For planet-hunting astronomers seeking twins or even cousins of Earth around other stars, the universe has just become much less lonely.
To qualify as close planetary kin, another world must be rocky and reside in the not-too-hot, not-too-cold “habitable zone” of its star, bathed in approximately as much starlight as Earth. There—if it possesses an atmosphere neither crushingly thick nor vanishingly thin—such a world could harbor a temperate climate where life-giving liquid water might pool in lakes, seas and oceans. Statistics from ongoing planet surveys suggest billions of worlds in our galaxy could meet these meager criteria, but so far less than a dozen candidates have been found that merit spine-tingling speculations about mirror Earths.
At least, that was the case until today. Writing in Nature, an international group of researchers details the discovery of seven worlds comparable with our own, orbiting a star 40 light-years away in the constellation Aquarius called TRAPPIST-1. Three of the planets orbit in TRAPPIST-1’s habitable zone and the other four could also conceivably sustain liquid water and life under certain atmospheric conditions. All appear to be roughly the same size, mass and composition as Earth.
For now the planets of TRAPPIST-1 are known only by their catalogue notations—TRAPPIST-1 b, c, d, e, f, g and h, labeled in order of their distance from their star. Soon that may change—the worlds will cry out for names as astronomers revel in their study and come to know them. TRAPPIST-1 is so cosmically close to us, so rich with promising worlds, that it is destined to be a touchstone for all future searches for habitable planets. And within a decade, some optimists say, studies of TRAPPIST-1 could provide compelling evidence for the existence of life beyond our solar system.
“This is the first time so many Earth-sized planets have been found in the habitable zone of a star,” says astronomer Michaël Gillon of the University of Liège in Belgium, leader of the TRAPPIST—for TRAnsiting Planets and Planetesimals Small Telescope—survey from which the star draws its name. (The survey itself is named for Belgium’s distinct variety of monk-brewed beer.) “For the first time, we won’t have only four terrestrial planets that we can study in detail—Mercury, Venus, Earth and Mars. We now have seven more… We could be on the edge of answering the most fundamental question ever from a philosophical point of view, of how frequent life is in the universe.”
The first hints of the momentous discovery arrived in September 2015, when the TRAPPIST team detected three planets in the system but also spied tantalizing hints that more lurked there unseen. To find them, the team enlisted the help of many other observatories, culminating in aid from the Spitzer Space Telescope, which stared at the star for a full 20 days in September and October 2016 to gather more data. Spitzer’s observations ultimately revealed four additional planets on October 27, 2016—a breakthrough the team celebrated with a round of Trappist beers.
Shrunken Stars, Shadowy Worlds
The star TRAPPIST-1 is scarcely bigger than Jupiter, but 80 times as heavy, and somewhere between 500 million to a few billion years old. It is what astronomers call an ultracool M dwarf—a star that is the smallest, dimmest and coldest any star can be. Our sun, for comparison, is a G dwarf—a stellar type that is 12.5 times bulkier, 2,000 times brighter and roughly 20 times rarer than M dwarfs; the universe, it turns out, prefers to make small stars rather than large ones. The star’s innermost world, TRAPPIST-1 b, completes an orbit in just 1.5 days whereas the outermost TRAPPIST-1 h resides in perhaps a 20-day orbit, five times closer to the star than Mercury’s distance from our sun. All this means TRAPPIST-1’s temperate planets, huddled close around the star like wanderers around a campfire, may be the norm throughout the cosmos, with planetary systems like our own as the outlier.
All seven worlds were detected because they transit, meaning they cross the face of their star as seen from Earth, casting shadows that astronomers measure as minuscule dips in the star’s light. The depth of the corresponding dip allows astronomers to estimate a transiting planet’s size whereas the dip’s recurrence over time provides a way to pin down a planet’s orbital period and distance from its star. For the TRAPPIST-1 system, because all its planets are so close together they also perturb one another in a resonant interplay of forces that creates measurable shifts in the timing of each world’s transit, causing each transit to arrive sooner or later than it otherwise would if unperturbed. “By measuring this change, we can determine the mass of the planets,” says Julien de Wit, a postdoctoral associate and TRAPPIST team member at Massachusetts Institute of Technology. “By knowing precisely the size and mass of the planets, we can determine their bulk density, and geophysicists can then help us better understand their interiors.” Based on these measurements, the TRAPPIST team estimates all of the planets possess a density similar to Earth, and could thus be rocky worlds or, alternatively, worlds with small, dense cores of metal and rock surrounded by thicker, lighter layers of water, ice and gas.
