Any future historian of 21st-century space science may well divide the subject into two eras: before the James Webb Space Telescope (JWST) and after. The telescope was built to transform our understanding of the cosmos by studying the first stars and galaxies, and within less than a year of operations, it has already delivered tantalizing and potentially revolutionary results from its observations of the early universe. Yet JWST’s work is poised to transform many other subfields of astronomy, none arguably more so than the study of exoplanets, worlds orbiting other stars. Astronomers now know of more than 5,000 exoplanets but know next to nothing about most of them—their composition, environmental conditions or even prospects for life. JWST is beginning to change that, thanks to its as-yet-unparalleled ability to directly observe these alien worlds, picking apart their light to discern finer details and occasionally even managing to snap an exoplanet’s picture against the overwhelming glare of its home star.
Such results remain a far cry from the astrobiological holy grail of finding and studying potentially Earth-like worlds but are enormously exciting nonetheless, given that JWST and its core science goals were conceived before exoplanets were even known to exist. “The exoplanet community is just giddy at the moment,” says Mark Clampin, director of the Astrophysics Division at NASA Headquarters in Washington, D.C.
JWST’s first year of science is scheduled from July 2022 through June 2023. Of that period, called Cycle 1, about a quarter of the telescope’s time is being devoted to exoplanets across about 75 programs. One of the most exciting applications of JWST is studying exoplanet atmospheres. Gold-plated and about as wide as a full-grown African elephant, the telescope’s infrared-tuned primary mirror allows it to probe the atmospheres of exoplanets to a degree never before possible. “With the Hubble [Space Telescope], we’ve done a decade of detecting water, which we found abundantly, but not much else,” says Nikole Lewis of Cornell University. “That was the only thing you could measure.” JWST can see water, too—as well as a much wider array of molecules including carbon dioxide, sodium, and more. Some of the compounds JWST can detect, such as methane, are closely associated with metabolic processes in Earth’s biosphere, making them possible biosignatures that could help reveal life’s presence on other potentially habitable worlds beyond the solar system.
In August astronomers revealed they had used JWST to detect carbon dioxide on an exoplanet for the first time by watching for signs of the gas in the light of a gas giant planet’s host star filtering through the world’s atmosphere. Known as transmission spectroscopy, this technique is incredibly useful not only for studying giant planets but also for investigating smaller ones that might be more like our solar system’s retinue of rocky worlds. “We needed to start with ‘Okay, do they have air?’” Lewis says. “Once we understand that, we can develop a better strategy of looking for biosignature gases.”
At the 241st meeting of the American Astronomical Society (AAS) in Seattle earlier this month, astronomers announced another transmission spectroscopy result from JWST. This time the telescope studied an Earth-sized world called LHS 475 b, which orbits a red dwarf star 41 light-years away from Earth. In this case, JWST actually confirmed the planet’s existence, which previously had been hinted at by NASA’s Transiting Exoplanet Survey Satellite (TESS). “We confirmed it was a planet by observing it with JWST,” says Sarah Moran of the University of Arizona, a collaborator on the result.
JWST observed two orbits of the planet around its star, but an additional observation expected in May will be needed to better parse the contents of the planet’s atmosphere. So far, however, the team “can say a lot of things about what the atmosphere is not like,” Moran says. “We know it’s not hydrogen dominated like Jupiter or Saturn. We think it probably does not have an Earth-like atmosphere. But it could have a carbon dioxide atmosphere like Venus or Mars, or it could have no atmosphere at all like Mercury.” Those results could help inform the study of other rocky planets around red dwarfs, which account for some three quarters of all stars in the Milky Way. “We’re in the very first stages of trying to measure atmospheres for rocky planets and trying to figure out if a planet can be habitable,” Moran says.
In terms of studying rocky planets, JWST is largely limited to worlds that orbit red dwarfs, which are dim enough to avoid overloading the telescope’s exquisitely sensitive optics. Such stars are known to be prone to intense flaring that could blast away the atmospheres of worlds like LHS 475 b, which orbit perilously close to their host stars in comparison with the much wider star-planet separations among our solar system’s rocky worlds. “There’s the possibility that absolutely all of their atmospheres have been blown away by their stars,” Lewis says. One major red dwarf target of interest, the TRAPPIST-1 system nearly 40 light-years from Earth, contains seven Earth-sized worlds. Several of these are in the star’s habitable zone—the region around the star in which sufficient planet-warming starlight might allow liquid water to exist. Early observations of TRAPPIST-1 are still underway, including ones seeking out atmospheres. Those results could go a long way toward revealing whether red dwarf worlds can actually be habitable. “Hopefully we’ll know by the end of Cycle 1,” Lewis says.
