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Reexamining a group of stars observed by NASA's Kepler spacecraft, astronomers say they have identified a trove of candidate planets that are both Earth-sized and potentially habitable. Scientists on the Kepler team had mistakenly pegged the candidates at about twice Earth's diameter and much too hot for life, the astronomers report in a paper posted 12 September on the arXiv preprint server (www.arXiv.org).
Philip Muirhead of the California Institute of Technology in Pasadena and his colleagues used infrared observations to make new, more accurate estimates of the sizes and masses of 84 stars less than half as massive as our sun, which Kepler had examined in visible light.
The properties of such low-mass stars aren't as well understood as those of sunlike stars. Accurate measurements are crucial, Muirhead says, because Kepler searches for planets by detecting tiny dips in starlight produced when an orb transits, or crosses in front of, its home star. Measurements of the size of a planet are only as good as those of the star it transits.
But Muirhead's team had an advantage: One member- Bárbara Rojas-Ayala, now at the American Museum of Natural History in New York City- had recently developed new techniques for estimating the temperatures of low-mass stars and the amounts of "metals" (elements heavier than helium) they contain. At the Palomar Observatory's 5.1-meter Hale Telescope near Escondido, California, the researchers recorded near-infrared spectra of 84 of the 87 low-mass stars around which Kepler had spotted candidate planets. After determining the metal abundance and tem-perature of the stars, the team used a stellar
evolution model to estimate the mass and radii of the stars.
The results might be called a case of "Honey, I've shrunk the stars!" The researchers found that, on average, the low-mass stars were about 190 kelvin cooler and only about half the diameter of estimates published in the Kepler mission's star catalog. That translates into candidate planets substantially closer to
Earth's diameter and more likely to lie in the habitable zone around their parent stars: the region where water would remain liquid on the surface of a solid body (see figure).
The Kepler catalog had identifi ed just one planet candidate in the habitable zone among the low-mass stars, with a diameter about twice that of Earth. Muirhead and colleagues' new tally reclassified that object as too cold for life, but it moved six others into the habit-able zone. All have diameters between 0.9 and 1.9 Earth diameters and thus are likely to be rocky like Earth, Muirhead says.
"This paper is a godsend," says planet hunter Geoffrey Marcy of the University of California (UC), Berkeley. "My group is immediately going to adopt these new stellar radii and masses in our work to determine the occurrence of planets around stars of different types."
The new findings for low-mass stars do not affect Kepler's search for planets around sun-like stars, Muirhead emphasizes. But because low-mass stars make up the majority of stars in the cosmos, he says, the results "strongly motivate us to redouble our efforts on these stars" - some of which lie only a few tens of
light-years from Earth.
Andrew Howard of UC Berkeley notes that the new study isn't likely to be the final word because additional refinements of stellar models might increase the size of low-mass stars. That's possible but probably unimportant, Muirhead says: "I think it's unlikely the refinements to the models will change the stellar radii as dramatically as our work has."
Philip Muirhead of the California Institute of Technology in Pasadena and his colleagues used infrared observations to make new, more accurate estimates of the sizes and masses of 84 stars less than half as massive as our sun, which Kepler had examined in visible light.
The properties of such low-mass stars aren't as well understood as those of sunlike stars. Accurate measurements are crucial, Muirhead says, because Kepler searches for planets by detecting tiny dips in starlight produced when an orb transits, or crosses in front of, its home star. Measurements of the size of a planet are only as good as those of the star it transits.
But Muirhead's team had an advantage: One member- Bárbara Rojas-Ayala, now at the American Museum of Natural History in New York City- had recently developed new techniques for estimating the temperatures of low-mass stars and the amounts of "metals" (elements heavier than helium) they contain. At the Palomar Observatory's 5.1-meter Hale Telescope near Escondido, California, the researchers recorded near-infrared spectra of 84 of the 87 low-mass stars around which Kepler had spotted candidate planets. After determining the metal abundance and tem-perature of the stars, the team used a stellar
evolution model to estimate the mass and radii of the stars.
The results might be called a case of "Honey, I've shrunk the stars!" The researchers found that, on average, the low-mass stars were about 190 kelvin cooler and only about half the diameter of estimates published in the Kepler mission's star catalog. That translates into candidate planets substantially closer to
Earth's diameter and more likely to lie in the habitable zone around their parent stars: the region where water would remain liquid on the surface of a solid body (see figure).
The Kepler catalog had identifi ed just one planet candidate in the habitable zone among the low-mass stars, with a diameter about twice that of Earth. Muirhead and colleagues' new tally reclassified that object as too cold for life, but it moved six others into the habit-able zone. All have diameters between 0.9 and 1.9 Earth diameters and thus are likely to be rocky like Earth, Muirhead says.
"This paper is a godsend," says planet hunter Geoffrey Marcy of the University of California (UC), Berkeley. "My group is immediately going to adopt these new stellar radii and masses in our work to determine the occurrence of planets around stars of different types."
The new findings for low-mass stars do not affect Kepler's search for planets around sun-like stars, Muirhead emphasizes. But because low-mass stars make up the majority of stars in the cosmos, he says, the results "strongly motivate us to redouble our efforts on these stars" - some of which lie only a few tens of
light-years from Earth.
Andrew Howard of UC Berkeley notes that the new study isn't likely to be the final word because additional refinements of stellar models might increase the size of low-mass stars. That's possible but probably unimportant, Muirhead says: "I think it's unlikely the refinements to the models will change the stellar radii as dramatically as our work has."
SOURCE : SCIENCE MAGAZINE VOL 333
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