Before fulfilling its audacious dream of interstellar flight, Breakthrough Starshot—the private effort funded by billionaire Yuri Milner to conduct high-speed robotic voyages to the stars within a generation—must first find a destination.
The project’s primary target is the triple star system Alpha Centauri, our nearest interstellar neighbor at just over four light-years away. Of its three stars, only the red dwarf Proxima Centauri is known to have a planet, an Earth-mass world in a star-hugging orbit where liquid water—and therefore life as we know it—could exist. Astronomers already have plans to closely study this planet, but may find it unwelcoming due to its bombardment with intense flares from its nearby host star. Many believe the system’s larger, brighter and more sunlike stars, the binary pair Alpha Centauri A and B, offer better prospects for life-friendly worlds, even though all previous planet hunts there have come up empty-handed. Thoroughly examining these two stars requires expensive new instruments and many nights on the world’s best, most in-demand telescopes—boons just as elusive as Alpha Centauri’s planets. For years, this relative lack of resources has rendered any worlds around Alpha Centauri A or B effectively invisible to us, lost in the overpowering glare of those stars.
Before the end of the decade, however, they may appear in plain view. This week, Milner’s Breakthrough Initiatives organization announced a partnership with the European Southern Observatory (ESO) to search for and image the planets of Alpha Centauri A and B as early as 2019. The partnership, in which Breakthrough purchases instrument upgrades and observing time on ESO’s Very Large Telescope (VLT) in Chile for an undisclosed sum, is only the first phase of the organization’s more ambitious plans to scour nearby stars for promising worlds that its Starshot probes might someday visit.
“It’s high time that humanity gets to know its neighboring star system better and finds out if it contains more planets,” Milner says. “This collaboration will develop state-of-the-art instruments to enhance the already impressive VLT in pursuit of that common goal.” Breakthrough representatives say the organization is already in discussions to augment its search with additional Southern Hemisphere observatories, and is also investigating possibilities for launching small, planet-finding space telescopes.
Breakthrough’s Big Bet
According to Olivier Guyon, an astronomer at the University of Arizona who helped craft Breakthrough’s planet-hunting goals, Milner’s support was crucial for persuading ESO and other risk-averse public institutions to take a chance on Alpha Centauri. “This is not just a billionaire funding something that no one else is interested in; many of us have been pushing for this for years without success, because this is an expensive and risky venture,” he says. “Most people say we probably won’t find anything, but I think we have to look anyway, and Breakthrough is providing enough resources and assuming enough risk to make that happen.”
Statistics on thousands of worlds from NASA’s planet-hunting Kepler mission and other surveys suggest practically all stars should harbor planets—with perhaps twenty percent of sunlike stars bearing one in the “habitable zone” where temperatures could allow life as we know it to thrive. But those same statistics also indicate that planets are less common around binary stars like Alpha Centauri A and B, and previous studies have largely ruled out large worlds like Jupiter or Neptune there. All this may mean there is a better-than-even chance the search will find no planets at all, says Markus Kasper, an astronomer and ESO science lead for the collaboration. “But there may be habitable planets [around Alpha Centauri A and B] that we could already see with current telescopes, so we will give it a try,” Kasper says.
With Breakthrough’s planned upgrades, Kasper says the VLT could detect a planet twice Earth’s size in the habitable zones of either Alpha Centauri A or B in about 60 hours of observations. No one yet knows whether such a “super-Earth” would be a habitable world like our own—or instead an airless rock or a gas-shrouded miniature version of Neptune. What is certain is that of all the worlds already detected around other stars, such midsize planets are both the most abundant and most mysterious. What’s more, Guyon and other astronomers say that further, as yet unfunded upgrades could also allow the VLT to spy any smaller, Earth-size planets in Alpha Centauri.
Even if planets are there to be found, “this will be a tough measurement, so it’s a long shot,” says Bruce Macintosh, an astronomer at Stanford University who has advised Breakthrough. “I’m always a bit of a pessimist about these sorts of things—looking at only two stars, even for common planets like super-Earths, is a gamble.”
Presuming that gamble pays off and the VLT spots a planet in our nearest neighboring star system, the discovery could catalyze an explosion of interest and follow-up investigation. Studying Alpha Centauri’s planets would likely shift from a fringe pastime to a staple of astronomy. Other telescopes operating at different wavelengths could attempt to reveal the planet’s exact orbit and size as well as its bulk composition and surface temperature. Ultimately, a nascent fleet of next-generation observatories could even probe the world for so-called “biosignatures”—gases such as oxygen and methane that, here on Earth anyway, are mostly produced by living organisms. And, of course, Milner and his merry band of Breakthrough researchers would strive to send robotic probes there.
Planets from the Vortex
The VLT is a sprawling research complex dominated by four eight-meter telescopes, each of which would be a world-class facility on its own. Telescopes with mirrors much smaller would struggle to resolve any faint planets flitting like fireflies around Alpha Centauri’s stars. But the VLT’s status as a workhorse for global ground-based astronomy is not due to massive hardware alone. It is perched high on a mountaintop in the desolate Atacama Desert of northern Chile, far from light-polluting cities and well above most clouds and other weather patterns. On moonless nights visitors can become disoriented and euphoric in the cold, thin air and all-consuming darkness, as countless stars emerge like nameless leviathans from the heavenly depths to overwhelm the constellations. In that sea of alien suns the stars of Alpha Centauri—among the brightest in the southern skies—shine like a familiar and beckoning beacon.
