The fifth planet - the Phaeton hypothesis

The Phaeton hypothesis from Wikipedia. An interesting history of a strong hypothesis, consistent with much data but now displaced by the accretion 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 Sergej Vladirimovich 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:

It veered too close to Jupiter and was torn apart by its 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 internal catastrophe

Today, the Phaeton hypothesis has been superseded by the accretion model.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.

Displaced but not yet fully buried.