One theory states that there was once a fifth planet orbiting the Sun between the orbits of Mars and Jupiter. It was a rocky planet and about 2 times the size of Earth. Although it was formed, it was broken up by Jupiter's gravity and its remains are now the Asteroid Belt.
There have been various proposed names for this Hypothetical planet, many named after gods or goddesses from various mythological stories.
A common name for this planet is Phaeton, suggested by Heinrich Wilhelm Matthäus Olbers. This is another name for an asteroid. It is named after the son of the Greek god Helios, Others call this planet Tiamat, after the primordial mmonster goddess of the ocean from Babylonian mythology.