The Long Shot

Feature / by Lee Billings /

Two rival scientific teams are locked in a high-stakes race to discover other Earth-like worlds—and forever change our own.

Alpha Centauri. Courtesy of ESO.

Viewing Alpha Centauri is easy if you live in the southern hemisphere—it’s the third-brightest star in the entire sky, a gleaming golden nail hammered into the foot of the constellation Centaurus, a few degrees away from the Southern Cross and the Coalsack Nebula. That point of light is actually two stars, Alpha Centauri A and B, both so close together they can only be individually resolved through telescopes. Each resembles our Sun; A is slightly larger and pale yellow, while B is slightly smaller and dusky orange. They’re 4.39 light years from Earth, orbiting each other in a roughly 80-year period, with an average separation slightly greater than the distance between our Sun and Uranus. A third, much smaller red dwarf star, Proxima Centauri, drifts in the system’s outskirts.

Fischer’s observation program relies on a fact of Newtonian mechanics: Just as a star exerts a gravitational pull on its planets, the planets pull on the star, making it wobble in sync with the planets’ orbits. These wobbles, or shifts in radial velocity (RV), are imperceptible to the eye, but can be detected by breaking up the star’s light into a spectrum. In their revolutions, a star’s planets play it like an instrument, producing a symphony of regularly shifting light we can see from Earth: As a planet pulls the star away from us, the light grows redder; as it pulls the star closer, the light grows bluer. By measuring the intervals and strengths of these shifts in the starlight’s wavelength, astronomers can discern a planet’s orbit and estimate its mass.

RV shifts are how the vast majority of extrasolar worlds have been discovered, but only because these planets, called “hot Jupiters,” are extremely massive and in hellishly close orbits around their stars. Their stellar wobbles are measurable in meters per second; seeing the much smaller centimeters-per-second wobble of an Earth twin is orders of magnitude more difficult. For the Alpha Centauri system, the feat is akin to detecting a bacterium orbiting a meter from a sand grain—from a distance of 10 kilometers. But by devoting hundreds of nights of telescope time to collecting hundreds of thousands of individual observations of just these two stars, Fischer believes she can eventually distill the faint RV signal of any Earth-like planets. It’s simply a matter of statistics and brute force. The planets wouldn’t reveal themselves as images in a telescope, but as steadily strengthening probabilistic peaks.

Despite her assurance that she doesn’t take risks, the hazards in Fischer’s project are considerable—some would say extreme. The RV precision and stability required to detect a planet like Earth is entirely unprecedented, perhaps impossible. If not undone by technical difficulties at the end, like Peter van de Kamp and so many other dreamers, Fischer may be thwarted by nature’s apparent indifference to wishful thinking: There may be no planets around Alpha Centauri at all.

The person who convinced her to devote years to a search for Alpha Centauri’s planets was her frequent collaborator Greg Laughlin, a 41-year-old multitalented theoretical astrophysicist at the University of California, Santa Cruz. In his spare time, Laughlin crafts software tools and observation programs for a global network of amateur planet-hunters, runs Monte Carlo simulations of the financial markets, and maintains a blog devoted to extrasolar planetology. Speaking to me from the air-conditioned comfort of his university office, he said the genesis of the project came on a quiet night in early July 2006.

“I was sitting at my kitchen table when I began thinking about the possibility of detecting any habitable planets around Alpha Centauri, doing some back-of-the-envelope calculations. B in particular looked promising, because it has lower mass and it’s a very calm, quiet star,” Laughlin said. “I couldn’t shoot it down—finding planets [in Alpha Centauri] with a low-budget project seemed alarmingly feasible.”

Alpha Centauri’s brightness and visibility in southern skies for 10 months each year means an observation program can proceed relatively quickly with few disruptions. Further, the two stars offer a natural calibration: An identical signal in both of them would likely indicate a flaw in the observational equipment.

Discoveries of massive, close-in planets with the RV technique come quickly—just a handful of strong periodic signals are needed. So most RV surveys, hoping to rapidly harvest the low-hanging planetary fruit, have spread themselves thinly over a large number of stars. Laughlin realized that by focusing observations on a single promising star, the signatures of smaller planets should gradually emerge. “Your signal, the mass of a planet in a given orbit, scales with the square root of the number of observations,” he said. “With four times as many observations, in theory you can detect planets that are half as massive. If you’re willing to average over not hundreds, but hundreds of thousands of measurements, you can probably detect planets with masses equal to or less than that of Mars”—that is, a tenth the mass of Earth.

The more Laughlin thought about it, the more foreordained Alpha Centauri appeared for such an extreme search strategy. It began to seem somehow destined. On human timescales, the stars appear fixed in the sky, but as our Sun moves through its 250-million-year orbit around the galactic center, it brings us to new neighbors. Every few hundred thousand years, the list of our nearest neighboring stars must be made anew.

“If we were plopped down at some random point in the galaxy, there’s only a 1 percent chance we’d find ourselves near stars so optimal for detecting small rocky planets like our own,” Laughlin said. “The hand of fate has dealt us a very interesting situation that has not existed for at least 99.9 percent of Earth’s history. It’s remarkable that Alpha Centauri is right next door just as humans emerge and develop the ability to make these measurements. I’m enamored with that coincidence.”

Within a month of his late-night reverie, Laughlin had written several blog posts exploring the idea. Soon he began running computer simulations with his student, Javiera Guedes, to determine whether planets were likely to form in stable, habitable orbits in the Alpha Centauri system. Everything checked out: In the models, planets readily formed in the not-too-hot, not-too-cold regions around both A and B, where life-giving liquid water could exist. Laughlin broached the idea of an observational program to Fischer, who quickly jumped on board, bringing Howard Isaacson, her student, with her.

Walp came with Fischer, too. Back at the observatory after our hike, Fischer returned to her room for a few moments, and Walp told me how he became involved with the project. The story resembles a random walk, with Walp borne toward CTIO by trajectories of coincidence. He dropped out of college to work in political consulting, but a midlife crisis sent him back to school to get a mathematics degree. Walp met Fischer in 1997, while working as a clerk in San Francisco State’s astronomy department.

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