Exoplanets are planets that lie outside of our solar system, and researchers can use telescopes to study their atmospheres.

But until the launch of NASA’s JWST, the largest optical telescope in space, researchers haven’t been able to determine whether rocky exoplanets—planets composed mostly of rocks rather than gas giants such as Jupiter and Saturn—are able to build and maintain atmospheres.

“We’ve used the Hubble Space Telescope for many years to study the atmospheres of gas giant exoplanets, but atmospheric signatures coming from rocky planets are so tiny that we needed a much more sensitive space telescope to see them,” said University of Michigan astronomer Ryan MacDonald.

Now, MacDonald is a co-author of a study that has discovered hints of water vapor potentially coming from a rocky exoplanet called GJ 486 b. However, it is also just as likely that the water vapor signal is coming from the red dwarf star it orbits. Whether the water vapor is associated with the star or the planet, this represents a significant leap forward in what astronomers can observe about rocky exoplanets, MacDonald said.

“It hasn’t really been possible for us to characterize rocky planet atmospheres before. It’s only with the greater collecting area and sensitivity of JWST that we can even start to address some very fundamental science questions about what rocky planets around other stars are even like,” said MacDonald, postdoctoral researcher and Sagan Fellow with the NASA Hubble Fellowship Program.

The team’s results have been accepted for publication in The Astrophysical Journal Letters.

Rocky exoplanets are most likely to be found orbiting red dwarf stars, the most common stars in the universe. 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 (meaning it lies in the habitable zone).

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 reestablish, an atmosphere in such a harsh environment.

To help answer that question, astronomers used JWST to study a rocky exoplanet GJ 486 b. It is too close to its star to be within the habitable zone, with a surface temperature of about 800 degrees Fahrenheit (430 degrees C). And yet, their observations show hints of water vapor. If the water vapor is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and close proximity to its star.

Water vapor has been seen on gaseous exoplanets before, but to date no atmosphere has been detected around a rocky exoplanet. 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, lead author of the study.

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. 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 analyze the resulting data. The results from all three are consistent in that they show a mostly flat spectrum with an intriguing rise at the shortest, bluest infrared wavelengths. The team ran computer models considering a number of different molecules and concluded that the most likely source of the signal was water vapor.

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

MacDonald was one of three theorists on the team to look independently at the data. He ran a code he developed during his doctoral research called an atmospheric retrieval code. This code computes millions of possible explanations to figure out the range of properties, whether they be atmospheric or stellar properties, that can explain the observations—the mostly flat spectrum with the intriguing rise that indicated water vapor.

“My analysis found two families of solutions that could produce an identical fit, even though they were completely different physical explanations,” he said. “One was this pure water atmosphere that other theorists on our team had also seen, but the second solution indicated the star could be the real culprit.

“However, you can’t choose in science what the data says. And what it says is that you could equally well explain the slope by having star spots with no atmosphere around the planet.”

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 JWST 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.

“As a kid, I was reading all these classic sci-fi stories, about people going and exploring planets around other stars, but it was purely in the realm of imagination. We didn’t even know if these planets existed,” MacDonald said. “In the last 20 years, we’ve been finding rocky planets around other stars, and now, we’re finally at the point where our imagination can finally actually be compared to the reality of what these planets are really like.

“Going forward, I like to hope there’ll be a nice feedback loop between scientific discoveries about exoplanets and science fiction like the stories that inspired me.”

Alicia Highland, an undergraduate research assistant and co-author of the study, helped sift through the data.

“The unwavering pursuit of the age-old question ‘Does life exist beyond our planet?’ by scientists has been the driving force behind our advancement in space exploration and astrophysics,” Highland said. “The insatiable curiosity and relentless determination to unravel the mysteries of the universe is what makes the humanity within science so remarkable and inspiring to me.”

The JWST is the world’s premier space science observatory. JWST will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. JWST is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.