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 primary occupants of Titan have been the Eternals, for approximately 750,000 years. Multiple civil wars ravaged the population several times. It 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.