NASA's Kepler confirms first planet in habitable zone of Sun-like star

NASA's Kepler mission has confirmed its first planet in the "habitable zone," a region where liquid water could exist on a planet's surface, the U.S. space agency announced on Monday. The newly-confirmed planet, Kepler-22b, is the smallest yet found to orbit in the middle of the habitable zone of a star similar to Sun. The planet is about 2.4 times the radius of Earth. Scientists don't yet know if Kepler-22b has a predominantly rocky, gaseous or liquid composition, but its discovery is a step closer to finding Earth-like planets. Previous research hinted at the existence of near-Earth-size planets in habitable zones, but clear confirmation proved elusive. Two other small planets orbiting stars smaller and cooler than Sun recently were confirmed on the very edges of the habitable zone, with orbits more closely resembling those of Venus and Mars. "This is a major milestone on the road to finding Earth's twin, " said Douglas Hudgins, Kepler program scientist at the NASA Headquarters in Washington. "Kepler's results continue to demonstrate the importance of NASA's science missions, which aim to answer some of the biggest questions about our place in the universe." Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or "transit," the stars. Kepler requires at least three transits to verify a signal as a planet. "Fortune smiled upon us with the detection of this planet," said William Borucki, Kepler principal investigator at the NASA Ames Research Center who led the team that discovered Kepler-22b. "The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season." The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the spacecraft finds. The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets. Kepler-22b is located 600 light years away. While the planet is larger than Earth, its orbit of 290 days around a Sun-like star resembles that of Earth. The planet's host star belongs to the same class as Sun, called G-type, although it is slightly smaller and cooler. Of the 54 habitable-zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed. This finding will be published in The Astrophysical Journal.