An Earthlike World in the Cosmic Neighborhood

Just down the block from our planet — circling one of the stars in Alpha Centauri — is a world very similar to the one we know best

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Image courtesy of Nature

Artist's rendering of Alpha Centauri Bb, orbiting its parent star

Exoplanets can be hard to keep straight. The eight planets in our solar system are nothing next to the 2,300-plus possible planets astronomers have discovered circling other stars. Even those so-called exoplaneteers themselves concede that names like HD209458 or 51 Pegasi or GJ1214 don’t exactly roll off the tongue. And they certainly don’t stick in anyone’s memory.

Things will be different with a major new planet discovery just announced in Nature, however. A team of exoplaneteers based at Switzerland’s Geneva Observatory have spotted an exoplanet orbiting in the Alpha Centauri star system — our sun’s nearest celestial neighbor, and a favored destination for generations of sci-fi writers. For once, scientists are talking about a star just about everyone’s heard of.

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But that’s not what makes this discovery so important. The new planet, designated Alpha Centauri B b (ok, not so memorable) is about the same size, and quite possibly the same composition, as Earth. The big similarities, alas, end there: with a “year” lasting less than four days and an orbit just 4% as far from its star as Earth is from the Sun, the planet is hot to sustain life. But in the search for a Mirror Earth — a true twin of our planet, with a balmy, biology-friendly climate — it’s a major step. Not only that, said co-discoverer Stephane Udry in a press conference: “Finding a low-mass planet [like this one] means there’s a large probability of finding additional planets, including planets in the habitable zone” — that is, the not-too-hot, not-too cold region where life might exists.
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Best of all, while most exoplanets lie tens or even hundreds of light-years from Earth, the Alpha Centauri system, consisting of three individual stars orbiting around one another, is just four light-years away. That means that this Alpha Cen B b and any other planets in the system will be far more accessible for followup observations that could look for signs of life. “The odds against having such a good target so close by,” said University of California, Santa Cruz exoplaneteer Greg Laughlin, also speaking at the press conference, “are a thousand to one.”

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The odds against spotting the new planet in the first place were also pretty high. The Geneva astronomers found it the same way co-author Michel Mayor found the very first exoplanet, back in 1995: they looked for subtle changes in starlight as the orbiting planet’s gravity pulled the star alternately toward Earth, then away, then back again, over and over.

That first planet, 50 Pegasi b, was a giant, half as massive as Jupiter, so its tug, and the changes in starlight it produced, were relatively easy to see. For an Earth-mass planet, however, the job is far tougher. The wobble is far more subtle — so subtle that pulsation in the surface of the star itself, caused by waxing and waning magnetic fields, can masquerade as planetary wobbles. To extract the true signal of the planet, the scientists had to take careful measurements of the star, Alpha Centauri B, three times a night for some 500 nights, before they were convinced they had something.

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Even so, they don’t claim to be 100% certain the planet is there — but they’re pretty sure. The thousand-to-one odds apply here too, says Udry — in favor of this being the real deal. Not everyone is quite so convinced, however. “In my opinion,” wrote Artie Hatzes of Germany’s Thuringian State Observatory in a Nature commentary, “the matter is still open to debate.”

It probably won’t be for long. Yale’s Debra Fischer has been looking at the Alpha Centauri system as well, albeit for a shorter time. “We’ve carried out simulations with our existing data,” she said in an e-mail, “and do not rule out the possibility that this signal is real. We’re looking forward to intensive follow-up when the star emerges [from behind its companion, Alpha Centauri A] in January.”

Both Fischer’s group and the Geneva team will continue to monitor the Alpha Centauri system, and Alpha Cen B in particular, for additional planets. Finding a true mirror Earth with the wobble method will be very tough. At best, Udry said, he could spot a planet with four times Earth’s mass out in the habitable zone, where a planet’s leverage on its parent star is less. Such a planet could still harbor life, theorists suspect, but finding Earth’s twin will probably take another leap in technology. Both Yale and Geneva are already working on that, trying to develop better spectroscopes to detect even subtler wobbles in stars.

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A longer-term goal is developing optical telescopes that could resolve the planet directly. More tantalizing still would be sending a space probe out for an in-person visit — and it’s not entirely crazy to think it could happen, though it would take some patience. “With current technology,” said Loughlin, “it would take 40,000 years to get there. Given our propensity for instant gratification, that’s not really in the cards.”

But if astronomers can pinpoint a truly habitable planet in the Alpha Centauri system, Loughlin suggested, “there could be a groundswell of interest in developing new technology that could get us there within a human lifespan.” Indeed, engineers have already looked into ways of doing just that. “They’re speculative and farfetched so far,” he admitted. But as the long-ago race to the Moon illustrated, with a strong enough incentive, the speculative and the farfetched can become real in an awfully big hurry.

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