Fifth planet (hypothetical) Fifth planet (hypothetical) In the history of astronomy, a handful of Solar System bodies have been counted as the fifth planet from the Sun. Under the present definition of a planet, Jupiter is counted as the fifth. There are three main ideas regarding hypothetical planets between Mars and Jupiter. During the early 19th century, as asteroids were discovered, they were considered planets. Jupiter became the sixth planet with the discovery of Ceres in 1801. Soon, three more asteroids, Pallas (1802), Juno (1804), and Vesta (1807) were discovered. They were counted as separate planets, despite the fact that they shared an
IAU definition of planet IAU definition of planet The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which: Among other things, this definition caused Pluto to no longer be a planet, a change from how it had been widely considered until that point. A non-satellite body fulfilling only the first two of these criteria (such as Pluto) is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB).