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
Iron planet In science fiction, such a planet has been called a "Cannonball". An extrasolar planet candidates that may be composed mainly of iron is KOI-1843 b Iron planet ] An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in the Solar System (as the other terrestrial planets are silicate planets), but larger iron-rich exoplanets may exist. Iron is the sixth most abundant element in the universe by mass after hydrogen, helium, oxygen, carbon, and neon. There are also theorized helium planets