by Lee Billings
For the first time ever, astronomers may have glimpsed light from a world in a life-friendly orbit around another star.
The planet candidate remains unverified and formally unnamed, little more than a small clump of pixels on a computer screen, a potential signal surfacing from a sea of background noise. If proved genuine, the newly reported find would in most respects not be particularly remarkable: a “warm Neptune” estimated to be five to seven times larger than Earth, the sort of world that galactic census-takers such as NASA’s Kepler and Transiting Exoplanet Survey Satellite missions have revealed to be common throughout the Milky Way. But even though it would be shrouded in gas and essentially bereft of any surface to stand on, its distance from its star would place it in the so-called “habitable zone” where liquid water could exist. No other planet has been directly seen in this starlight-drenched region around any other star, because of the associated glare. And this world’s celestial coordinates would be straight out of astronomers’ wildest dreams—it would orbit a near-twin of the sun called Alpha Centauri A, which also happens to be a member of a triple-star system that, at just shy of 4.5 light-years away, is the closest one to our own.
Because of its proximity, the system’s other members—a slightly smaller sunlike star called Alpha Centauri B, and the diminutive red dwarf star Proxima Centauri—are also high-priority targets for astronomers, who have already indirectly detected the presence of two worlds around Proxima (including one that is likely rocky and within that star’s habitable zone). Whether looking for real estate across town or around another star, location really is everything. The Centauri system is so close that it offers a unique front-row seat for scientists seeking to study the atmospheres and surfaces of any worlds that exist there, especially to seek out possible signs of life. And astronomers long ago learned that planets are, in some respects, like household pests: where one is seen, others are likely to be found. Which is why, as tentative as they may be, the burgeoning crop of Centauri worlds hints at untold discoveries that are yet to be made and could profoundly transform views of our place in the universe.
The new findings were reported Wednesday in the journal Nature Communications. They come from an international consortium of planet hunters called Breakthrough Watch, via the inaugural science run of a one-of-a-kind “direct imaging” instrument called NEAR (New Earths in the AlphaCen Region), which operates on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile. The effort is named for its chief funding organization, Breakthrough Initiatives—the brainchild of the Silicon Valley billionaire Yuri Milner, who also sponsors related projects to search the heavens for signs of alien civilizations and to send pint-sized interstellar probes to the Alpha Centauri system.
“Alpha Centauri presents us with a magical opportunity, because there is no better place in the sky to try to directly image small, potentially habitable planets,” says study co-author Pete Klupar, Breakthrough Initiatives’ chief engineer. “This was in some sense low-hanging fruit—for just $3 million, we were with our international partners able to build an instrument to take advantage of ESO’s billions of dollars invested in its telescopes. But it’s also like going after a needle in a haystack, which is why no one has ever done this before. Governments tend to build survey instruments, to look at large numbers of stars and guarantee a return on investment, whereas NEAR was purpose-built to just do this one, risky thing.”
“When we collaborate on a global scale, we discover new worlds, and we keep advancing,” Milner says. “The identification of a candidate habitable-zone planet in our celestial backyard will continue to power our curiosity.”
The candidate’s tantalizing signal emerged from 100 hours of observations on the VLT, stretched across a total of 10 nights in the spring of 2019. By June of that year, as the Breakthrough Watch team members sifted through their observations, they began to realize they might have found something. Kevin Wagner, the study’s lead author and a postdoctoral Sagan Fellow at the University of Arizona, first saw the telltale evidence of a planet-like blip cresting far above NEAR’s instrumental noise. It happened while he was remotely processing a batch of data during a family vacation in Lake Jocassee, S.C. Measuring its brightness and sandwiching it between limits on planet masses and sizes calculated in previous studies by other groups, the Breakthrough Watch team estimated that—if the blip were indeed a planet—it would most likely be somewhere between Neptune and Saturn in size. By November, he and his colleagues were certain the find was worth publishing, even if it proved not to be a world at all. (It would not be the first time our neighboring star system has fooled astronomers. Peer-reviewed claims of a small planet around Alpha Centauri B in 2012 evaporated a few years later, found to be products of stellar noise.)
“In a way, I hope we haven’t detected anything this time, too,” Wagner says. “Because what I’m most excited to find is an Earthlike planet in the habitable zone. The presence of a Neptune in the habitable zone of Alpha Centauri A would not rule out something smaller nearby, but it would limit some of the area in which we could hope for rocky worlds to exist there in the first place.”