According to TRAPPIST collaborator Amaury Triaud of the University of Cambridge, from the surface of one of the middle worlds TRAPPIST-1 would be a salmon-colored orb appearing 10 times larger than the sun in Earth’s sky but no brighter than our star appears at dusk. You would feel rather than see most of its light, which peaks in warm, invisible rays of infrared. Every now and then a neighboring planet would drift by overhead, the nearest ones appearing twice the size of Earth’s full moon in the sky. But the star itself would appear immobile, because all seven of TRAPPIST-1’s planets are in such close-in orbits they are tidally locked, spinning once per orbit so that they eternally turn the same hemisphere to face the star, leaving the other in constant darkness. Astronomers used to believe tidal locking would be a death knell for life due to air freezing out on a planet’s dark side, but now suspect even a thin, Mars-like atmosphere could sustain atmospheric currents to transport heat between the hemispheres. If they have atmospheres at all, whatever the weather may be on TRAPPIST-1’s worlds it is probably always windy.
The “Ultracool” Opportunity
The cosmic prevalence of M dwarfs and the compact, shrunken architecture of their accompanying worlds combined to make them ideal targets for astronomers seeking transiting planets. Because transits only occur when a planet’s orbit is aligned essentially edge-on with our line of sight, they are quite rare, and astronomers must monitor hundreds of stars to find even a few transiting worlds. But the closer a planet is to its star, the greater its chance of transiting, and M-dwarfs are both the most numerous stars as well as those most likely to harbor close-in planets. What’s more, planets transiting across smaller stars can be easier to see because they block a greater proportion of the star’s light—and no ordinary stars are smaller than ultracool M dwarfs. Detecting such transits is surprisingly cheap—TRAPPIST consists of just two computer-controlled 60-centimeter telescope that target nearby ultracool M dwarfs.
In fact, a planet transiting a nearby ultracool M dwarf will block such a relatively large fraction of the star that an otherwise-elusive phenomenon can be readily seen: a ring of light surrounding a planet’s silhouette, created by starlight blasting through and around its upper atmosphere. Using NASA’s orbiting Hubble Space Telescope or its James Webb Space Telescope launching next year, astronomers could measure the fluctuating colors of such rings to learn what sorts of molecules the light hits as it passed through, effectively sniffing the air of transiting planets across the light-years. Astronomers consider this a shortcut for finding life on exoplanets, compared with parallel efforts to directly image Earth-size planets in habitable orbits around bright, nearby sunlike stars—a challenge so great its solution likely lies decades in the future. Gillon had all this in mind when he designed the TRAPPIST survey as well as its soon-to-debut successor, an array of four ground-based one-meter telescopes called SPECULOOS (named, fittingly, after a type of cookie popular in Belgium).
“This has been the whole purpose of our experiments, to focus on ultracool M dwarfs in the solar neighborhood to find and study the atmospheres of Earth-size transiting planets,” Gillon says. “These are the only targets for which such measurements are possible with current technology, with Hubble and Webb.”
Looking for the Breath—or Death—of Life
Already, Gillon and his team have used Hubble to look for and rule out the presence of very thick hydrogen-dominated atmospheres around at least two of the planets, the innermost TRAPPIST-1 b and its adjacent neighbor TRAPPIST-1 c. The team is also using Hubble to seek out signs of water vapor on the planets. If astronomers used Webb to watch more intently, capturing 20 or 30 transits per planet in the TRAPPIST-1 system, Gillon says, the telescope would be able to tease out the presence (or absence) of several atmospheric gases that, here on Earth, are vital diagnostics of our planet’s habitability and biosphere.
Like Hubble, Webb could see water vapor, but far more definitively—enough to perhaps surmise the existence of global oceans. Moreover, it could see carbon dioxide and estimate each planet’s world-warming greenhouse effect, and thus its average temperature. And it would look closely for ozone—a by-product of oxygen produced by photosynthetic plants—as well as methane, an organic compound emitted by anaerobic bacteria. Methane and oxygen are unstable when mixed together—their mutual existence in a planet’s air would suggest they are being continually replenished by some unseen source. “If you have a combination of ozone and methane in the presence of carbon dioxide and water, you have only one obvious explanation,” Gillon says. “It is life.”
