UC SANTA BARBARA (US) — A group of potential Earth-size planets appear to orbit in the habitable zone of smaller, cooler stars than our sun.
The planets are part of a group of Earth-size candidates, some of which are in the habitable zone, a region where liquid water could exist on a planet’s surface. NASA’s Kepler mission announced the discovery this week.
The Kepler space telescope also found six confirmed planets orbiting a sunlike star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our solar system.
Kepler looks for planet signatures by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them. This is known as a transit. Since transits of planets in the habitable zone of sun-like stars occur about once a year and require three transits for verification, it is expected to take three years to locate and verify Earth-size planets orbiting sun-like stars.
The recent discoveries are part of several hundred planet candidates identified in new Kepler mission science data, released Feb. 2. The findings increase the number of planet candidates identified by Kepler to date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter; and 19 are larger than Jupiter.
Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size —up to twice the size of Earth—to larger than Jupiter.
The findings are based on the results of observations conducted from May 12 to Sept. 17, 2009, of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400 of the sky.
“The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy,” says William Borucki of NASA’s Ames Research Center, the mission’s science principal investigator. “We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone, some of which could have moons with liquid water.”
Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates. Kepler-11, located approximately 2,000 light years from Earth, is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury.
The only other star with more than one confirmed transiting planet is Kepler-9, which has three. The Kepler-11 findings are reported in the journal Nature.
“A statistical analysis of the sample of multiple planetary candidates shows that planets tend to come in packs, meaning that planets are likely to reside in systems with several planets, rather than only one,” says Avi Shporer, a physicist at the University of California, Santa Barbara, who is involved in the Kepler mission.
“Another interesting result based on the large sample of planet candidates discovered by Kepler is that when accounting for the geometrical probability for a planet to transit its host star, it turns out that close to 20 percent of all stars are orbited by planets, meaning that a significant fraction of the stars in the sky are orbited by alien worlds,” says UC Santa Barbara physicist Tim Brown, who also is involved in the mission.
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