In Greek mythology, Theia, goddess or divine, (sometimes written Thea or Thia), also called Euryphaessa, wide-shining, was a Titan. The name Theia alone means simply, "goddess"; Theia Euryphaessa (Θεία Εὐρυφάεσσα) brings overtones of extent (εὐρύς eurys "wide", root: εὐρυ-/εὐρε-) and brightness (φάος phaos "light", root: φαεσ-).
According to the giant impact hypothesis, there was once a Mars-sized body referred to as Theia orbiting in our solar system. The planet was named after the Greek Titan who gave birth to the Moon goddess Selene—a fitting name considering that the planet Theia is thought to be responsible for the birth of our moon.
Theia would have formed in about the same orbit as Earth, but about 60° ahead or behind. When the protoplanet had grown to be about the size of Mars, its size made it too heavy for its orbit to remain stable. As a result, its angular distance from Earth varied increasingly, until it finally it crashed into the Earth.
The collision would have occurred circa 4.533 billion years ago when Theia would have hit the Earth at an oblique angle, and destroyed herself in the process. Theia's mantle and a significant portion of the Earth's silicate mantle were thrust into space. The left over materials from Theia mixed with the materials from the Earth and eventually formed the Moon.