Few astronomers believe, however, that such a slam-dunk result would arrive easily—if at all. More likely, many say, the data gathered first by Hubble and later by Webb would paint a far more incomplete and muddled picture that could only be resolved by subsequent generations of observatories. For instance, TRAPPIST-1’s status as an ultracool M dwarf could complicate the search for oxygen on its planets, says David Charbonneau, a pioneering planet hunter at Harvard University who runs another transit survey, MEarth, targeting larger, brighter M dwarfs. “Optical wavelengths are the best place to look for signs of oxygen and its abundance in a planet’s atmosphere, but TRAPPIST-1 is very faint in optical light, and you need those stellar photons to go through the atmosphere of the planet,” he says. Instead, Webb would look for signs of ozone on TRAPPIST-1’s planets, which appear strongest in infrared light where the star is brighter—but according to Charbonneau, such measurements alone cannot easily distinguish oxygen’s abundance to help constrain its origins.
Knowing the amount of oxygen in a planet’s atmosphere could be particularly crucial for learning the true nature of any M dwarf world, because theorists have found a number of “false positives” for producing the gas. Some of these abiotic pathways would not only produce an excess of oxygen, but would also render a planet totally lifeless. M dwarf stars slowly burn their nuclear fuel, allowing them to shine and support habitable worlds for hundreds of billions—even trillions—of years. But this longevity comes at a high price: For the first billion or two years of their existence, M dwarfs can shine even brighter than the sun as they slowly develop into full-grown stars, roasting any planets in the regions that will eventually become their habitable zones. Oceans on such planets could boil to steam, and intense ultraviolet light could break apart the water molecules in the upper atmosphere, flinging the lighter hydrogen into space and leaving a crushing high-pressure atmosphere of almost pure oxygen behind. Further complications arise from intense flares and winds from the adolescent star, which could entirely erode the atmospheres of nearby planets unprotected by geomagnetic fields—or instead reduce a thick, steamy atmosphere to a thinner, more habitable state.
TRAPPIST-1 as Test Bed
According to Gillon and his colleagues, TRAPPIST-1’s planets could have avoided the worst of these effects if they formed farther out from the star and gradually drifted in to their present positions—a well-studied phenomenon called migration thought to be nearly universal occurrence in the disks of gas and dust in which planets form like embryos in a womb. But Rory Barnes, a planet theorist at the University of Washington, notes planet migration typically occurs in the first 10 million years of a star’s life, far too early to avoid the billion-year baking in what would later become an M dwarf’s habitable zone. Even so, Barnes says, if one of the innermost planets of TRAPPIST-1 had its atmosphere baked away, it could be replenished by volcanism produced by the star’s gravitational tugging on the world’s innards, which would heat the interior. Or that same “tidal heating” could instead cause the planet to turn itself inside out, becoming an uninhabitable mass of churning magma.
“Realistically, the most exciting thing about observing this system with Webb may be that it will give us a handle on how these tidal effects influence atmospheres, particularly for the closer-in planets where the effects can be enormous,” Barnes says. “You could imagine seeing that the atmospheres of TRAPPIST-1 b or TRAPPIST-1 c are not Earth-like at all, just completely apocalyptic and dominated by whatever the planets spew out of their guts to paint the surface and the clouds.”
Shawn Domagal-Goldman, a planetary scientist at NASA Goddard Space Flight Center, says the focus on searching for life shouldn’t distract from the amazing broader potentials for the TRAPPIST-1 system. “We’ve been talking about these kinds of planets around these sorts of stars for years—whether they have oceans, what their “bio-signatures” could look like and how they might form false positives for life,” he says. “What’s so cool about this system is it provides us with an almost ideal scenario for testing all of these ideas in the near term.”
Gillon agrees. “We will compare these planets not only to Earth and to our solar system’s other terrestrial planets, but also to themselves. They must have formed from the same disk but at different distances from the star, so we could study their differing chemical history and atmospheres to probe their history and formation and evolution…. I don’t know if we will detect oxygen and methane and so on, but I am sure we will extend our detailed theory of how planets formed in our solar system to places far beyond.”
To Sara Seager, a planetary theorist at MIT, this newfound system may best be seen as a crucible in which a new, deeper understanding of far-distant worlds and their prospects for life may be forged. “TRAPPIST-1 is where we will test our hopes and desires and fears about planets orbiting these very cold, very low-mass M dwarf stars,” she says. “The hope is that we will find water vapor, and infer there is a liquid water ocean upon a rocky world. The desire is that we will find compelling signs of life. The fear is that we’ll find oxygen, and not know if it’s from life or not. That’s a great fear to have—to do that we’d still be pretty happy. A more worrisome fear is that none of these planets can be habitable at all—that we will do our very best to study this system only to learn that ultracool stars are problematic. Part of the thrill of these discoveries is the potential that they hold—not necessarily what is there right now. M dwarfs are probably not the end—they are just the beginning.”