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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, this celestial body is Jupiter.

Previous fifth planets

There are three main ideas regarding hypothetical planets between Mars and Jupiter.

Asteroids

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 orbit as defined by the Titius-Bode law. Between 1845 and 1851, eleven additional asteroids were discovered and Jupiter had become the twentieth planet. At this point, astronomers began to classify asteroids as minor planets. Following the reclassification of the asteroids in their own group, Jupiter became the fifth planet once again. With the redefinition of the term planet in 2006, Ceres is now considered a dwarf planet.

The Disruption Theory

A hypothetical planet between Mars and Jupiter has long been thought to have occupied the space where the asteroid belt is currently located. Scientists in the 18th century dubbed this hypothetical planet Phaeton. Though today the Phaeton hypothesis has been largely discarded by the scientific community (after being superseded by the accretion model), some fringe scientists regard this theory as credible and even likely. This resurgence is largely based on Zecharia Sitchin's readings of Sumerian mythology , regarding the supposed presence of a planet called Tiamat in the writings of ancient astronomers. Also a Planet V has been hypothesized by NASA scientists John Chambers and Jack Lissauer, this planet however would not have created the asteroid belt, in most only a portion of it, see also #The Planet V Theory.

The theories today regarding the formation of the asteroid belt from the destruction of a hypothetical fifth planet are usually collectively referred to as the Disruption Theory. This theory states that there was once a major planetary member of the solar system circulating in the present gap between Mars and Jupiter, which was variously destroyed when:

it veered too close to Jupiter and was torn apart by the gas giant's powerful gravity.
it was struck by another large celestial body.
it was destroyed by a hypothetical brown dwarf, the companion star to the Sun known as Nemesis.
it was shattered by some great internal catastrophe.

In 1988, Donald W. Patten wrote a book entitled Catastrophism and the Old Testament outlining the theory that a planet he called Astra overtook Mars and, upon reaching the Roche Limit, broke apart much like the comet Shoemaker-Levy 9 did when it reached Jupiter's Roche Limit in 1994.

The Planet V Theory

Based on simulations, NASA space scientists Chambers and Lissauer have proposed the existence of a planet between Mars and the asteroid belt, going in a successively eccentric and unstable orbit, before 4,000 Mya. They connect this planet, which they name Planet V, and its disappearance with the Late Heavy Bombardment episode of the Hadean era. Chambers and Lissauer also claim this Planet V most probably ended up crashing into the Sun. This theory is distinguished from the Disruption Theory in that it does not address the creation of the asteroid belt from the remains of the destroyed planet.

Fifth planet in fiction

The concept of a fifth planet which had been destroyed to make the asteroid belt, as in the Disruption Theory, has been a popular one in fiction, especially in James P. Hogan's popular Giants book series. See Asteroids in fiction. The juvenile novels written by Robert Heinlein for Scribner's in the 1950s refer to Planet V as the Ruined Planet, or as Planet Lucifer.