An artist's impression of Kepler-22b, a "Super-Earth" within its star's habitable zone.

Until recently, our solar system was the only planetary system humans knew. It was even thought by some that our solar system was unique in the universe. This changed in the early 1990s, when astronomers began to discover the first exoplanets, worlds orbiting other stars.

Since the first discovery, astronomers have found over a hundred exoplanets. Astronomers now believe that planetary systems are plentiful throughout the universe.