Planets that orbit stars other than the Sun. Since 1995, the number of such planets known has increased from zero to more than 100. Prior to the discovery of extrasolar planets, theoretical studies suggested that most planetary systems would look like the solar system, with small rocky planets orbiting near the star and the larger gas giants further out, all in nested concentric quasicircular orbits. These theories have been disproven, and in their place a much richer understanding of planet formation and evolution has emerged.