Apr 16, 2008 12:43 PM
Editor's Note: JR Minkel was in St. Louis this week for the annual "April meeting" of the American Physical Society. See his other blog posts on the pioneer anomaly, dark matter, the Higgs boson and the timeline for the Large Hadron Collider, and check back for frequent updates.
ST. LOUIS—Earthlike planets would be hard to spot, but not impossible, according to a talk this morning at the American Physical Society meeting that summarized the search planets outside the solar system (exoplanets) that are habitable.
Most of the nearly 300 known exoplanets are enormous gas giants, much larger than Jupiter or Saturn. Right now we don't know of any potentially rocky planets orbiting at the right distance from their stars to support liquid water—our default sign of potential habitability. There were a couple of preliminary reports of Earthlike planets, but they haven't panned out.
The most direct way to identify another Earth would be to spot it visually, but that's extremely difficult. Say you were a few light-years away trying to spot Earth in orbit around the sun. Because the sun is so bright, it's like trying to pinpoint a firefly next to a searchlight, physicist Sara Seager of the Massachusetts Institute of Technology said in her talk.
Future space telescopes might be able to cut through the glare by blotting out individual stars with a specially shaped shade placed in the scope's field of view.
Something that we can do right now is hunt for planets transiting (crossing) in front of their stars, which makes the searchlight effect much smaller. Even in this case, however, astronomers would still be far more likely to find a "super-Earth"—a rocky body closer to 10 times Earth's mass—next to a dim star than to find a true Earthâ€“sun combo.
Researchers can measure the light from transiting exoplanetâ€“star systems (as well as from a star's wobble caused by the gravity of an orbiting planet) to infer the object's mass and radius, which gives them a good idea whether it's solid or not, if not the exact composition. And in a technique that's come into its own over the past year, they can use transit data to deduce the chemical composition of the planet's atmosphere in a similar way. Water vapor on a small world would be a dead giveaway of liquid water, Seager said.
Then there's the big question: What about life? The trouble here is nobody knows what different forms life could take. Still, molecular oxygen or ozone in the atmosphere would be a strong sign, she said, because they're too unstable to last on their own. Seasonal variations in methane would also be telling.
Seager summed things up this way: "Do I expect to find an Earth in my lifetime? Absolutely yes," she said. "But I also expect to live a long time."
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