So Many 'Earths'

共享题干题【2015年真题】                                         （一） So Many 'Earths'
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     The Milky Way ( 银河) contains billions of Earth-sized planets that could support life that's the finding of a new study. It draws on date that came from NASA's top planet-hunting telescope.
     A mechanical failure recently put that Kepler space telescope out of service. Kepler had played a big role in creating a census of planets orbiting some 170, 000 stars. Its data have been helping astronomers predict how common planets are in our galaxy. The telescope focused on hunting planets that might have conditions similar to those on Earth.   
     The authors of a study, published in The Proceedings of the National Academy of sciences, conclude that between 14 and 30 out of every 100 stars, with a mass and temperature similar to the Sun, may host a planet that could support life as we know it. Such a planet would have a diameter at least as large as Earth's, but no more than twice that big. The planet also would have to orbit in a star's habitable zone. That's where the surface temperature would allow any water to exist as a liquid.
    The new estimate of how many planets might fit these conditions comes from studying more than 42, 000 stars and identifying suitable worlds orbiting them. The scientists used those numbers to extrapolate ( 推算 ) to the rest of the stars that the telescope could not see. The estimate is rough, the authors admit. If applied to the solar system, it would define as habitable a zone starting as close to the Sun as Venus and running to as far away as Mars. Neither planet is Earthlike (although either might have been in the distant past). Using tighter limits, the researchers estimate that between 4 and 8 out of every 100 sunlike stars could host an Earth-sized world. These are ones that would take 200 to 400 days to complete a yearly orbit.
    Four out of every 100 sunlike stars doesn't sound like a big number. It would mean, however, that the Milky Way could host more than a billion Earth-sized planets with a change for life.