On June 13 NASA’s official tally of exoplanets—planets beyond our solar system—shot past 4,000. The master list includes alien worlds around both tiny neutron stars and swollen, dying suns, planets in the crowded center of our galaxy the Milky Way, and planets floating alone in the depths of interstellar space. One location is conspicuously missing, however: Alpha Centauri, the star system next door to our own. Because of a confounding set of circumstances, this intriguing target has remained a cipher—at least, until now.
Technology is improving, and the stars are aligning, literally, to reveal potentially habitable worlds that may be hiding around Alpha Centauri. “The level of interest in finding them is definitely increasing,” says astrophysicist Ruslan Belikov of NASA’s Ames Research Center.
Over the past few weeks, Belikov’s team and another group led by Markus Kasper of the European Southern Observatory have been trying to glimpse Alpha Centauri’s putative worlds with the help of two different giant telescopes in Chile. Using new high-precision starlight-sifting instruments, researchers are gearing up for even more subtle searches. If those efforts come up empty, several groups are developing low-cost satellites to extend the planet hunt into space. And if NASA fails to fund those projects, there is another backup plan: two private efforts—the billionaire-backed Breakthrough Initiatives and the crowdfunded Project Blue—are gearing up to make the missions happen anyway.
“There’s a risk, because we don’t know if we will see anything. In my view, the risk is balanced by the reward of getting an image of Earth 2.0,” Belikov says.
The lure of Alpha Centauri is easy to understand: For the scientists yearning to find living worlds beyond our own, this system seems almost too good to be true. It contains not just one star resembling our sun but two of them, called Alpha Centauri A and B. Star A is the sun’s near twin in temperature, size, luminosity and composition; star B is just a tad smaller and cooler. They are both about 10 percent older than our solar system, leaving plenty of time to for any alien evolution to do its thing. Alpha Centauri is also the very closest system in the galaxy, lying just 4.37 light—years away and providing a uniquely intimate perspective on any possible planets there. As an added bonus, Alpha Centauri A and B have a third companion, a dim red dwarf named Proxima Centauri, which has an Earth-size world of its own.
Unfortunately, the things that make Alpha Centauri so appealing in concept also make it extremely challenging in practice. One of the most successful ways to find exoplanets is to look for a slight wobble in a star’s light. An orbiting planet will tug its parent star back and forth, making the starlight appear alternately a little bluer and a little redder than normal. For a solitary star, detecting those color variations is an exceedingly delicate process. In a binary system such as Alpha Centauri, it becomes much more so. The light from the two adjacent stars tends to blend together, obscuring the wobble signal. Things get trickier still if the two stars appear extremely close together in the sky—and by a stroke of bad luck, stars A and B reached the tightest point in their 80-year orbit in 2016.
Lily Zhao, a young planet hunter at Yale University, recently summarized the consequences of those problematic circumstances. Stitching together a full decade of radial-velocity measurements, Zhao and her colleagues can only say that Alpha Centauri A has no planets more than about 50 times the mass of Earth and that Alpha Centauri B has no planets of more than eight Earth masses. A plethora of smaller, Earth-size worlds could be zipping around either star, and we would have no idea. “That has pretty much marked the limit of the searches until now,” Zhao says.
Frustrated by such vagueness, a few brave astronomers have tried to squint through the muddled data to spy more precise answers. The attempts have not gone well. In 2012 a group led by Xavier Dumusque, then at the University of Geneva, announced the discovery of an Earth-mass planet around Alpha Centauri B to great fanfare. But three years later, an independent analysis showed no sign of the planet, suggesting it was just a data artifact. Dumusque conceded that “probably the planet is not there.”
Nevertheless, Zhao declares that “the prospects are great” for the next round of planet prospecting around Alpha Centauri. For one thing, the separation between stars A and B is increasing, and by 2020, it will be wide enough that powerful telescopes will be able to observe each star individually. More importantly, vastly improved new versions of instruments called spectrographs will allow far more sensitive searches for wobbles than those that were possible with the HARPS detector used by Dumusque and his colleagues.
