Finding Earth-like planets around any star would be a stunning feat, and either Kepler or CoRoT may deliver such news before too long. But how much more exciting still if we find a planet like this around a star as close as Centauri B? After all, the Centauri stars are our closest stellar neighbors, close enough (a mere 40 trillion kilometers!) to conjure up the possibility of a robotic mission there and, if we play our propulsion cards right in the future, perhaps a manned trip as well.

A Radial Velocity Long Shot

But can we pick up the faint signature of a terrestrial world in this system, given that it would be akin to ‘detecting a bacterium orbiting a meter from a sand grain — from a distance of 10 kilometers’? The phrase is Lee Billings’, from his fine essay in SEED called The Long Shot, on an ongoing project to do just that. Most radial velocity surveys are spread out over numerous stars, picking off close-in worlds whose traces should be obvious in short periods of time.

Gregory Laughlin (UC-Santa Cruz), on the other hand, armed with a planet hunter’s insights, a passion for the Centauri system, and a realization that patience could tease out faint signals like these, traded ideas with Debra Fischer (San Francisco State) on the possibility of devoting years to an Alpha Centauri search. Fischer is now hard at work, using a telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. She works with a decommissioned spectrometer and other vintage equipment. Call it ‘Alpha Centauri on the cheap.’


It may take promising early data to get even this modest setup funded after National Science Foundation funds run out in November, but we’ll take the funding problem one step at a time. For now, the precision work continues, with software Fischer herself coded being used to filter out distortions of weather, instrumentation and stellar activity on the target stars to hunt for the minute shifts in wavelength that could signal the breakthrough discovery. If she pulls this off, Fischer’s patience may become legendary.

Image: Centauri planet-hunter Debra Fischer. Credit: NASA.

A Parallel Hunt, and Controversy

We may be talking three to five years here, and in the meantime, Fischer’s work is being paralleled by Michel Mayor and Stéphane Udry using the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory facilities at nearby La Silla. But ‘matched’ isn’t the best word — Mayor’s team isn’t as fixated on Alpha Centauri as Fischer’s because HARPS can’t be committed to a single, intensive project. If we’re talking an Earth-mass planet in Centauri B’s habitable zone, Fischer should find it first. A larger world may be claimed by Mayor.

You must read this essay. Billings is a wonderful writer whose scientific clarity is matched by a novelist’s eye for detail. And he’s working with a fascinating bunch of scientists, including Philippe Thébault, whose recent papers, discussed in these pages, have made the case that planets could not form around Centauri A and B because the relative velocities of planetesimals there would prevent their further growth. You have to love a scientist who speaks so openly about his own results, which differ sharply from Greg Laughlin’s conclusions on the Centauri system. Here’s Thébault on the matter, as quoted by Billings:

“If you ask a doctor about a fatal diagnosis he makes for a cancer patient, of course he wishes he is wrong. It is the same for me. It appears to be very difficult to form planets around close binary stars. I don’t wish for such a universe—I wish for another universe where planet formation is always very easy. I hope that Greg is right and that I’m wrong.”

We won’t know for a while, but it’s interesting that the binary system HD196885, a close match to Alpha Centauri, has a known gas giant at 3.5 AU. With refreshing candor, Thébault says he can’t explain that result: “…It shouldn’t be there.” On the flip side, the astronomer Peter van de Kamp spent decades looking for planets around Barnard’s Star, only to learn that the effect he was observing was an aberration in his instruments. The hunt continues in search of hard data.


Image: Alpha Centauri in context. Note that the three Centauri stars appear here as a single light source. Credit: Akira Fujii/David Malin.

What Centauri Means

Billings has his own thoughts on the significance of a find, and he also asks planet hunter Geoff Marcy for his take on the project:

The discovery of habitable worlds around any star would be front-page news, but finding them around our next-door neighbors would catalyze a scientific and cultural revolution, an immense rising wave of effort to learn whether our sister stars’ habitable planets were in fact inhabited. The ripples would spread beyond science to touch and change our literature and art, our politics and religion, perhaps even aiding our struggles to unite, survive, and expand as a species. The chain of chance that brought us into existence would swing to point outward to the stars, strengthening our resolve to someday reach them.

“If planets are found around Alpha Centauri, it’s very clear to me what will happen,” Marcy said. “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital camera, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”

Well, maybe. Or maybe we’ll start with something closer to home, such as putting new emphasis on a planet hunter mission that can get spectroscopic observations of planetary atmospheres around these stars, a hellishly difficult challenge, but considerably less expensive than a one-shot flyby. That’s a question that can only be resolved by technical advances in coming years. While we work on the relevant technologies, what an encouraging thought that early in the next decade, we should have hard evidence for the presence or absence of Centauri planets. Billings again:

Alpha Centauri is today what the Moon and Mars were to prior generations—something almost insurmountably far away, but still close enough to beckon the aspirational few who seek to dramatically extend the frontiers of human knowledge and achievement. For centuries, it has been a canonical target of the scientific quest to learn whether life and intelligence exist elsewhere. The history of that search is littered with cautionary tales of dreamers whose optimism blinded them to the humbling, frightful notion of a universe inscrutable, abandoned, and silent.

Let’s hope today’s dreamers will be vindicated in their hopes for planets in this fascinating system. The confirmation of a terrestrial world there would add robustly to our theories of planetary formation. The study of its atmosphere could tell us whether our nearest neighbor also sheltered life. And what was true decades ago still holds: As an inspirational target driving propulsion research for the ultimate next generation mission, there’s nothing like Centauri.