Astra.

Space  There is a theory that the asteroid belt was the product of a notional "fifth" planet which existed between the orbits of Mars and Jupter until a cataclysm led to its destruction.

"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 the 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 the gas giant's 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.

The planet is most often called Phaeton. But sometimes Astra.