A new explanation for forming "super-Earths" suggests that they are more likely to be found orbiting red dwarf stars - the most abundant type of star - than gas giant planets like Jupiter and Saturn. The theory, by Dr. Alan Boss of the Carnegie Institution's Department of Terrestrial Magnetism, describes a mechanism whereby UV radiation from a nearby massive star strips off a planet's gaseous envelope exposing a super-Earth. The work explains recent extrasolar planet discoveries by the microlensing method.

Boss had previously shown that red dwarf stars are likely to form gas giant protoplanets rapidly by the disk instability mechanism, whereby the gaseous disk forms spiral arms and self-gravitating protoplanets that would become Jupiters in the absence of any interference.

However, most stars form in regions where massive O stars eventually form. Such stars emit immense amounts of ultraviolet (UV) radiation, which strips off the disk gas around young stars, exposing their outer protoplanets to UV and stripping away their gaseous envelopes.

"It dawned on me that because UV stripping depends on the mass of the central star, super-Earths should be found on much smaller orbits around a red dwarf than around the Sun," Boss says. "This idea naturally predicts red dwarfs that form close to massive stars will end up with super-Earths orbiting at the distances where super-Earths have been found by microlensing."

Red dwarfs that form in the absence of massive stars will not suffer UV stripping and hence will form gas giant planets at these distances, instead of super-Earths. Such stars are in the minority so red dwarfs should be orbited mostly by super-Earths at asteroidal distances and beyond. This prediction agrees with the microlensing detections to date.

It remains to be seen if Boss's theoretical predictions will be verified by the ongoing microlensing searches and by the space-based planet detection missions being planned by NASA and the European Space Agency. Determining the compositions of super-Earths will be a major challenge with important implications for their habitability.