For centuries, planets beyond our solar system—called exoplanets—existed only in theory and science fiction. It seemed nearly impossible to detect planets light-years away, since the ...

This story appears in the December 2009 issue of National Geographic magazine ... To date, astronomers have identified more than 370 “exoplanets,” worlds orbiting stars other than the sun.

Astronomers have discovered an exoplanet that might have favorable ... What distinguishes a super-Earth? As "National Geographic" explains, HD 20794 d is significantly larger than Earth—its ...