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Theia was a planet-sized body about as large as the planet Mars that more or less had shared the same orbit as Earth until its destruction 4.5 billion years ago, resulting in the creation of our Moon.

Earth's "twin" is usually referring to Venus with almost the same size and mass as Earth, but not a whole lot more in common.

Thea was a titaness in Greek mythology.
Source(s):
http://en.wikipedia.org/wiki/Theia_(plan?
the planetary body that gave birth to the moon.

http://en.wikipedia.org/wiki/Theia
The mythical figure sometimes spelled as Thea