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 relatively tiny worlds would appear billions of times fainter than their parent stars. But in the last two decades astronomers have successfully developed indirect detection methods, most of which rely on measuring the effects of orbiting planets on far-off stars.

In 1992 astronomers reported the first planet-size masses around a dead star, the pulsar PSR1257+12, which sits 2,000 light-years away. Three years later came news of the first known exoplanet, a Jupiter-like gas giant orbiting its star closer than Mercury circles our sun. That world was detected around the sunlike star 51 Pegasi, a mere 50