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SAN JOSE, Calif. — One of the most common kinds of planets detected by NASA's Kepler telescope appears to be a type that doesn't exist in our own solar system, a leading astronomer on the Kepler team said Friday.
This type of planet has a size in the range between two and four times Earth's diameter, but it shouldn't be called a "super-Earth" or a "mini-Neptune," said Berkeley astronomer Geoff Marcy, one of the world's most experienced planet-hunters. For now, he's calling them "sub-Neptunes."
Based on an analysis of the Kepler planets' sizes and densities, sub-Neptunes should have a rocky core that's swathed in a thick layer of hydrogen and helium gas. That combination distinguishes them from rocky planets like Earth, as well as gas giants like Jupiter and ice giants like Neptune.
"They dominate the planet census, and yet none of them are found in the solar system," Marcy said here during a symposium at the annual meeting of the American Association for the Advancement of Science.
Marcy said the analysis suggests that rocky planets can't get much larger than 1.5 to two times Earth's width. But that doesn't mean we should give up on finding alien analogs to Earth, he said. The Kepler mission's scientists already have identified scores of planets that are less than twice Earth's width, and they say our Milky Way galaxy must have lots more such worlds.
"There are billions of Earth-size planets, and many of them exist in the habitable zone," said NASA researcher Bill Borucki, the Kepler mission's principal investigator. "The question is, why hasn't SETI picked up the signal?"
Another member of the Kepler science team, Natalia Batalha of San Jose State University and NASA's Ames Research Center, showed off a list of 29 potential super-Earths that lie within their parent stars' habitable zones, where liquid water and possibly life could conceivably exist.
One of the aims of the Kepler mission is to identify potentially habitable Earth-class planets, a category known as eta-Earth.
"We now have a very highly reliable sample of small-planet candidates in the habitable zone of both M- and K-type stars [red and orange dwarfs] that will enable an eta-Earth determination for this class of stars," Batalha said.
She added that similar determinations may be made for some of the small planets that Kepler has detected around sunlike stars, known as G-type stars. However, it's still debatable whether the candidates on Kepler's current list should be classified as rocky planets in the traditional sense, or as sub-Neptunes.
Batalha's list doesn't yet include any Earth-size planets in Earthlike orbits around sunlike stars, but after Friday's symposium, she hinted that it may not be long before such long-sought worlds start popping up in the Kepler database.
"There are going to be more," she told NBC News.