JWST also sports an exciting add-on called a coronagraph, a device for blocking most of the light of stars so that fainter accompanying planets can be seen (this was crucial for JWST’s first-ever exoplanet image, which researchers unveiled last September). The starlight-suppressing power of the telescope’s coronagraph is insufficient for revealing any small, potentially habitable worlds, but recent work has shown the coronagraph should allow JWST to see worlds down to the size of Jupiter or Saturn that are orbiting red dwarf stars at or beyond five times the Earth-sun distance (five astronomical units, or AU). That’s roughly the position of Jupiter in our own solar system.
This analysis comes from Kellen Lawson of NASA’s Goddard Space Flight Center and his colleagues, who at the recent AAS meeting debuted stunning infrared views of a sprawling debris disk encircling a young star some 32 light-years from Earth. “In the past, direct imaging has been limited to 10 or so Jupiter masses,” Lawson says. “Here we’re sensitive to a [single] Jupiter mass.” That will allow JWST to look for rough analogues of Jupiter around other stars in a way not possible before. “Our hope is, with JWST, we can constrain the presence of planets in this regime,” Lawson says. Such directly imaged planets can be directly pinpointed in their orbits around their stars, giving a prime opportunity “to follow-up and get a ton of really incredible data.”
Astronomers are also excited about the exoplanet capabilities of another telescope, the European Space Agency’s (ESA’s) Gaia observatory. Launched to space in 2013 primarily to map the motions and positions of billions of stars in our galaxy, the telescope is also expected to find thousands of exoplanets. At the AAS meeting, Sasha Hinkley of the University of Exeter in England—who leads one of JWST’s early exoplanet imaging programs—announced that, using Gaia and the Very Large Telescope (VLT) in Chile, his team had seen an unusual planet some 130 light-years from Earth that appeared to be undergoing nuclear fusion. “It’s burning deuterium,” he says, referring to a hydrogen isotope that achieves starlight-powering nuclear fusion at lower temperatures than normal hydrogen. Further studies of the system, Hinkley says, could help astronomers draw less blurry lines among stars, planets and brown dwarfs—the latter being a loosely defined class of objects that fall between planets and stars in mass. The 130-light-year-distant planet, spotted thanks to Gaia witnessing a wobble in its host star’s motion caused by the unseen world’s gravitational pull, could be one of many upcoming exoplanets found by the telescope, some of which could also be interesting targets for JWST.
The quest for Earthlike worlds, however, seems set to define JWST’s exoplanet legacy despite being largely beyond the telescope’s reach. “That’s where all this work is headed,” Hinkley says, “and that’s why a majority of people are in this game.” In 2026 a new ESA mission called PLATO will launch, with finding such worlds as its primary goal. PLATO will stare at vast swathes of the sky to, for the first time, hunt in earnest for Earth-like worlds around sunlike stars within about 1,000 light-years of the solar system. A few tens of such planets are expected to be found during the telescope’s four-year primary mission, says Ana Heras of ESA, the mission’s project scientist. “We really don’t know what the occurrence rate is [for Earth-like planets],” she says. PLATO will go some way to telling us how many, if any, there are in our corner of the galaxy.
JWST will not be able to closely study such worlds. Nor will its successor, the Nancy Grace Roman Space Telescope, set to launch by 2027, be capable of doing so. But Roman will play a crucial role alongside its other scientific objectives: testing the advanced coronagraph technology that will be needed to produce images of potentially habitable Earth-like worlds around stars like our sun. That technology is meant to then be employed on JWST and Roman’s successor, the newly dubbed Habitable Worlds Observatory, which is set to launch no sooner than the late 2030s on a mission to produce the first-ever images of potentially habitable Earths. That telescope must be “about 100 times more stable” in space than JWST to achieve such a goal, says Bruce Macintosh, director of the University of California Observatories. “That’s not a negligible challenge.”
The road to this eventuality is a long one. “We’re at the beginning of a journey here,” Clampin says. But even leaving aside any talk of holy grails, JWST’s transformative early exoplanet results remain a thrill for scientists. The best, however, is still yet to come. “People need to be patient,” Lewis says. “The first cycle is all about picking the low-hanging fruit. We’re going to start going crazy in the next few cycles.”