There is no shortage of other possible explanations for the weak signal, which is essentially a thermal wisp of infrared photons—that is, of heat—that seems to originate from a source at the outer edge of Alpha Centauri A’s habitable zone. In visible light, a sunlike star outshines a small, rocky planet by a factor of billions. But in infrared the star is dimmer and the planet is at its brightest, so this contrast ratio is “only” measured in millions. For decades, the difficulty of achieving even this more modest measurement limited direct imaging to hot giant planets orbiting far from their stars. That is, until NEAR was built. It is a mid-infrared coronagraph, a specialized instrument designed to blot out the bulk of a star’s thermal glow at a tight wavelength of 10 microns. Augmented by adaptive optics to compensate for the blurring turbulence of Earth’s atmosphere, in operation it switches its focus between Alpha Centauri A and B every tenth of a second, using observations of each star to help calibrate those of the other. It progressively winnows out starlight, and stacks frame after frame to allow any faint planetary light to accumulate and eventually be seen. But rather than betraying the presence of a planet, any resulting blip could instead be a far-distant background object, a clump of starlight-warmed dust or an asteroid belt circling around a star, or even the errant play of stray photons leaking from beamlines and spraying across sensitive optics inside the instrument. Wagner and his co-authors have already ruled out the first possibility (no known background star or galaxy can account for the blip), but the others remain in play to various degrees.
Confirmation of the blip’s planetary status should have been relatively straightforward: simply attempt to observe it again after sufficient time has passed; if it is in fact a planet, its orbital motion will have swept it to a new and very different position around its star. Subsequent, more time-intensive studies with NEAR could then crudely measure the blip’s colors to help eliminate the “dust cloud” hypothesis. But this was not to be—not yet, anyway—as the ensuing COVID pandemic shut down astronomical observatories and most everything else around the globe. Wagner says the team has applied for additional time to use NEAR on the VLT, but the proposal has yet to be approved.
“The timing is such a shame,” says Debra Fischer, a veteran planet hunter at Yale University. She is unaffiliated with the study, but her work with her student Lily Zhao has placed the best-yet constraints on the planets that may or may not exist in the Alpha Centauri system. “If it’s in the habitable zone around Alpha Centauri A, that’s an Earthlike orbit, so observing six months later would probably have nailed it,” Fischer says. “Without that, this isn’t a planet detection paper, it’s a demonstration of NEAR’s capability to monitor Alpha Centauri in the mid-infrared. But if this turns out to be right—oh my God, it’s huge.”
Brave New Worlds
For now, NEAR is the only coronagraph on Earth with a realistic chance of imaging Alpha Centauri’s hidden worlds. But other instruments and facilities are already waiting in the wings to apply their own scrutiny to the system. Fischer’s high-precision EXPRES radial velocity spectrograph—and an even more advanced European counterpart, ESPRESSO—are both already operational. They could help indirectly confirm the planet candidate and others, and could estimate their masses, by watching for periodic wobbles each world’s orbital tugging induces upon its host star. A related technique, astrometry, could do much the same thing, pinpointing planetary masses by measuring how each world’s gravitational influence slightly shifts its star’s position in the plane of the sky. Such observations using the Atacama Large Millimeter Array in Chile, or even a modest, Breakthrough-funded dedicated space mission, could occur later this decade.
NASA’s James Webb Space Telescope, slated to launch in late October, would also be capable of directly imaging the candidate planet given one full day of observing time, according to a recent study led by one of Webb’s foremost scientists, Charles Beichman of the California Institute of Technology. “Because Alpha Centauri A is a twin of our own sun and less than five light-years away, it really is our closest solar neighbor,” Beichman says. “That makes it first among equals, of all the stars in the sky. No other system will lend itself to more detailed possible studies over the next several decades.”
The space agency’s follow-up mission to Webb, the Nancy Grace Roman Space Telescope, will also carry a coronagraph as a technology demonstration that could (with certain tweaks now being actively considered) potentially snap pictures of the candidate.
And, around the same time Roman may launch, a new generation of sophisticated coronagraphs mated to gargantuan ground-based observatories should begin operations that could in mere minutes produce images of Centauri planets that would currently require hours upon hours of NEAR’s time on the VLT. Armed with starlight-gathering mirrors 30 meters or more across, ESO’s European Extremely Large Telescope and its American counterpart, the Giant Magellan Telescope, could both in theory gather enough light from a habitable-zone Neptune around Alpha Centauri A to study its atmosphere, sniffing out what familiar or alien chemistry occurs there. (A third behemoth, the U.S.’s Thirty Meter Telescope, is presently planned for a site in the Northern Hemisphere from which Alpha Centauri would not be visible.) Finally, NASA and other space agencies are now studying concepts for multibillion-dollar space telescopes for the 2030s and beyond. Some of these could image and search for signs of life on small rocky planets around Alpha Centauri as well as many other nearby stars.
All of which means that, even if this latest candidate from Alpha Centauri proves spurious, it still signals something quite real: a looming sea change, in which planet-hunting astronomers shift from safe, statistical surveys to the more daring in-depth study of individual worlds, some of which might harbor life.
“Whether this thing is real is, to me, almost secondary,” says study co-author Olivier Guyon, an innovator in direct imaging and chair of Breakthrough Watch. “Because either way it shows we’re clearly opening a new era in the history of astronomy where, finally, after more than 20 years of hard work, we can at last perform direct imaging of another star’s habitable zone. This is the ‘game on’ moment for the field.”