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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 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.
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 20th century dubbed this hypothetical planet Phaeton. Today the Phaeton hypothesis, superseded by the accretion model, has been discarded by the scientific community; however, some fringe scientists regard this theory as credible and even likely.
Based on simulations, NASA space scientists John Chambers and Jack Lissauer have proposed the existence of a planet between Mars and the asteroid belt, going in a successively eccentric and unstable orbit, 4 billion years ago. 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. Unlike the Disruption Theory's fifth planet, "Planet V" is not credited with creating the asteroid belt.
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.