Saturn's moon of Titan is a hollow planetoid around a Celestial-built microfusion star. It was established by the Celestials to power a research facility to keep watch on genetic experimentation on Earth.
The Eternals of Earth relocated from their city of Titanos some time after the raising of the Demiurge. They dubbed their home New Titan and became known as the Titanians.
Titan is currently a relatively thriving colony of several hundred people and acts as an informal 'point of contact' to dissuade major intergalactic powers from colonizing the Solar System. The Titanians have taken it upon themselves to protect Earthlings from influences from beyond the Solar System.