Atmosphere, the mass of gases surrounding a planet or any other celestial object. Earth's atmosphere is categorized by temperature, into troposphere, stratosphere, mesosphere, and thermosphere. Atmospheres are known to surround most of the solar system's planets and at least one natural satelliteSaturn's Titan. Stars, such as the sun, also have atmospheres. The atmospheres of Earth and some of the other planets contain liquid and solid particles (for example, water droplets and dust) dispersed within the gases.

An atmosphere is held in place by the gravitational attraction of the celestial object it surrounds. Although the term atmosphere means "sphere of air," only the atmosphere of Earth is known to contain the particular combination of gases known as air. Other planets have atmospheres of different gases. Jupiter's atmosphere, for example, appears to consist primarily of hydrogen and helium, together with small amounts of methane and ammonia. The atmospheres of stars contain large quantities of hydrogen and helium.

There are several theories about the origin of planetary atmospheres. One theory is that most of the planets at one time had atmospheres similar to the present atmospheres of Jupiter and Saturn. However, intense solar radiation changed the conditions on the planets closest to the sun. Virtually all of Mercury's atmosphere was driven off. Venus, Earth, and Mars lost large portions of their atmospheres, particularly the lighter elements and compounds, leaving atmospheres with heavier gases predominating.

An atmosphere is densest near the surface of the celestial object it surrounds. At increasing distances from the surface, the atmosphere becomes less and less dense until it cannot be distinguished from the vacuum of space.