)/Josh Diaz (APL))

Know Thy Star, Know Thy Planet

Exoplanet researchers have only recently started to appreciate that aberrations on the surface of a star, such as dark spots or bright faculae, can contaminate the spectra of an exoplanet. They can make it look like a molecule is present when in reality it isn’t. The effect is particularly stark with M dwarf stars, like GJ 486 b’s, and it’s a possibility the team couldn’t shake.

“We see a signal, and it’s almost certainly due to water,” Moran said. “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.”

M dwarfs are cool enough that water vapor can exist in their photospheres. Much of the water is broken into its components, hydrogen and oxygen, but some can remain stable near star spots, which are cooler than the surrounding environment.

That water vapor is possibly what Webb detected. When the team ran models to test this possibility, the results closely matched the spectra Webb returned.

“We didn't observe any evidence of the planet crossing any star spots during the transits, but that doesn't mean that there aren't spots elsewhere on the star,” said study co-author Ryan MacDonald, an astrophysicist at the University of Michigan. “And that's exactly the physical scenario that would impart these signals into the data and could wind up looking like a planetary atmosphere.”

Future Webb observations could shed more light on the issue. In an upcoming program this summer, researchers will use the telescope’s Mid-Infrared Instrument to observe GJ 486 b’s dayside. If the planet lacks an atmosphere, or has only a thin one, the hottest part of the dayside should stay directly under the star. But if there is an atmosphere, the hottest point should shift as the atmosphere circulates the heat.

The team’s greatest hope, however, may be proposing a future observation using Webb’s Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument. At the invisible infrared wavelengths of light that Webb observes, the two potential scenarios — an exoplanet with a water-vapor-rich atmosphere or a humid star — are virtually indistinguishable. However, peek just outside visible wavelengths, which NIRISS does, and the two situations quickly diverge.

“It’s joining the two instruments — NIRISS and NIRSpec — that will really pin down whether or not this planet has an atmosphere,” Stevenson said.

Until then, the mystery will persist.

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