Zhao is working on a high-resolution spectrograph called EXPRES (for Extreme Precision Spectrometer) on the Discovery Channel Telescope in Arizona. A similar device called ESPRESSO (for Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) has been installed on the Very Large Telescope (VLT) in Chile, which has a clear view of Alpha Centauri in the southern sky. These spectrographs are designed to pick up wobbles as slight as 10 centimeters per second. That sensitivity is 10 times better than their predecessors and right at the level needed to pick up the back-and-forth tug caused by an Earth-like planet orbiting around either of Alpha Centauri’s stars.
Both ESPRESSO and EXPRES are already monitoring stars across the sky. There are no plans yet for an Alpha Centauri–centric dedicated search, but if—when—new results emerge, Zhao promises, they will be trustworthy. “Everyone has been upping the ante on how sure you can be of a detection before you can publish it,” she says.
Lighthouses and Fireflies
Presuming that the wobble searches are successful in finding planets around Alpha Centauri, they will still leave a lot of unknowns about these alien worlds. Such studies will indicate how massive the planets are and how they orbit but will reveal little about their physical qualities. They will reveal nothing at all about what the planets look like. The only way to obtain that kind of information is to observe these bodies directly. For the truly ambitious explorers of Alpha Centauri, the goal is to capture a “pale blue dot” image: a blip of light containing the first portrait of what could be a truly Earth-like planet around another star.
No instrument yet created is sensitive and precise enough to do that. “Imaging an exoplanet nearby a bright star is like trying to see a firefly nearby a lighthouse a few hundred kilometers away. In the case of Alpha Centauri, we have two lighthouses, so the problem is even worse,” says Franck Marchis of the SETI Institute, who is the science operations lead for Project Blue. But slightly larger worlds are now within reach, and looking for them can already tell us a lot.
Odd as it may sound, the first order of business is figuring out what kinds of planets are not present around Alpha Centauri. Because of the proximity between stars A and B, stable planets can exist only rather close to each star, no more than about 2.5 times as far out as Earth’s distance from the sun. Any giant, Jupiter-size planets orbiting in those inner sanctuaries would have long ago destroyed any smaller, potentially Earthlike planets orbiting in the habitable zone—the clement region where liquid water (and hence, life as we know it) could exist.
As of yet, wobbles have shown no sign of giant planets around Alpha Centauri, which is encouraging. “We know that there isn’t a large planet that would crowd out a planet in the habitable zone,” Belikov says. Still, he is worried by the prospect of “semi-giants”—Neptune-class worlds that are small enough to have possibly avoided detection until now but that could still be large enough to spoil the prospects for life.
To that end, Belikov and his colleagues have made the first serious attempt to directly image planets around Alpha Centauri. They booked two rounds of observing time on the Gemini Planet Imager instrument on the Gemini South Telescope atop Cerro Pachón in Chile. This summer’s observing run got thoroughly clouded, but Belikov intends to try again next spring. A similar search he performed last year came up empty, which is good news: so far, there is no hint of pesky Neptune-size planets in the Alpha Centauri system, either.
The even better news is that Belikov’s observations are already being superseded. Kasper is closing in on smaller planets, just slightly heftier than Earth, using a VLT instrument called VISIR (for VLT Imager and Spectrometer for Mid-Infrared), which was rebuilt and rebranded as NEAR (for Near Earths in the AlphaCen Region). NEAR is the result of an unusual collaboration between the European Southern Observatory, which runs the VLT, and the private Breakthrough Initiatives, which provided funding for critical equipment upgrades. It is the first device built and operated specifically to find planets around Alpha Centauri.
During NEAR’s inaugural run in late May and early June, Kasper’s team, too, was plagued by bad weather. Still, the researchers managed to squeeze in 100 hours of observations, collecting six terabytes of raw data. “We’ll be sensitive to planets that are more than two times the radius of the Earth,” Kasper says. That is well beyond what Belikov can do and a factor of five better than the best of the wobble studies.
It will not be long before the world learns what, if anything, NEAR has seen around Alpha Centauri. The results should be out in October, Kasper says. If the data look good, he hopes to do another observing run sometime after March 2020, when Alpha Centauri will again be well placed for viewing in Chile.
