The Earth Next Door

by Lee Billings

Astronomers find an exoplanet that could be habitable—and it’s as close to us as it could possibly be

It was just over 20 years ago—a blink of a cosmic eye—that astronomers found the first planets orbiting stars other than our sun. All these new worlds were gas-shrouded giants like Jupiter or Saturn and utterly inhospitable to life as we know it. But for years each discovery was dutifully reported as front-page news, while scientists and the public alike dreamed of a day when we would find a habitable world. An Earth-like place with plentiful surface water, neither frozen nor vaporized but in the liquid state so essential to life. Back then the safe bet was to guess that the discovery of such a planet would only come after many decades, and that when a promising new world’s misty shores materialized on the other side of our telescopes, it would prove too faraway and faint to study in any detail.

Evidently the safe bet was wrong. On Wednesday astronomers made the kind of announcement that can only occur once in human history: the discovery of the nearest potentially habitable world beyond our solar system. This world may be rocky like ours and whirls in a temperate orbit around the sun’s closest stellar neighbor, the red dwarf star Proxima Centauri just over four light-years away. Their findings are reported in a study in Nature.

Although technically still considered a “candidate” planet awaiting verification, most astronomers consulted for this story believe the world to be there. Scarcely more than the planet’s orbital period and approximate mass are known, but that is enough to send shivers down spines. Proxima Centauri shines with only about a thousandth of our sun’s luminosity, meaning any life-friendly planets would have to huddle close. The newfound world, christened “Proxima b” by scientists, resides in an 11.2-day orbit where water—and thus the kind of life we understand—could conceivably exist. And it is likely to be little more than one third heavier than Earth, suggesting it offers a solid surface on which seas and oceans could pool. In a feat of discovery that could reshape the history of science and human dreams of interstellar futures, our species has uncovered a potentially habitable planet right next door.

“Succeeding in the search for the nearest terrestrial planet beyond the solar system has been an experience of a lifetime, and has drawn on the dedication and passion of a number of international researchers,” says the study’s lead author Guillem Anglada-Escude, an astronomer at Queen Mary University of London who spearheaded the observations. “We hope these findings inspire future generations to look beyond the stars. The search for life on Proxima b comes next.”

For some, Proxima b is a fitting capstone to the astronomical revolution that began when the first exoplanets were found. “For more than 20 years the history of exoplanets has been defined by studying stars tens to hundreds of light-years away, when the Holy Grail—a small, rocky, potentially habitable planet—was just waiting to be discovered around our closest neighbor,” says astronomer Debra Fischer, a veteran planet hunter at Yale University who has led independent surveys of the Alpha Centauri system. “When we launch our first robotic explorers to stars beyond the solar system, we know where we should send them!”

Caleb Scharf, director of astrobiology research at Columbia University, says the the new planet represents “a tremendously important psychological moment for the field, as well as for our species. Discovering who lives in the house next door can change perspectives and priorities—and that's what Proxima b will do.”

Although it is barely more than four light-years away, Proxima Centauri is too faint to be seen with the naked eye. It drifts at the outskirts of the twin sunlike stars Alpha Centauri A and B, forming a stellar trio that appears as a single gleaming point in the southern constellation of Centaurus. The tiny star is fated to slowly slip farther away from us on the Milky Way’s celestial currents, but will remain the closest one bearing a planet for perhaps the next 40,000 years.

“For the first time, we have an exoplanet within our reach that could be a host to biological organisms,” says study co-author Mikko Tuomi, an astronomer at the University of Hertfordshire. “And that makes Proxima b not only one of the most fascinating discoveries astronomers have made but also one of the most important that can be made.”

Even so, it is a discovery that almost didn’t happen. “People seem to think we just found the planet. But no, we have believed it was there for years,” Anglada-Escude says. “We just had to build an argument to convince others it exists.”

