Phaeton (or Phaëton, less often Phaethon) is the hypothetical planet posited to have existed between the orbits of Mars and Jupiter whose destruction supposedly led to the formation of the asteroid belt. The hypothetical planet was named for Phaethon, the son of the sun god Helios in Greek mythology, who attempted to drive his father's solar chariot for a day with disastrous results and was ultimately destroyed by Zeus.
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The asteroid 3200 Phaethon, sometimes incorrectly spelled Phaeton, is a Mercury-, Venus-, Earth-, and Mars-orbit crossing Apollo asteroid with unusual properties.
Phaeton hypothesis
According to the discredited Titius–Bode law, a planet was believed to exist between Mars and Jupiter. After observing the discoveries made by the German astronomer and professor Johann Daniel Titius (1729–1796), Johann Elert Bode himself urged a search for the fifth planet. When Ceres, the largest of the asteroids in the asteroid belt (now considered a dwarf planet), was serendipitously discovered in 1801 by the Italian Giuseppe Piazzi and found to match the predicted position of the fifth planet, many believed it was the missing planet. However, in 1802 astronomer Heinrich Wilhelm Matthäus Olbers discovered and named another object in the same general orbit as Ceres, the asteroid Pallas.
Olbers proposed that these new discoveries were the fragments of a disrupted planet that had formerly orbited the Sun. He also predicted that more of these pieces would be found. The discovery of the asteroid Juno by Karl Ludwig Harding and Vesta, by Olbers, buttressed the Olbers hypothesis.
In 1823, German linguist and retired teacher Johann Gottlieb Radlof called Olbers' destroyed planet "Phaëthon," linking it to the Greek myths and legends about Phaethon and others. His ideas were similar to those later advocated by Immanuel Velikovsky. Despite Radlof's precedence, Russian authors of the 20th century claimed that, "The hypothetical planet of Olbers' was left nameless for a century and a half. Only in 1949 did the well-known Soviet astronomer S[ergej] V[ladirimovich] Orlov give it the name Phaeton... This name has become established."
Theories regarding the formation of the asteroid belt from the destruction of a hypothetical fifth planet are today collectively referred to as the "disruption theory". This theory states that there was once a major planetary member of our solar system circulating in the present gap between Mars and Jupiter, which was variously destroyed when:
Today, the Phaeton hypothesis has been superseded by the accretion model. Most astronomers today believe that the asteroids in the main belt are remnants of the protoplanetary disk, and in this region the incorporation of protoplanetary remnants into the planets was prevented by large gravitational perturbations induced by Jupiter during the formative period of the solar system.
Other hypotheses
Some scientists and non-scientists continue to advocate for the existence and destruction of a Phaeton-like planet.
Zecharia Sitchin suggested that the goddess known to the Sumerians as Tiamat in fact relates to the planet that was destroyed by a rogue planet known as Nibiru, creating the asteroid belt. His work is widely regarded as pseudoscience.
The astronomer and author Tom Van Flandern held that Phaeton exploded through some internal mechanism. In his "Exploded Planet Hypothesis 2000", he lists possible reasons for its explosion: a runaway nuclear reaction in uranium in the core, a change of state as the planet cooled down creating a density phase change, or through continual absorption of heat in the core from gravitons.
In 1972, Soyuzmultfilm studies produced an animated short film titled Фаэтон - Сын Солнца (Phaeton: The Son of Sun), directed by Vasiliy Livanov (Василий Ливанов), in which the subject of the asteroid belt is treated as the remains of a planet.
Phaeton in literature
Several works of fiction feature a supposed planet (sometimes named Phaeton) existing in the past between the orbits of Mars and Jupiter, which somehow became the Solar System's asteroid belt.