From ScienceDaily:
The planets of our solar system come in two basic flavors, like vanilla and chocolate ice cream. We have small, rocky terrestrials like Earth and Mars, and large gas giants like Neptune and Jupiter. We’re missing the astronomical equivalent of strawberry ice cream — planets between about one and four times the size of Earth. NASA’s Kepler mission has discovered that these types of planets are very common around other stars.
New research following up on the Kepler discoveries shows that alien worlds, or exoplanets, can be divided into three groups — terrestrials, gas giants, and mid-sized “gas dwarfs” — based on how their host stars tend to fall into three distinct groups defined by their compositions.
“We were particularly interested in probing the planetary regime smaller than four times the size of Earth, because it includes three-fourths of the planets found by Kepler. That’s where you’ll find rocky worlds, which are the only kind that we would consider potentially habitable,” says lead author Lars A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA).
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Planets between 1.7 and 3.9 times the size of Earth were dubbed gas dwarfs since they have thick atmospheres of hydrogen and helium. The rocky cores of gas dwarfs formed early enough to accrete some gas, although they did not grow as large as gas giants like Jupiter.
See also: Copernicus, you are not going to believe who is using your name. Or how.
“Behold, countless Earths sail the galaxies … that is, if you would only believe …”