An atmosphere is the gaseous layer that surrounds an astronomical body, such as a planet or a star. The ability of a planet to retain a substantial atmosphere depends on the strength of the planet's gravitational field and its temperature. Mercury, for example, has (virtually) no atmosphere because its gravity is quite feeble and it is also very hot. Pluto, on the other hand, develops a thin atmosphere near perihelion because, although its gravity is weak, its temperature is low enough to prevent gas molecules, released from the surface, from escaping quickly into space. Of the moons of the Solar System, only Titan has a dense atmosphere.