Sunday, October 16, 2011

Star Measurements Hint at Many More Abodes for Life

     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 (
     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."



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