Scientists haven’t found a rocky exoplanet with air. But now they have a plan.
Perhaps surprisingly, the majority of stars in the galaxy are not sun clones but smaller orbs of gas and plasma known as red dwarfs, about half the size of Earth’s star.
Astronomers have had their sights set on these stars as tantalizing places to look for habitable worlds for a while now. Not only are they the most populous stars, but their planets are easier to study from a practical standpoint. Current atmosphere-detecting methods work best when planets orbit fairly close to their stars in space. Red dwarf stars’ relatively cooler temperatures present an opportunity for worlds to be closer without getting fried.
Despite red dwarfs’ potential, no one knows for sure whether their worlds can have atmospheres, or what chemicals could be within their air.
Scientists will soon begin to answer those questions with the James Webb Space Telescope, a partnership of NASA and its European and Canadian counterparts. A new large-scale program will budget about 500 hours for observing rocky exoplanets orbiting small red stars, specifically to search for atmospheres.
Though the survey is designated for Webb’s fourth research cycle, which starts next July, observations may begin sooner, said Néstor Espinoza, an astronomer heading the program’s implementation team.
“This is one of those high-risk, high-reward programs,” he told Mashable. “Imagine that for all of the targets, we detect atmospheres. Then you answer the question, ‘Yes, atmospheres are very common around these stars. That means maybe life can emerge.’ On the other hand, if you found out that none of them have atmospheres, that would be pretty sad, but also pretty interesting. It would mean that our planetary system is actually really, really special.”
Most astronomers agree that detecting atmospheres is crucial in the quest for habitable worlds. NASA has playfully called Earth’s own atmosphere its “security blanket”: Without it, the type of life flourishing here wouldn’t exist. This cocoon holds oxygen in the air and filters out harmful ultraviolet radiation from the sun, all while keeping our world warm. Furthermore, it creates pressure that allows liquid water to pool on the surface.
Scientists have found signs of atmospheres surrounding many of the 5,700 exoplanets discovered so far, but all of them have been around gas giant planets, like Jupiter, with air mostly made of hydrogen. The hunt for a more terrestrial world swaddled in a protective atmosphere has so far eluded astronomers, though Webb has recently helped scientists find some reasonable bets, such as 55 Cancri e, GJ 486 b, and LHS 1140 b.
Jennifer Lotz, who directs Webb and Hubble’s operations at the Space Telescope Science Institute in Baltimore, recently decided to initiate the rocky exoplanet survey using the director’s discretionary time, in the same way revolutionary science campaigns like the Hubble deep field images came to fruition. In addition to Webb’s work, the new survey will include about 250 orbits of ultraviolet observations by the Hubble telescope to help characterize the red dwarf stars’ activity.
Hubble will play a pivotal role in revealing whether one of these terrestrial worlds could even hold onto an atmosphere so close to a star that’s constantly bombarding it with radiation.
“It’s a beautiful message to put out there,” Espinoza said. “People think Webb is the successor to Hubble, but that’s not really true. They complement each other. It’s kind of the perfect dream team to do this job.”
“It’s kind of the perfect dream team to do this job.”
Secondary eclipse technique
Since Webb opened for business, researchers have frequently used a technique called transmission spectroscopy to study exoplanets. When these worlds cross in front of their host star, starlight gets filtered through their atmospheres. Molecules within the atmosphere absorb certain light wavelengths, or colors, so by splitting the light into its basic parts — like a rainbow — astronomers can detect what light segments are missing to discern the molecular makeup of an atmosphere.
But that method has had its drawbacks. If the starlight were completely uniform, that would be one thing, but red dwarf stars, also known as M dwarfs, can get stellar spots just like the sun, causing variability in the signals. This problem, called stellar contamination, has recently led Webb scientists to embrace another technique, known as secondary eclipse observations.
With secondary eclipses, it’s a game of hide-and-seek. Scientists measure the signals of the red dwarf and planet when the planet is at the star’s side. Then, when the planet’s orbit takes the world behind the star, scientists collect the star’s light signal alone. By subtracting the star from the total, the researchers can then isolate the light coming from just the planet. Teams will use a particular wavelength filter that can detect carbon dioxide, thought to be a likely atmospheric ingredient.
Scientists will also take thermal measurements to get an early sense of whether an atmosphere could be present. If the temperature is lower than expected, it’s a strong indication that a thick atmosphere is distributing energy from the planet’s dayside — the hemisphere facing the star — to the nightside.
Webb’s Survey of Rocky Worlds
The new campaign will allow scientists to survey 10 to 20 rocky worlds, with an emphasis on planets between 200 and 450 Kelvin. For comparison, Earth is 288 Kelvin, or an average 59 degrees Fahrenheit. Most of the targets will be less than twice the size of Earth. The Space Telescope Science Institute will put out a call for advisers to help determine the target list.
For Kevin Stevenson, an astrophysicist at Johns Hopkins Applied Physics Laboratory, the wishlist would include GJ 486 b, a world he has previously studied, and LTT 1445 A b, one of the nearest strong cases for having air. He believes the Webb campaign will advance humans’ understanding of rocky worlds by a decade.
“This survey program will be the next major step towards answering the question, ‘Are we alone?'” Stevenson said in an email. “Without an atmosphere, it’s hard to see how life, at least as we know it, can survive on a planet.”
“This survey program will be the next major step towards answering the question, ‘Are we alone?'”
If most M-dwarf-orbiting planets turn out not to have an atmosphere, then a future instrument, the Habitable Worlds Observatory, becomes all the more important in the search for life beyond this solar system, he said. The Hubble-esque telescope is expected to focus on terrestrial worlds orbiting sun-like stars.
But a breakthrough could have a huge impact on how Webb and other flagship observatories are used going forward, said Sarah Moran, an exoplanet scientist at the University of Arizona’s Lunar and Planetary Laboratory.
“If the program is able to conclusively determine that one of these rocky worlds around an M dwarf star has an atmosphere, it could really set the stage not only for the rest of JWST’s science operating lifetime,” she said in an email, “but also maybe that of the next great observatory.”