The Pale Red Dot

Tuomi was the first to see hints of the world in 2013 while planet-hunting with Anglada-Escudé. As relatively junior researchers, the two astronomers had struggled to secure prized telescope time to search for planets and instead were seeking overlooked worlds by carefully reanalyzing public data released by other teams. Proxima b’s faint signal first appeared in several years’ worth of combined observations from two planet-hunting spectrographs, HARPS and UVES, which are operated on telescopes in Chile by the European Southern Observatory (ESO). Both HARPS and UVES had monitored Proxima Centauri for years, watching for the star being tugged to and fro by any unseen pirouetting planets, but the teams running them had claimed no detections. Orbiting worlds impose a distinctive periodic wobble on their stars, sometimes so gently that they sway their suns slower than the pace of a crawling baby. The 11-day wobble Tuomi thought he saw in the combined HARPS and UVES Proxima Centauri data was slightly stronger—1.4 meters per second, an adult’s average walking speed. Along with several colleagues, Tuomi and Anglada-Escudé quickly wrote and submitted a paper reporting the potential planetary signals.

But many things can move a star. For example, heaving waves and vortices of magnetized plasma flowing on its surface can mimic or mask the wobble caused by a small planet. And even the best planet-finding spectrographs are subject to calibration errors that can cause further confusion. In 2012 the HARPS team had announced a wobble possibly caused by a small rocky world around Alpha Centauri B—but that planet ultimately proved illusory, a phantom produced by starspots, stellar rotation and questionable statistical analysis. It was a cautionary tale: If the HARPS team had been fooled by Alpha Centauri B, one of the most quiescent stars in the sky, hopes seemed slim for finding planets around neighboring Proxima, which continually erupts with “superflares” that can easily scuttle careful observations.

Proxima’s reputation as a cantankerous flare star had kept it from the top of the HARPS team’s priorities—and had also cast doubt on Tuomi’s and Anglada-Escude’s claims. Their paper was rejected; reviewers found their evidence unconvincing. Anglada-Escude responded by spending the next two years developing the “Pale Red Dot” observing campaign, named for the famed “Pale Blue Dot” Voyager 1 image of Earth from deep space popularized by astronomer Carl Sagan. It would be an audacious departure from most previous planet hunts, which tended to favor skimming large numbers of stars for easier, more obvious worlds rather than hammering away at any single target.

Anglada-Escude and the rest of the Pale Red Dot team persuaded ESO to give them a 20-minute chunk of Proxima-focused time on HARPS for 60 consecutive nights in the spring of this year. The team bolstered their HARPS work with concurrent observations from two other telescopes to monitor for flares and starspots that could masquerade as planets. As the observing campaign began, Anglada-Escude also worked with study co-author and Carnegie Institution for Science astronomer Paul Butler—a grizzled planet hunter who helped found the field 20 years ago—to successfully extract the 11-day wobble from the UVES data alone. Pairing the old data with the new, the signal soared above the sea of stellar noise, cresting into unquestionable statistical significance. Within the first 10 days of the months-long observing run, the Pale Red Dot team knew they had found Proxima b. “I’m totally convinced,” Butler says. “Nature is malicious and tries to hide things, but you don’t accidentally find two perfectly matching signals from two separate instruments.”

Members of the HARPS team, whose data proved so crucial to the discovery, are convinced as well. “The signal is significant and is due to a planet,” says Christophe Lovis, an astronomer at the Observatory of Geneva who developed the HARPS team’s data-analysis software. “It is the recent, high-cadence data sets that make the difference…. [The Pale Red Dot team] simply tried their luck, and it worked.”

Goldilocks Dreams and Nightmares

Perhaps the most surprising thing about Proxima b is that its existence is not really surprising at all.

In recent years, thanks in large part to the work of the HARPS team as well as results from NASA’s planet-hunting Kepler space telescope, astronomers have converged on a statistical estimate for the number of potentially life-friendly worlds in our galaxy. Somewhere between 15 to 30 percent of the Milky Way’s stars, it seems, should harbor “Goldilocks” worlds—planets neither too big to be smothered by thick atmospheres nor too small to lose their precious air to space, in not-too-hot, not-too-cold orbits where liquid water could exist on their rocky surfaces.

Similar to the way granules of rock outnumber massive boulders on a sandy beach, wee stars like Proxima Centauri are far more abundant than larger ones like our sun. Small stars are more efficient with their nuclear fuel as well, shining hundreds of billions—even trillions—of years longer than sunlike stars. So we should expect most Goldilocks worlds to exist around red dwarfs like Proxima Centauri. But should we expect them to actually be habitable? Of this, not even the experts are sure.

