An atmosphere, a term derived from the 17th century New Latin ‘atmosphaera’ combining Greek words ‘atmos’ meaning ‘vapor’ and ‘sphaira’ meaning ‘sphere,’ is a layer of gases enveloping a planet or another celestial body of significant mass. This gaseous layer is held in place by the gravitational pull of the body it surrounds. It is more likely to be retained if the celestial body’s gravity is high and the atmospheric temperature is low.

The Earth’s atmosphere, primarily composed of nitrogen, also contains vital elements like oxygen, used by most organisms for respiration, and carbon dioxide, essential for photosynthesis in plants, algae, and cyanobacteria. Additionally, it provides protection to living organisms from harmful solar ultraviolet radiation. The current composition of the Earth’s atmosphere has evolved over billions of years due to biochemical modifications by living organisms.

The term ‘atmosphere’ can also be applied beyond planetary contexts. For instance, the term ‘stellar atmosphere’ pertains to the outermost region of a star, usually starting from the opaque photosphere and extending outwards. In stars with sufficiently low temperatures, compound molecules may even form in their outer atmosphere. Consequently, an atmosphere, while typically associated with planets, can also describe the gaseous environs of diverse astronomical bodies.