About 500 light-years away in the constellation Cygnus lives a star, which, though smaller and redder than the sun, has a planet that may look awfully familiar.
With a diameter just 10 percent bigger than Earth's, the newly found world is the first of its size found basking in the benign temperature region around a parent star where water, if it exists, could pool in liquid form.
Scientists on the hunt for Earth's twin are focused on worlds that could support liquid surface water, which may be necessary to brew the chemistry of life.
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Statistically speaking, Earth-sized planets orbiting in stars' so-called habitable zones -- not too far away for water to freeze, not too close for it to vaporize -- should be common, recent studies show.
But observations are difficult to come by. NASA's Kepler space telescope spent four years staring at about 150,000 target stars looking for slight and repeated dips in their light caused by orbiting planets passing by, or transiting, relative to the telescope's line of sight.
A planet the size of Earth positioned about as far from a host, sun-like star and as far away as Earth orbits the sun would block just 80- to 100 photons of starlight out of every million -- and do so only once every 365 days, notes astronomer Thomas Barclay, with the Kepler science team at NASA's Ames Research Center in Moffett Field, Calif.
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An Earth-sized planet circling a smaller star is an easier target. The newly found world, designated Kepler-186f, obscures about 400 photons of starlight out of every million as it transits its parent star -- and repeats the cycle every 130 days.
"I wouldn't say this is the ‘bingo' planet, but this is really one of the major milestones on the road," Barclay told Discovery News. "This isn't an Earth twin, but perhaps it's an Earth cousin."