Despite resembling Earth in mass and exposure to starlight, Proxima b “is not an Earth twin,” says Franck Selsis, an exoplanet atmospheres expert at the University of Bordeaux. The same 11.2-day orbit that places Proxima b in its star’s habitable zone also subjects it to a rogue’s gallery of deleterious effects that could eradicate a biosphere—or prevent one from forming in the first place.

For Rory Barnes, an astrobiologist at the University of Washington whose gloomy outlook on habitability has gained him a reputation as a “destroyer of worlds,” all those potential obstacles suggest Proxima b may not be the life-friendly planet we’re looking for. “In general, any planet should be considered unlikely to support life,” Barnes argues. “This one has different requirements than our own, and probably has more hurdles to overcome than Earth did.”

Chief among them, Barnes says, is the fact that red dwarfs tend to have violent, unstable youths—rather like a human being who lives for millennia but consequently suffers centuries of turbulent adolescence. Because they are so small, such stars are thought to form very slowly, spending many millions of years accreting mass and shining far brighter than they do for the rest of their lives. If Proxima b formed where we see it today, Barnes says, to be habitable “it would have to somehow avoid being baked to a Venus-like runaway greenhouse state for hundreds of millions of years.” Barnes, however, also offers one possible remedy using another bit of creative Goldilocks reasoning: a not-too-thick, not-too-thin blanket of hydrogen that could act as sunscreen for the young planet, gradually evaporating under the harsh starlight and only dissipating after Proxima Centauri settled into adulthood.

Extreme tides produced by Proxima Centauri’s pull on its diminutive companion could be another killer. Those tidal effects could cause Proxima b to rotate just once per orbit, effectively “locking” one side of the world in darkness while the other faces the star—although many researchers now believe most conceivable atmospheres would circulate heat between the two sides to keep hope for a biosphere alive. More troubling is a phenomenon called “tidal heating,” friction produced inside a planet by flexure from its star’s tidal tug. If Proxima b’s orbit is (or ever was) significantly elongated, swooping close to the star at one end and far out on the other, the resulting tidal heating could boil off any ocean all on its own without any help from starlight.

Jim Davenport, a postdoctoral fellow at Western Washington University, believes Proxima b’s biggest obstacle to Earth-like conditions is likely to be Proxima Centauri’s continual flares, which can be ten times more energetic than any ever observed on our sun. Harsh x-rays and ultraviolet radiation from the flares could strip Proxima b of its atmosphere, leaving it barren and airless. But hope remains in this case, too—a protective magnetic field much like Earth’s or a thick and steamy atmosphere could conceivably fend off the worst of the flares. “To play on Hamlet, there are more things in heaven and exoplanets than are dreamt of in our textbooks,” Davenport says. “But right now we just don’t know.”

Sooner or later, that will change. Already the discovery is fueling new interest in searches for radio- or laser-based messages beamed toward Earth from any technologically talkative aliens on the planet as well as futuristic proposals to send robotic probes voyaging to our nearest star system—even if these efforts are unlikely to deliver results in the near future. In the shorter term, as news of Proxima b reverberates through the scientific community, astronomers are preparing a full-court press to observe and study it.

Meet the Neighbors

Due to Proxima b’s short orbital period, Anglada-Escude says, planet-hunting spectrographs besides HARPS and UVES could conceivably confirm the planet’s existence in a matter of weeks. More extensive studies of Proxima Centauri’s wobbles could then better constrain the planet’s mass and orbit, placing tighter limits on the possibilities for life there and potentially revealing more planets.

But the greatest hope among the planet hunters is that Proxima b transits, by chance passing across the face of its star as seen from Earth so as to cast a minuscule but measurable shadow toward our waiting instruments. David Kipping, an astronomer at Columbia University, is now leading a team searching for signs of Proxima b’s possible transit in recent observations of Proxima Centauri taken by the Canadian Space Agency’s MOST space telescope. “We are optimistic that it transits,” Kipping says. “If it does, it ticks that last box and becomes almost as optimal as possible, and that seems so perfect it gives me pause for concern.” Accounting for the star’s fluctuating brightness due to its flares, Kipping says, will delay a conclusive result until sometime in September. If convincing signs of a transit appear in the MOST data, astronomers will likely seek airtight confirmation using bigger hardware—NASA’s infrared Spitzer space telescope.

