Spotted by the Kepler space telescope, the planet is the smallest yet found to orbit in the middle of the habitable zone -- the region in space where liquid water could exist on a planet's surface of a star similar to our sun. The planet, dubbed Kepler-22b is about 2.4 times the radius of Earth and an orbit of 290 days around a sun-like star resembles that of our world. The planet is orbiting the same class of star as our sun, called G-type, although it is slightly smaller and cooler, NASA stated.
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"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, Kepler program scientist in a statement. There are 48 planet candidates in their star's habitable zone.
While this is a decrease from the 54 reported in February, NASA said the Kepler team has applied a stricter definition of what constitutes a habitable zone in the new catalog, to account for the warming effect of atmospheres, which would move the zone away from the star, out to longer orbital periods. Of habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed.
NASA said Kepler also has discovered more than 1,000 new planet candidates, nearly doubling its previously known count. Ten of these candidates are near-Earth-size and orbit in the habitable zone of their host star. Candidates require follow-up observations to verify they are actual planets. NASA's Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates.
Just last week NASA said Kepler discovered an exoplanet orbiting one of the brightest stars in the Kepler field of view. Coined Kepler-21b, the planet is approximately 1.6 times the radius of Earth and nearly ten times the mass of Earth. Circling its host star every 2.8 days, Kepler-21b orbits at a distance of six million km - nearly ten times closer than Mercury orbits the sun. The surface temperature is calculated to be 2,960 Fahrenheit. While this temperature is nowhere near the habitable zone in which liquid water might be found, the planet's size is approaching that of Earth.
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Kepler has made a number of other key space discoveries.
In August 2010, Kepler discovered two Saturn-sized exoplanets crossing in front of, or transiting, the same star. At the time NASA said in addition to the two confirmed giant planets, Kepler spotted what appears to be a third, much smaller transit signature in the observations of the sun-like star designated Kepler-9, which is 2,000 light years away from Earth. The planets were named Kepler-9b and 9c.
Also on 2010 Kepler spotted five planets orbiting stars beyond our own solar system. The five planets are called "hot Jupiters" because of their deep mass and extreme temperatures, NASA said. They range in size from about the same size as Neptune to larger than Jupiter and have orbits ranging from 3.3 to 4.9 days, NASA stated. The orbs likely have no known living organisms because NASA estimates their temperatures to range from 2,200 to 3,000 degrees Fahrenheit, hotter than molten lava and all five orbit stars hotter and larger than Earth's sun. In June, mission scientists announced the mission has identified more than 700 planet candidates that it had not confirmed as planets.
The satellite has been peering at a patch of space, scanning over 150,000 stars since 2009.
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