Two stars orbiting a common gravitational center. It was long thought that no planets could form or keep stable orbits in such systems because the stars would pull each others protoplanets out of orbit with their gravity. But several planets have been observed in such systems and modern calculations show that wide binaries can have separate planetary formation and orbits around both stars while close binaries can have a common protoplanetary disk and then stable orbits for those planets. Only a few "mid-spaced" binaries are unsuitable for planets, accounting for only a few percent of them, most of which are extremely eccentric.