Artwork of a synestia. This is a hypothetical torus of molten rock that is believed to form when two similarly sized rocky bodies collide - one may have arisen when the Earth was struck by a Mars-sized world called Theia, inside of which the Moon was born. As the rapidly rotating structure cools, magma droplets solidify on its outer edges and rain inwards, where they gather together to form a central body. Further out, smaller fragments form which do not fall inwards but instead coalesce into a smaller body orbiting the central one.