About 25 percent of Kepler's stars have a super-Earth that is 1.25 to twice as big as Earth in orbits of 150 days or less. The same percentage of stars have a mini-Neptune that is twice to four times as big as Earth in orbits up to 250 days long, reports astronomer Francois Fressin, with the Harvard-Smithsonian Center for Astrophysics.
Bigger planets are much more rare. Three percent have large Neptune-class planets four to six times as big as Earth and five percent have gas giants six to 22 times Earth's mass in orbits of 400 days or less.
Fressin and colleagues also determined that except for gas giants, planets form around all types of stars, including red dwarfs, the most common type of star in the galaxy.
"When you get into the subject of habitability, which is a very sticky subject, then it might matter that you're around a red dwarf instead of a sun-like star," California Institute of Technology astronomer John Johnson told Discovery News.
"It could be that the 20 or 30 factors that go into habitability that we know of on the Earth might not be met on other planets at all, he said. For example, it might be very rare to have a planet like Earth that is only partially covered with water, instead of completely submerged or with no water at all.