Phaeton (alternatively Phaethon or Phaëton) was the hypothetical planet theorized by the Titius–Bode law to have existed between the orbits of Mars and Jupiter, the destruction of which supposedly led to the formation of the asteroid belt (including the dwarf planet Ceres). 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. However, his name was also used historically for Jupiter itself as well.
According to the hypothesized Titius–Bode law, a planet was believed to exist between Mars and Jupiter. After learning of the regular sequence discovered by the German astronomer and mathematician J.D. Titius (1729–1796), astronomer Johann E. Bode urged a search for the fifth planet corresponding to a gap in the sequence. (1) Ceres, the largest asteroid in the asteroid belt (now considered a dwarf planet), was serendipitously discovered in 1801 by the Italian Giuseppe Piazzi and found to closely match the "empty" position in Titius' sequence, which led many to believe it to be the "missing planet". However, in 1802 astronomer Heinrich W.M. Olbers discovered and named the asteroid (2) Pallas, a second object in roughly the same orbit as (1) Ceres.
Olbers proposed that these two discoveries were the fragments of a disrupted planet that had formerly orbited the Sun, and predicted that more of these pieces would be found. The discovery of the asteroid (3) Juno by Karl Ludwig Harding and (4) Vesta by Olbers, buttressed his hypothesis.
In 1823, German linguist and retired teacher J.G. Radlof called Olbers' destroyed planet Phaëthon, linking it to the Greek myths and legends about Phaethon and others. The idea was similar to those later advocated by Immanuel Velikovsky but only in that the catastrophe was in recent times. 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 Sergej Vladirimovich Orlov give it the name Phaeton."
In 1927, Franz Xaver Kugler wrote a short book titled Sibyllinischer Sternkampf und Phaëthon in naturgeschichtlicher Beleuchtung (The Sybilline Battle of the Stars and Phaeton Seen as Natural History). The central idea in Kugler’s book is that the myth of Phaethon was based on a real event: Making use of ancient sources, Kugler argued that Phaeton had been a very bright celestial object that appeared around 1500 BC which fell to Earth not long afterwards as a shower of large meteorites, causing catastrophic fires and floods in Africa and elsewhere.
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 destroyed by one or more of the following hypothetical processes:
In 1953, Soviet Russian astronomer I. I. Putilin suggested that Phaeton was destroyed due to centrifugal forces, giving it a diameter of approximately 6,880 kilometers and a rotational speed of 2.6 hours. Eventually, the planet became so distorted that parts of it near its equator were spun off into space. Outgassing of gases once stored in Phaeton's interior caused multiple explosions, sending material into space and forming asteroid families. However, his theory was not widely accepted. Two years later in 1955, Odessan astronomer K. N. Savchenko suggested that Ceres, Pallas, Juno, and Vesta were not fragments of Phaeton, but rather its former moons. Phaeton had an additional fifth satellite, assumed to be the size of Ceres, orbiting near the planet's Hill sphere, and thus more subject to gravitational perturbations from Jupiter. As a result, the fifth satellite became tidally detached and orbited the Sun for millions of years afterward, making periodic close misses with Phaeton that slowly increased its velocity. Once the escaped satellite re-entered Phaeton's Hill sphere, it collided with the planet at high speed, shattering it while Ceres, Pallas, Juno, and Vesta assumed heliocentric orbits. Simulations showed that for such a Ceres-sized body to shatter Phaeton, it would need to be travelling at nearly 20 kilometers per second.
The disrupted planet hypothesis has also been supported by French-Italian mathematician and astronomer Joseph-Louis Lagrange; Canadian geologist Reginald Daly in 1943; American geochemists Harrison Brown and Clair Patterson; Soviet academics Konstantin Savchenko, Alexander Zavaritskiy, Vasily Fesenkov, Ivan Putilin, and Otto Schmidt; British-Canadian astronomer Michael Ovenden; and American astronomer Donald Menzel. Ovenden suggested that the planet be named "Krypton" after the destroyed native world of Superman, as well as believing it to have been a gas giant roughly eighty-five to ninety Earth masses in mass and nearly the size of Saturn.
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 that never formed a planet, and that in this region the amalgamation of protoplanets into a planet was prevented by the disruptive gravitational perturbations of Jupiter during the formative period of the Solar System.
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 a planet that was destroyed by a rogue planet known as Nibiru, creating both Earth and the asteroid belt. His work is widely regarded as pseudoscience.
The astronomer and author Tom Van Flandern held that Phaeton (which he called "Planet V", with V representing the Roman numeral for five and not to be confused with the other postulated former fifth planet not attributed to the formation of the asteroid belt) exploded through some internal mechanism. In his "Exploded Planet Hypothesis 2000", he lists possible reasons for its explosion: a runaway nuclear reaction of uranium in its 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. Van Flandern even suggested that Mars itself may have been a moon of Planet V, due to its craters hinting to exposure to meteorite storms and its relatively low density compared to the other inner planets.
In 1972, Soyuzmultfilm studios produced an animated short film titled Phaeton: The Son of Sun (Russian: Фаэтон - Сын Солнца), directed by Vasiliy Livanov, in which the asteroid belt is portrayed as the remains of a planet. The film also has numerous references to ancient astronauts.
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.
Presque toutes ces petites planètes circulent entre les orbites de Mars et Jupiter. On admet qu'elles représentent les fragments dispersés d'une grande planète qui se serait désintégrée [inter alia]