Forget looking for UFOs. Scientists have found a more practical way to guesstimate which planets are most likely to harbor extra-terrestrial life, simply by determining which planets are most similar to Earth. They declared Saturn's moon Titan to be one of the most habitable alien worlds, along with the exoplanet Gliese 581g, which is believed to reside some 20.5 light-years away from Earth. Scientists factored in a planet's size, density, and distance from its parent star in making their conclusions. Other factors considered were whether the world has a rocky or frozen surface, whether it has an atmosphere, and whether it has a source of energy, either through light from a parent star or via tidal flexing, in which gravitational interactions with other objects create energy in the form of heat.