True to its name, NEAR is bringing us nearer to the goal of finding Earth 2.0—but there is still a lot more distance to go. A planet twice the size of our own (the instrument’s lower limit) might very well have a thick, choking atmosphere, inhospitable to life. “The current efforts are falling short of reaching the sensitivity to find true Earth analogues,” says Olivier Guyon, an astronomer at the University of Arizona and the lead scientist for Breakthrough’s planet-hunting program, Breakthrough Watch. “So we will certainly keep looking at the Alpha Centauri system with better, more powerful instruments when they become available.”
One way to improve is to rise above our planet’s starlight-scattering atmosphere and take the search into space. Belikov has been pushing this argument for several years in the form of a mission concept called Alpha Centauri Exoplanet Satellite, or ACESat. It would incorporate a telescope with a 45-centimeter-wide mirror and a starlight-blocking device called a coronagraph to blot out the overwhelming lighthouse glare of Alpha Centauri A and B, potentially uncovering the 10-billion-times-fainter firefly glow of nearby small planets.
In 2014 NASA provided some funding for the ACESat technology, though the agency later passed on it as a mission. “The technology was not mature back then, but it’s getting much more mature now,” Belikov insists. He is trying again at NASA with a new proposal called the Alpha Centauri Direct Imager, or ACEND. He is also hedging his bets, working on a similar concept with Project Blue. As always with these kinds of high-frontier projects, money is at least as much of an issue as technology.
At a projected total cost of about $50 million, the Project Blue satellite is a pittance as compared with the $9-billion budget of the James Webb Space Telescope. Raising those kinds of funds for a single-purpose private mission has not been easy, however. Project Blue originally aimed to launch in 2021, but “I don’t think that is doable with the current state of funding,” Marchis says. “We need the support of a wealthy donor or group of donors to improve the technological readiness and start building the instrument.”
Breakthrough Initiatives has the advantage of a built-in network of such donors, including its billionaire co-founder, Yuri Milner. As part of Breakthrough Watch, the organization is studying a small coronagraph-carrying space telescope akin to ACESat. Breakthrough Watch is also pursuing a separate mission called TOLIBOY, tentatively set for a 2021 launch, which would search for planets by precisely mapping the motions of Alpha Centauri A and B using a technique called astrometry.
All of these projects will lay the groundwork for even grander efforts to come. Belikov is working to adapt ACESat technology for NASA’s upcoming space-based Wide Field Infrared Survey Telescope, or WFIRST, set for launch in the mid-2020s, and for the successor to the Hubble Space Telescope, forecast to arrive in the 2030s. Kasper is helping to apply lessons from NEAR to a new instrument, METIS (for Mid-Infrared ELT Imager and Spectrograph), which will be installed on the 39-meter Extremely Large Telescope, currently under construction in Chile. The TOLIBOY satellite is intended as a precursor to a larger, more capable space-based astrometry mission called, naturally, TOLIMAN.
All of these approaches, together, should finally fill in the blanks that surround Alpha Centauri. “Everything that we know points to the possibility and, I would say, likelihood of potentially habitable planets existing around A and B,” Belikov says. That limited knowledge is about to be put to the test. If Alpha Centauri turns out to be barren, this will suggest that Earth-like planets are not as common as once thought. Perhaps double stars such as Alpha Centauri—which make up the majority of all stellar systems—are not promising places to find life. Or perhaps we will find that Alpha Centauri has planets that are similar to Earth in size but nothing at all like it in environment. Our home planet may be a rare outlier in a hostile universe.
Then again, the sighting of a dim blue dot of exoplanetary light around Alpha Centauri could tell an entirely different story. “If we find that one of those planets is like our Earth, meaning it has continents and oceans, it will become a new, potentially living world at our doorstep,” Marchis says. Finding another Earth right next door would imply that there are millions or billions more of them across our galaxy. “That will be a highlight of astronomy for the next century and possibly a way to expand the interest of our whole civilization for space exploration.”