First Earth-sized planets netted

Planet hunt finds two orbs of terrestrial proportions orbiting a distant sunlike star

The newest exo-apples of the planet-hunting Kepler space telescope’s unblinking eye are two rocky, Earth-sized planets hovering around Kepler-20, a sunlike star 950 light-years away.

The first two Earth-sized exoplanets, Kepler-20e and Kepler-20f, are depicted here compared to Earth and Venus. Scientists think Kepler-20f could have an atmosphere, but suggest that extremely hot conditions on Kepler-20e would have evaporated any atmosphere. Tim Pyle

Though snuggled too close to their star to be habitable, these first Earth-sized worlds confirmed by the Kepler team are another big step forward for the planet hunters, who recently found a planet somewhat larger than Earth orbiting a sunlike star at a distance hospitable to life. Finding habitable distant worlds — Earth-sized planets at the right distance from their stars to allow the presence of liquid water — is the team’s ultimate goal.

“The hunt is on to find a planet that combines the best of both of these worlds — a true Earth twin,” says David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and a coauthor of a study describing the small planets that appears online December 20 in Nature.

One of the planets, the pragmatically named Kepler-20e, is a bit smaller than Venus — 0.87 times as wide as Earth — and completes a trip around the star every 6.1 days. The other, Kepler-20f, is 1.03 times as wide as Earth, and a year on that planet would last just 19.6 days. Because the planets are so small, they’re probably made of ingredients similar to Earth’s.

Depending on where and how it formed, Kepler-20f could even have developed a water vapor atmosphere, says planetary scientist Jonathan Fortney of the University of California, Santa Cruz. “If it started out with the amount of water we had on Earth and Venus, it’s probably long gone — just like it is on Venus,” he says. “But if that planet had a tremendous amount more water, then it might have some left over.”

The Kepler-20 system is a quintet comprising three large planets (Kepler-20b, c and d) and the two Earth-sized ones, all tucked in nearer to their star than Mercury is to the sun. Moving out from Kepler-20, the five spheres alternate in size, with the runts of the planetary litter bracketed on either side by their bigger siblings.

“It’s one of the most shocking architectures we’ve seen,” Charbonneau says. “Exoplanets have had a lot of surprises, but this is going to be very difficult to explain.”

The strange — but stable — configuration is encoded in the blips and blinks the planets produce as they pass in front of their sun, which is one of more than 150,000 in a field of stars the telescope stares at. Different-sized blips correspond to different-sized planets, and watching the star for long enough reveals how frequently each planet completes its journey.

Currently set to wrap up at the end of 2012, the mission could be extended for several more years if limited budgets allow. More observing time will let scientists monitor Kepler’s starry patch for long enough to detect Earth-sized planets in longer, habitable orbits.

“The spacecraft doesn’t know about politics and financial difficulties — it will continue to beam data back to Earth until at least 2015, even if no one is listening,” says astronomer Debra Fischer of Yale University. “You just have to keep the lights on, and keep the science team intact. The next three years are where they’re going to detect the Earths at habitable distances.”

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