The Search for Earth Look-Alikes Is Getting Serious
One promising planet turned out not to have an atmosphere. But there are six more where it came from.
Updated at 5:30 p.m. ET on March 29, 2023.
Several years ago, astronomers pointed a telescope at another star and discovered something remarkable: seven planets, each one about the same size as Earth. The planets were quite close to their small star—all seven of their orbits would fit inside Mercury’s. And yet, because this star is smaller, cooler, and dimmer than our own, at least three of those rocky worlds are in the habitable zone, at the right temperature for liquid, flowing water. Earthlike size and sunniness don’t guarantee that you’ll find ET, but if you were looking for signs of alien life beyond this solar system, this corner of the universe would be a promising place to start.
The system, which orbits a star known as TRAPPIST-1, is unusual; scientists had never found one like it before, nor have they since. We can’t see the exoplanets, which are named b, c, d, e, f, g, and h; from 40 light-years away, they were just tiny blips in telescope data. Artists at NASA have illustrated them, their imaginations guided by details of the worlds in our system, including Earth’s clouds and oceans, but the exoplanets have fundamentally remained a mystery. So when the James Webb Space Telescope, the newest and most powerful telescope out there, was launched, experts and space enthusiasts alike were anxious to point it toward this cosmic alphabet and get a real glimpse of the worlds within.
Now the first results are out: The Webb telescope has observed b, the innermost planet, and found … nothing. No signs of carbon dioxide, a key component of our atmosphere, and which Webb is designed to detect even from many light-years away. And good evidence that there was no significant atmosphere at all. “We’re surprised,” Tom Greene, an astrophysicist at NASA who led the team behind the new research, told me. “I was a little disappointed.”
The good news is that we still have six other planets to check out, and the worlds that are farther away from their star might be more likely to have a substantial atmosphere. That means we have six more chances to find an atmosphere around a rocky world, and perhaps even detect the presence of compounds associated with life as we know it. More observations would also give us a richer understanding of whether stars like the one in the TRAPPIST-1 system, known as red dwarfs, are promising candidates in the search for habitable planets in the cosmos. This has big implications: Red dwarfs far outnumber sunlike stars in the Milky Way, and they’re likely to have rocky planets too. If even one TRAPPIST-1 planet has the conditions that we know are needed for life, it would suggest that the galaxy could be teeming with habitable worlds—and Earth might not be so special.
Other astronomers I spoke with shared Greene’s disappointment at TRAPPIST-1b’s lack of an atmosphere, but some aren’t surprised at all. Since the existence of the system was announced to the public in 2017, scientists have developed countless models for the planets, and the predictions were split. “Some people thought that the planet would have no atmosphere at all, and some folks thought that it would have maybe a Venuslike atmosphere that was mostly made of carbon dioxide,” Jonathan Fortney, an astronomer at UC Santa Cruz who worked with Greene on the new research on b, told me.
Before Webb came along, the Hubble Space Telescope observed most of the TRAPPIST planets, including b, and found no evidence of light and puffy atmospheres made of hydrogen. This was just fine with astronomers, because such a Neptunelike atmosphere wouldn’t be conducive to the kind of life that arose here on Earth. Scientists wanted to detect heavier gases such as carbon dioxide, methane, and oxygen—a trio that, at least on Earth, indicates life respiring beneath the clouds—and for that, they needed the Webb telescope.
Greene and his team used Webb to assess b’s atmosphere in a new way: They measured heat in the form of infrared light radiating from the planet. A cooler result would suggest the presence of an atmosphere, circulating the star’s heat around the globe. A hotter one would mean a bare surface, absorbing the energy and then reflecting it back, like asphalt after a warm day. The Webb data revealed the latter case to be true; with a day-side temperature of about 450 degrees Fahrenheit, TRAPPIST-1b is “just about perfect for baking pizza,” as NASA put it, but it’s also an airless ball of rock.
The planet might have had an atmosphere many eons ago, but its star likely took it away, Megan Mansfield, an astronomer at the University of Arizona who also uses Webb to study exoplanets, told me. Red-dwarf stars are cool stars, technically speaking—they are far less luminous than the sun—but they love to flare, blasting radiation out into space. “Those kinds of things can strip the atmosphere off a planet,” Mansfield said, especially one orbiting this close. TRAPPIST-1b might still have a very tenuous atmosphere, too ephemeral for Webb to detect, like the wisp of gas that envelops Mercury—but that’s not the kind of Earthlike environment researchers are hoping to discover in that system.
So astronomers will move down the line of planets to c, d, e, f, g, and h. Greene said he was more optimistic about detecting atmospheres around TRAPPIST-1’s other planets—at least before the disappointing discovery on b. But it’s too early to lose hope. Perhaps conditions are more comfortable farther out, where “there’s more space for that intense radiation and flaring from the star to spread out,” Mansfield said.
The Webb telescope has already observed c, and the results should be out soon, Greene told me. If it also turns out to be an atmospheric dud, that might not be a reason for astronomers to worry. Same with d, even, because it orbits at the edge of the habitable zone. But e? Then they’ll be nervous. Planets e, f, and g stand the best chance of being Earthlike, with not only an atmosphere but also an ocean. “Every data point we get, just like the one we just got now, will help to refine those theories of what habitability means for planets in [red dwarf] systems,” Nikole Lewis, an astrophysicist at Cornell, told me. Poor, barefaced b might even help researchers determine whether the more promising planets have water: Lewis said that the lack of atmosphere means that the Webb telescope can study the surface of the planet, searching for the chemical signature of water molecules in the light it reflects. A strong-enough signal would give astronomers hope that the substance exists elsewhere in the TRAPPIST system under better conditions—hope that, perhaps, one of these worlds could be a home.
Not for us, of course. A trek to the TRAPPIST system remains the stuff of science fiction. For the time being, humanity is tied to our calm, bright star, and to the planets and moons around it. We’ll build our fancy telescopes and train them on other worlds in the galaxy, wondering whether they have silky clouds of their own, and something, or someone, gazing up at them from the ground.