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.
A primordial atmosphere is an atmosphere made from the original material in a protoplanetary disk and retained since the formation of a planet. Gas giants, such as Jupiter and Saturn, have primordial atmospheres.
A secondary atmosphere is an atmosphere produced by the outgassing of volatile compounds from the rocky material of a terrestrial-type planet. Venus, Earth, and Mars, for example, all possess secondary atmospheres.