But a study released this week found that data gleaned from NASA’s now defunct Kepler spacecraft and the W. M. Keck Observatory found that the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. The study was discussed at NASA's Ames Research Center at the second Kepler Science Conference.
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For NASA, the researchers said, the fact that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. Scientists from the University of California, Berkeley, and University of Hawaii performed the study. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."
The researchers were quick to caution that Earth-size planets in Earth-size orbits do not necessarily support life, even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold.
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms," said Geoffrey Marcy, UC Berkeley professor of astronomy in a statement. "We don't know what range of planet types and their environments are suitable for life."
NASA launched the now crippled Kepler space telescope in 2009 to look for planets that cross in front of, or transit, their stars, which causes a slight diminution – about one hundredth of one percent – in the star's brightness. From among the 150,000 stars photographed every 30 minutes for four years, NASA's Kepler team reported more than 3,000 planet candidates. Many of these are much larger than Earth – ranging from large planets with thick atmospheres, like Neptune, to gas giants like Jupiter – or in orbits so close to their stars that they are roasted, the researchers stated.
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For its study the researchers said they focused on the 42,000 stars that are like the sun or slightly cooler and smaller, and found 603 candidate planets orbiting them. Only 10 of these were Earth-size, that is, one to two times the diameter of Earth and orbiting their star at a distance where they are heated to lukewarm temperatures suitable for life. The team's definition of habitable is that a planet receives between four times and one-quarter the amount of light that Earth receives from the sun.
If the stars in the Kepler field are representative of stars in the solar neighborhood, then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye. Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars. In January, the team reported a similar analysis of Kepler data for scorched planets that orbit close to their stars. The new, more complete analysis shows that "nature makes about as many planets in hospitable orbits as in close-in orbits," said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. "
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