The researchers created models for 14 different types of solid planets that might exist in our galaxy. The 14 types have various compositions and the team calculated how large each planet would be for a given mass. Some are pure water ice, carbon, iron, silicate, carbon monoxide and silicon carbide; while others are mixtures of various compounds.

"We’re thinking seriously about the different kinds of roughly Earth-size planets that might be out there, like George Lucas, but for real," said Marc Kuchner of NASA's Goddard Space Flight Center.

The team took a different approach from previous studies. Rather than assume planets around other stars are versions of the planets in our solar system, they considered all types of planets that might be possible, given what astronomers know about the composition of protoplanetary disks around young stars.

The research that included Kuchner, Sara Seager of the Massachusetts Institute of Technology, the late Catherine Hier-Majumder of the Carnegie Institution of Washington, and Burkhard Militzer of Carnegie is to be reported in the Oct. 20 issue of the Astrophysical Journal.