Peering at a rocky planet 26 light-years away, the James Webb Space Telescope (JWST) has detected signs of water vapor. The discovery would mark the first time astronomers have ever succeeded in discerning an atmosphere on a rocky planet outside our own solar system. Finding water vapor on a small world would also be a big step forward in the search for habitable planets beyond Earth, because water is essential to life as we know it.
However, an equally likely explanation for the water vapor has sown ambiguity in the potential landmark result. Spots of magnetic activity on the planet’s host star might as well be the source of the water vapor. To finally unravel the mystery, further observations with a variety of instruments are needed.
“Just knowing that water could exist on a rocky planet around another star would be a big deal,” said Ryan MacDonald, an astrophysicist at the University of Michigan. At the same time, he says, “knowingly it’s good to play devil’s advocate a little” rather than overpromising when a result turns out to be wrong. a preprinted paper detailing MacDonald and his colleagues’ analysis of the water vapor was posted May 1 and the study has been accepted for publication in the Astrophysical Journal Letters.
The team originally planned to look for carbon dioxide signatures in the atmospheres of rocky planets. The researchers focused on GJ 486 b, a rocky planet orbiting close to a red dwarf star in the constellation Virgo. Using JWST’s Near Infrared Spectrometer (NIRSpec) instrument, they watched as the planet crossed its star’s face as seen from Earth — a phenomenon called a transit. This allowed the team to capture a fraction of the starlight passing through the planet’s upper atmosphere, assuming the world has an atmosphere at all. Such light is especially valuable to astronomers because it can carry imprints of various molecules in a planet’s air. For example, water vapor preferentially absorbs light of certain wavelengths or colors. Using the light from two transits of GJ 486 b to form a rainbow-like “spectrum” — a technique called transmission spectroscopy — revealed dark absorption lines that, like a bar code, can be read to indicate the presence of water vapor there to reveal.
With an estimated surface temperature of 800 degrees Fahrenheit, GJ 486 b is similar to Venus and certainly not in the range of what would be considered habitable or Earth-like. It orbits so close to its parent star that the planet’s atmosphere could easily have been eroded long ago by stellar outbursts 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 pointing to atmospheric detection. “I was even more surprised when I compared it to my atmospheric models, and it matched water so well,” says Moran.
At first, 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.
A competing scenario arises from the fact that red dwarf stars are much smaller, fainter and cooler than our sun. This means that starspots on their surfaces — dark, highly magnetized regions on all stars that exhibit lower temperatures than their surroundings — are particularly cold and may be low enough to sustain water vapor formation. In 2018, years before the launch of JWST, a team of researchers from the University of Arizona realized that red dwarf star spots can be a tricky source of contamination which may mimic real atmospheric signals from companion exoplanets. With this in mind, Moran and her colleagues statistically calculated how well an atmospheric origin explained, or “fits,” the water vapor signal versus the fit of a stellar model that assumed star spots. The result was an almost identical fit for each scenario. Statistically speaking, if you want to be as sure as the experts whether this particular planet harbors water vapor, you can just toss a coin.
Part of the ambiguity is because of water’s remarkable physical properties. If the JWST instrument had picked up a strong signature of carbon dioxide molecules, that could be uniquely attributed to the planet, MacDonald says. “Water just turns out to be an unfortunate molecule that is very stable over a very wide range of temperatures,” he says.
While NIRSpec would have been sufficient for detecting carbon dioxide, water vapor detection sits perilously on the edge of that instrument’s capabilities. Without conducting observations using an assortment of instruments that cover a wider range of wavelengths, the conclusions are likely to remain ambiguous, Moran says.
“This is the very first year of sightings,” says MacDonald. “We’re kind of figuring out how to model the planets, how to model the stars, how to do the observations. It was always a bit messy in the beginning.” Still, he’s optimistic that the team is on an upswing learning curve to find optimal observing strategies for using JWST to learn more about the atmospheres of minor planets.
If it turns out that water vapor comes from the planet and not from the star, that would mean that GJ 486 b has an atmosphere. And if a planet with such a high surface temperature and dangerously close to its parent star can maintain an atmosphere, then presumably cooler worlds in milder orbits should offer even better chances of habitability. Even if starspots are found to be the source of the signal, Moran says it will give researchers a chance to learn more about the magnetic fields and other quirks of stellar astrophysics that can cause water vapor to form on red dwarfs themselves.
“I’m not surprised that this result is ambiguous,” said Jacob Bean, an astrophysicist at the University of Chicago who was not part of the research team. Transmission spectroscopy, he says, is being challenged by thin atmospheres, such as those possibly around GJ 486 b. Instead, Bean says, a technique called thermal emission could yield a less ambiguous result. In this approach, astronomers directly measure a planet’s infrared glow, usually by watching the world pass behind it and be eclipsed by its star, which allows the planet’s heat signature to be distinguished from that of the star. A smeared thermal emission across both the illuminated day side and the dark night side of a world would suggest a medium for transporting heat from incident 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, which Bean says will “bring a lot of clarity to the situation.” But while thermal emission may be able to show with greater certainty whether there is an atmosphere around the planet, it won’t be able to reveal much about the chemical composition of that possible atmosphere. “We’re still at the edge of what we can understand,” says Mansfield. “I think it’s still good to do all those different kinds of measurements.”
Making observations over a much wider range of wavelengths is the key takeaway, agrees Kevin Stevenson, an astronomer at Johns Hopkins University. Getting the best data on small, rocky exoplanets cannot be answered by just one type of observation. “I think the combination of getting transits and eclipses will give you the most information,” he says.
Within about a year, astronomers should collect enough data to definitively state whether GJ 486 b has an atmosphere, Stevenson predicts. “Then, of course, we can track other planets and get a better picture of the population as a whole,” he says. “This is really just the beginning.”