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Newfound Exoplanets Are Most Earth-Like Yet

NASA's planet-hunting Kepler spacecraft finds two worlds that have sizes and orbits similar to ours
January 6, 2015 |By Lee Billings

After five years of searching, researchers using data from NASA's exoplanet-hunting Kepler spacecraft have discovered what look to be two of the most Earth-like worlds yet. Dubbed Kepler 438 b and Kepler 442 b, both planets appear to be rocky and orbit in the not-too-hot, not-too-cold habitable zones of their stars where liquid water can exist in abundance. Astronomers announced the planets along with six other newfound small, temperate worlds today at a meeting of the American Astronomical Society in Seattle. Their findings will be published in The Astrophysical Journal. The discoveries double the number of known potentially habitable exoplanets. They also push Kepler's tally of vetted, confirmed worlds to just over 1,000, marking a milestone in the mission's epochal search for alien Earths.

This artist's conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning "planetary nebula." Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars.

Both planets are many hundreds of light-years away and orbit stars smaller and dimmer than our sun. Like most of Kepler's finds, they were discovered via transits—the shadows they cast toward our solar system as they cross the blazing faces of their stars. Transits allow astronomers to measure a planet's size, orbit and exposure to starlight. Kepler 438 b is only about 12 percent larger than Earth, and basks in 40 percent more starlight; Kepler 442 b is 30 percent larger and receives about 30 percent less light. Both spheres may be somewhat warmer than Kepler's two previous premier rocky worlds, Kepler 186 f and Kepler 62 f, each of which gets significantly less starlight—similar to that received by Mars. “We can't say for sure whether these planets are truly habitable—only that they are promising candidates for habitability," says study co-author David Kipping, an astronomer at the Harvard–Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass.

When Kepler launched into orbit in 2009 to survey a patch of sky containing some 150,000 stars, one of its primary goals was to find mirror Earths, worlds about the same size as our own in approximately 365-day orbits around sunlike stars. The task was expected to take just over three years because many things can cause stars to dim besides transiting planets, and astronomers would need to glimpse the periodic recurrence of any mirror Earth's transit not once or twice but three times to be convinced that any particular dimming was due to a planet.

The sunlike stars had other plans, however, proving to be more variable in brightness than mission planners had anticipated, muddying Kepler's search for the faint shadows. Kepler's scientists consequently focused on smaller, dimmer, quieter stars and asked for more time to gather additional data. The mission was extended in 2012 but was struck by stabilizer equipment failures in spring of 2013 that sent Kepler's once-steady gaze drifting askew, bringing its survey to a premature close. Last year the spacecraft was resurrected as the "K2" mission after researchers devised a new method to aim the telescope. K2 is a more limited transit survey and has scant hope of finding a mirror Earth. The last, best option for the mission to succeed in discovering Earth twins was to sift through Kepler's archival data from 2013 and earlier, which is filled with thousands of unconfirmed candidate planets. "I think these latest planets are about as good as we're going to get from the Kepler data," Kipping says. "I would like to be surprised—and I'm hopeful—but I'm not sure we'll find planets closer to Earth twins than the objects we present in our paper."

Read more: http://www.scientificamerican.com/article/newfound-exoplanets-are-most-earth-like-yet/
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