To glimpse Alpha Centauri’s planets, Breakthrough and ESO plan to upgrade an instrument called VISIR (VLT Imager and Spectrometer for mid-Infrared), which scans the sky at a thermal wavelength of 10 microns. At that wavelength an Earth-like planet would glow like a lightbulb, although it would still appear millions of times fainter than its sunlike star—but that’s still better than billions of times fainter, as it would be in visible light.
A device called a coronagraph can be built into a telescope to block most of the photons from a distant star’s glow, allowing the dim light from a planet to pass into the telescope’s sensors and create a glare-free image. Breakthrough and ESO, working with the University of Liège in Belgium and Uppsala University in Sweden, plan to develop and install a special, high-contrast “vortex” coronagraph on VISIR. The device is a centimeter-wide disk of synthetic diamond, etched with concentric ripples that twist starlight like a corkscrew and funnel it to the disk’s edges. This “enables peering very close to the star, as close as a telescope’s resolving power allows,” says Olivier Absil, an astronomer developing the coronagraph at Liège. Using VISIR on the VLT, “we will be able to unleash the full potential of the vortex coronagraph for the first time,” Absil says.
For one of the VLT’s massive eight-meter mirrors, such a coronagraph would allow a relatively clear view of warm planets in the habitable zones of both Alpha Centauri A and B. Or rather it would, if Earth’s starlight-smearing atmosphere was not in the way. Modern observatories use “adaptive optics”—computer-controlled deformable mirrors that change shape thousands of times per second—to cancel out the worst effects of atmospheric turbulence on starlight. Once upgraded with its coronagraph as well as a calibration module from Kampf Telescope Optics in Munich, VISIR will be mounted on the VLT’s eight-meter Unit Telescope 4, which has the observatory’s most sophisticated adaptive optics.
The same technologies for imaging Alpha Centauri’s planets at the VLT could conceivably be exported to other large southern telescopes equipped with adaptive-optics systems, namely the twin 6.5-meter Magellan Telescopes and the eight-meter Gemini South telescope. That would bolster the search, boosting its sensitivity and the likelihood of uncovering any lurking planets. Breakthrough representatives say the organization is now discussing partnerships with the leadership of both observatories. “The 10-micron VLT project—and Magellan/Gemini if they go forward—is likely to be humanity’s first attempt at directly imaging an Earth analogue around another sunlike star,” says Christian Marois, an astronomer and Breakthrough adviser at the National Research Council of Canada. “The next decade will be quite exciting for the search of habitable planets, and Alpha Centauri is the first logical star system to look at.”
Outside of Breakthrough’s ground-based efforts, other projects could also reveal Alpha Centauri’s planets. A new generation of ground-based “Extremely Large Telescopes” will debut in the 2020s. Boasting supersize mirrors more than 30 meters wide, these observatories could image Alpha Centauri’s worlds with relative ease at mid-infrared wavelengths. Such gargantuan telescopes would build on the technologies now being developed by Breakthrough and other organizations, and would offer hope of detecting biosignatures and other gases in planets’ atmospheres to reveal whether they are habitable—or even inhabited.
NASA’s 6.5-meter infrared James Webb Space Telescope, launching in 2018, could also conceivably monitor the star system in the mid-infrared range—but probably won’t. As a general-purpose observatory, much of Webb’s limited lifetime is already committed to other astronomical investigations. Moreover, because Alpha Centauri A and B are so bright and close together, Webb’s coronagraph could only block the light of one star while the light from the other beats down for tens of hours on the telescope’s delicate, irreparable sensors—a risk that mission operators are unlikely to take.
Beyond these, other space-based resources could investigate Alpha Centauri in visible, reflected light rather than infrared—providing crucial multiwavelength analysis that could pin down the true nature of any discovered worlds. NASA’s successor to Webb, a 2.4-meter space telescope called WFIRST slated to launch in the mid-2020s, could potentially observe Alpha Centauri. But like Webb it already has a full docket of other research priorities, and will probably be similarly challenged by the brightness and proximity of the two target stars.
Instead, glimpsing the planets in visible light might require the exact opposite of a government-funded megaproject. Ruslan Belikov and Eduardo Bendek, two scientists at NASA Ames Research Center, have outlined innovative plans for a small space telescope with a half-meter mirror that could launch before the end of the decade on a dedicated mission to obtain basic images of any Alpha Centauri planets. The concept, with an estimated cost of several tens of millions of dollars, has proved attractive enough to garner the attention of private investors. Breakthrough is also reportedly investigating a small space mission of its own, a telescope devoted to watching for wobbles of Alpha Centauri A and B rather than directly imaging planets. Such wobbles, produced by the gravitational tugging of unseen worlds, could be used to pin down each planet’s precise mass and orbit without the need to first snap a planetary portrait.
“I see all of these efforts as very complementary, and we should do all of them if we can, because together they serve to paint a more complete picture of any planets that might exist around Alpha Centauri,” Belikov says. “Of course there is a part of me that wants [our concept] to be first, but that is negligible compared to my curiosity about what’s out there and the benefits a discovery with any method would bring…. I don’t see this as a race, but rather as a collective concerted search. When a colleague or a friend scores, I cheer.”