A transit would be a treasure trove for astronomers. The planetary silhouette would allow them to directly measure Proxima b’s size, precisely pin down its mass and even calculate its density and estimated composition. Moreover, starlight limning the edges of a transiting Proxima b could allow astronomers to determine the presence and bulk composition of the world’s atmosphere, if it has one. Such observations would likely require the observational heft of NASA’s 6.5-meter James Webb Space Telescope, launching in October of 2018.

Even if Proxima b proves not to transit, it still offers a unique opportunity for a coming generation of extremely large ground-based telescopes presently under construction around the world. Set to debut in the 2020s and armed with light-gathering mirrors stretching 30 meters or more across, such telescopes could conceivably obtain direct images—actual pictures—of Proxima b, unveiling otherwise unavailable information about its composition and history. The biggest of these next-generation behemoths will be ESO’s European Extremely Large Telescope (E–ELT), which could begin operations in Chile as early as 2024. “Proxima b might the only habitable zone planet [or at least one of the very rare ones] that could be imaged with the E–ELT,” Selsis says. “Even without talking about life, this could represent a revolution in planetary science.” Observations of the planet in transit—or direct images from a garguantuan ground-based telescope—could reveal whether the planet has a thick, watery atmosphere. If it does, Selsis says, “we would then know that red dwarf planets can keep their water despite stellar activity and be habitable. That would be fantastic.”

Looking later in the 20s0s and past James Webb, NASA’s next large space observatory will be WFIRST, a souped-up version of the agency’s wildly successful Hubble telescope with a wider, infrared-optimized field of view. Present plans call for WFIRST to fly with a high-performance coronagraph, an instrument capable of blocking a star’s light so that faint accompanying planets can be directly imaged. Alas, WFIRST’s coronagraph is optimized for stars like our sun, not red dwarfs like Proxima Centauri. According to Jeremy Kasdin, the Princeton University astronomer who leads development of the project’s coronagraph, WFIRST “will not be able to see Proxima b due to its closeness to its host star and the planet’s low intensity at the telescope’s wavelengths.”

For now, this means the goal of thoroughly probing Proxima b and other nearby worlds for convincing signs of life—so-called “biosignatures”—may remain out of reach for decades. “The longer-term goal of directly imaging these planets is to see if their atmospheres are conducive to or even influenced by a biosphere, to look for gases like oxygen that are very far from thermodynamic equilibrium, gases that on Earth are produced by living things,” says Butler, the planet-hunting veteran who has been pursuing this dream for most of his life. “People ask me, ‘how will you ever prove a planet has life?’ If you take a spectrum of a potentially habitable planet and see water and some gas out of equilibrium, you flip that question from ‘prove there is life’ to ‘prove there isn’t.’ My great hope is that this will happen in a generation.”

As generations go, many younger astronomers are less patient. Instead of waiting for another large space telescope even further in the future after WFIRST, some now say Proxima b has changed the rules. Just as its discovery required a dedicated, intensely focused observation campaign, seeking signs of life there might best be served by pushing for single-purpose space telescopes that are smaller, cheaper and faster than NASA’s lumbering multipurpose flagship missions. Already, some maverick NASA researchers have suggested such an approach for Proxima Centauri’s neighboring, more sunlike stars, Alpha Centauri A and B. “We may be in a new race now,” Anglada-Escude says. “Building a massive observatory to take pictures of planets around a hundred stars is very expensive. But people now know exactly what to look for, so you can design your telescope and instruments to look only at this planet, and optimize them for that single task.”

Sara Seager, an astrophysicist at Massachusetts Institute of Technology who helped plan WFIRST and other next-generation missions, sees the result as a profound new opportunity for exoplanetary science. “This gives permission for those of us in the field to put all our eggs in one basket, rather than throwing darts randomly at the sky,” Seager says. “Before Proxima b you’d scarcely imagine sending up a space telescope for one star, but now it’s imaginable. There are downsides: If we all pick the wrong thing, what happens then? Might there be less work for people to do, because we will be focusing on fewer objects? We could end up like the particle physicists, with thousands of authors for one paper. But this is the path to finding the most promising planets around the very nearest stars.” -Scientific America