 The idea of a counter-Earth has never been a serious scientific hypothesis in modern times.Phaëton is a name given to a supposed planet existing in the past between the orbits of Mars and Jupiter, which no longer exists, having become the Solar System's asteroid belt.Proposed not long after the discovery of multiple asteroids at the beginning of the 19th century, the idea that the asteroids were fragments of a single planet was gradually abandoned over the course of the middle decades of the 20th century in favor of the conclusion that no planet had ever accreted in the region of the asteroid belt in the first place.