Peering at a rocky planet 26 light-years away, the James Webb Space Telescope (JWST) has spied signs of water vapor. The discovery would mark the first time that astronomers have ever managed to discern an atmosphere on a rocky planet outside our own solar system. Finding water vapor on a small world would also be a major step forward in the search for habitable planets beyond Earth because water is essential to life as we know it.
An equally likely explanation for the water vapor has thrown ambiguity into the potentially milestone result, however. Spots of magnetic activity on the planet’s host star could just as well be the water vapor’s source. Ultimately untangling the mystery will require further observations with a variety of instruments.
“Just knowing that water could exist on a rocky planet around another star would be a huge deal,” says Ryan MacDonald, an astrophysicist at the University of Michigan. At the same time, he says, “in science, it’s good to play a bit of devil’s advocate” rather than overpromise a result that turns out to be incorrect. A preprint paper detailing MacDonald and his colleagues’ analysis of the water vapor was posted on May 1, and the study has been accepted for publication in the Astrophysical Journal Letters.
The team had originally planned to search for signatures of carbon dioxide in rocky planet atmospheres. The researchers targeted GJ 486 b, a rocky planet orbiting close to a red dwarf star in the Virgo constellation. Using JWST’s Near Infrared Spectrometer (NIRSpec) instrument, they watched as the planet crossed the face of its star, as seen from Earth—a phenomenon called a transit. This allowed the team to collect a fraction of the starlight that passed through the planet’s upper atmosphere—assuming the world has an atmosphere at all. Such light is especially precious to astronomers because it can carry imprints of various molecules in a planet’s air. Water vapor, for instance, preferentially absorbs light of certain wavelengths, or colors. Using the light from two transits of GJ 486 b to form a rainbowlike “spectrum”—a technique called transmission spectroscopy—revealed dark absorption lines that, much like a barcode, can be read to reveal water vapor’s presence there.
With an estimated surface temperature of 800 degrees Fahrenheit, GJ 486 b is comparable to Venus and certainly not in the range of what would be considered habitable or Earth-like. It orbits so close to its host star that the planet’s atmosphere could have easily been eroded long ago by stellar flares and other outbursts. Given such harsh conditions, the study’s lead author, Sarah Moran, a planetary scientist at the University of Arizona, says she was surprised to see signals indicating atmospheric detection. “I was even more surprised when I compared it with my atmospheric models, and it fit so well with water,” Moran says.
Initially the researchers thought they must be seeing water vapor high in the planet’s atmosphere. “But we immediately stepped back and said, ‘What are the other explanations?’” Moran says.
One competing scenario emerges from the fact that red dwarf stars are much smaller, dimmer and cooler than our sun. This means that star spots on their surfaces—dark, highly magnetized regions on all stars that exhibit lower temperatures than their surroundings—are especially chilly and can be low enough to sustain the formation of water vapor. In 2018, years before JWST’s launch, a team of researchers at the University of Arizona realized that red dwarf star spots could be a troublesome source of contamination that potentially mimics genuine atmospheric signals from accompanying exoplanets. With this in mind, Moran and her colleagues statistically calculated how well an atmospheric origin explained, or “fit,” the water vapor signal versus the fit from a stellar model that presumed star spots. The result was a nearly identical fit for each scenario. Statistically speaking, if you want to be as certain as the experts about whether this particular planet harbors water vapor, you can simply flip a coin.
Some of the ambiguity is because of water’s remarkable physical properties. If the JWST instrument had picked up a strong signature of molecules of carbon dioxide, MacDonald says, it would be uniquely attributable to the planet. “Water just turns out to be an unfortunate molecule that is very stable across a very wide range of temperatures,” he says.
While NIRSpec would’ve been sufficient for detecting carbon dioxide, the detection of water vapor sits precariously on the edge of that instrument’s capabilities. Without conducting observations using an assortment of instruments covering a wider wavelength range, Moran says, the conclusions will likely remain ambiguous.
“This is the very first year of observations,” MacDonald says. “We’re kind of figuring out how to model the planets, how to model the stars, how to do the observations. It was always going to be a little bit messy at the beginning.” Still, he’s optimistic that the team is on the upswing of a learning curve to figure out optimal observational strategies for using JWST to learn more about the atmospheres of small planets.
Should it turn out to be correct that water vapor is coming from the planet and not the star, that would mean that GJ 486 b has an atmosphere. And if a planet with such a high surface temperature and perilously close orbit to its host star can maintain an atmosphere, then presumably cooler worlds in more clement orbits should offer even better chances for habitability. Even if it turns out that star spots are the signal’s source, Moran says, this gives researchers an opportunity to learn more about the magnetic fields and other quirks of stellar astrophysics that allow water vapor to arise on red dwarfs themselves.
“I’m not surprised that this result is ambiguous,” says Jacob Bean, an astrophysicist at the University of Chicago, who was not a part of the research team. Transmission spectroscopy, he says, is challenged by thin atmospheres, such as the one potentially surrounding GJ 486 b. Instead, Bean says, a technique called thermal emission could provide a less ambiguous result. In this approach, astronomers directly gauge a planet’s infrared glow, usually by watching as the world passes behind and is eclipsed by its star, which allows the planet’s heat signature to be discerned from that of the star. A smeared-out thermal emission across both a world’s illuminated dayside and its dark nightside would suggest some medium for transporting heat from infalling starlight—that is, an atmosphere.
In the coming months, a team led by astronomer Megan Mansfield of the University of Arizona will make such thermal emission observations of GJ 486 b using JWST—bringing, Bean says, “a lot of clarity to the situation.” But while thermal emission might be able to show with more certainty whether there’s an atmosphere surrounding the planet, it won’t be able to reveal much at all about that possible atmosphere’s chemical composition. “We’re still kind of right at the edge of what we can understand,” Mansfield says. “I think it’s still good to do all these different types of measurements.”
Conducting observations over a much wider wavelength range is the key takeaway, agrees Kevin Stevenson, an astronomer at Johns Hopkins University. Getting the best data on small, rocky exoplanets won’t be answered by just one type of observation. “I think the combination of obtaining transits and eclipses will give you the most information,” he says.
Within the next year or so, astronomers should gather enough data to definitively declare whether GJ 486 b has an atmosphere, Stevenson predicts. “Then, of course, we can follow up on additional planets and get a better sense of the population as a whole,” he says. “This is really just the beginning.”