Planets about one and four times the Earth's size common around other stars

Researchers have claimed that NASA's Kepler mission has discovered that planets between about one and four times the size of Earth are quite 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.

Lead author Lars A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA), said they 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, adding that's where rocky worlds would be found, which are the only kind that they would consider potentially habitable.

Buchhave and his colleagues measured the amount of elements heavier than hydrogen and helium, which astronomers collectively call metals, in stars with exoplanet candidates. Since a star and its planets form from the same disk of material, the metallicity of a star reflects the composition of the protoplanetary disk.

The team took follow-up spectra of more than 400 stars hosting over 600 exoplanets. Then, they conducted a statistical test to see if the sizes of the planets fell into natural groups, along with the stellar metallicities.

They found two clear dividing lines-one at a size 1.7 times as large as Earth and the other at a size 3.9 times larger than Earth. They infer that these boundaries also mark changes in composition. Planets smaller than 1.7 Earths are likely to be completely rocky, while those larger than 3.9 Earths are probably gas giants.

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.