Titan is Saturn's largest moon. It's occasionally deemed Saturn IV. What makes this piece of idle rock so significant is the fact that is has a dense atmosphere, an extremely abnormal trait in a satalite; and more notably, the only other object in space where surface liquid has been found. Often described as being a planet-like-moon, Titan's diameter is rougly 48% the size of Earth's.
The planet actually depicts a scene somewhat similar to Venus prior to the Space Age. Information regarding its chemical makeup was gathered during the Cassini-Huygens mission in 2004. The largest shock was the fact that Titan supported liquid hydrocarbon lakes primarily in its Polar Regions - the only confirmed location of these lakes in the Universe outside of Earth. Geologically, the planet is very young, hosting only a few dozen impact craters.
The atmosphere of Titan is primarily Nitrogen, thus rendering it uninhabitable for humanoids, but it certainly does propose potential. Microbial extroterrestrial life is a very real possibility on Titan, meaning that, essentially, we have a case of Aliens on Titan.