Earth-like Planets May Be Less Common than We Think

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A view of Earth's vast Pacific Ocean

Unless searchers who scan the cosmic airwaves pick up signals from extraterrestrials, the discovery of life on other planets will take a while, and it will take a series of incremental steps. Step one — making sure there are planets outside our solar system in the first place — has long since been accomplished. Thanks to searches from the ground and, more recently, by the orbiting Kepler spacecraft, astronomers know that, at the very least, hundreds of planets — and almost certainly far more than that — orbit other stars.

Step two — finding a world similar to Earth, in an orbit in which temperatures allow life-giving water to exist as a liquid — is all but inevitable given the large and growing planetary sample group. When that happens, scientists will move on as quickly as they can to step three: using powerful telescopes to look for chemicals in the planet's atmosphere that betray the presence of life. The problem is, there's one extra step — call it two-and-a-half — that few scientists had really thought about much.

Water is abundant all over the Milky Way. Our solar system is chock-full of it, from Earth's ocean to the vastly greater amounts in the form of ice locked up in comets, moons and dwarf planets like Pluto. But that doesn't necessarily mean that all solar systems are like that. And a new paper published online suggests they may well not be.

The study, led by Princeton astrophysicist Nikku Madhusudhan, focuses on a star targeted by the ground-based Wide Area Planet Search, or WASP. Known as WASP-12, the star has a giant planet orbiting around it, dubbed, straightforwardly enough, WASP-12b. Last year, Madhusudhan discovered something odd about the planet: it has an unexpectedly large amount of carbon compared with, say, Jupiter, its closest analogue in our solar system. Since carbon is a major building block of all life on Earth, a carbon-rich planet might well signal an entire carbon-rich solar system. That could certainly include an Earth-like planet with all the ingredients for life, and, quite plausibly, life itself.

But carbon isn't the only element in play; oxygen counts too. For a lot of technically mind-numbing reasons, it's not easy to measure carbon directly in a star or planet; instead astronomers measure the carbon-to-oxygen ratio and judge from there. The ratio in the star itself, WASP-12, is about the same as in our sun. That should mean a similar balance in any planets circling the star, since the entire solar system formed from the same swirling mass of dust and gas. On WASP-12b, however, things are off-kilter, with carbon far more plentiful relative to oxygen. Since the planet couldn't go shopping for extra carbon elsewhere, the explanation must be that its carbon level isn't unusually high, but its oxygen level is unusually low. Where, Madhusudhan wondered, had all the oxygen gone?

The best guess, he and his co-authors believe, is that some other element is vacuuming it up, and the leading culprit is the carbon atoms themselves, which may have combined with oxygen to form carbon monoxide. You wouldn't need to capture all the oxygen this way to explain WASP-12b's low oxygen content — at least not at first. Instead, the investigators' models suggest, once the carbon starts snagging the oxygen, the process spins essentially out of control, stopping only when there's little or no oxygen left.

That discovery is fascinating stuff for planet scientists but not necessarily for the rest of us. The implications of the finding, however, are less arcane than they seem. Carbon plus oxygen, slowly warmed in a star's habitable zone, ought to be a simple recipe for getting life started (provided there's hydrogen to mix with the oxygen to form water, of course). But the new findings introduce an x-factor into the mix: you also have to keep the carbon and oxygen from getting too cozy, lest there be no O left to create the hoped-for H2O.

"Generally," says Madhusudhan, "we just assume that if a planet is in the habitable zone of its star, it could be capable of supporting life." Instead, he now realizes, you could just as easily get a planet dominated by methane and other nasty carbon-based substances.

If that process is far more common than the one that played out on Earth, the search for life — or at least, life as we know it — could be in vain. But it's not time to give up yet. "The only way to get a sense of how common this situation is," says Madhusudhan, "is to look at a lot of planets and find out how many of them are as carbon-rich." He and his collaborators are conducting a survey even now with the Hubble telescope, trying to answer the question. It shouldn't be long before we have a better sense of how damp the Milky Way really is.