planetary science or planetology, study of planets and planetary systems as a whole. Planetary science applies the theories and methods of traditional disciplines such as astronomy, geology, physics, chemistry, and mathematics to the study of the origin, composition, and distribution of matter in planetary systems, especially the evolution and structure of planets and their natural satellites. The study of planetary systems initially focused on the earth and moon. It was extended to include the other solar planets and their satellites with advances in astronomy, and increased in significance as the lunar and planetary probes of the last half of the 20th cent. returned more and more data. Planetary science burst out of the confines of the solar system with the discovery of extrasolar planets, or exoplanets, in 1995; additional milestones have included the identification of extrasolar multiple-planet systems in 1999, of an extrasolar planet nearly as old as the universe in 2003, of an extrasolar planet smaller than Mercury in 2013, and of a nearly earth-sized extrasolar planet orbiting a star in the habitable zone in 2014. Planetary science is also concerned with the study of life on other planets (see exobiology).