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
Formation and evolution of the Solar System formation of the Solar System. This period of heavy bombardment lasted several hundred million years and is evident in the cratering still visible on geologically dead bodies of the inner Solar System such as the Moon and Mercury. The oldest known evidence for life on Earth dates to 3.8 billion years ago—almost immediately after the end of the Late Heavy Bombardment. Impacts are thought to be a regular (if currently infrequent) part of the evolution of the Solar System. That they continue to happen is evidenced by the collision of Comet Shoemaker–Levy 9 with Jupiter in 1994, the 2009 Jupiter