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
Fifth Planet (novel) Fifth Planet (novel) Fifth Planet is a science fiction novel written by astrophysicist Sir Fred Hoyle and his son Geoffrey Hoyle. Another star is due to pass close to the sun, close enough for conventional spacecraft to reach it. The first planets observed are four gas giants, but then an inner 'Fifth Planet' is found. Signs of chlorophyll are detected, suggesting that it supports life. Rival Soviet and US expeditions are launched to visit it. (The world balance as it existed in 1963 is assumed to be still in place.) One important aspect of the plot is that the Soviet