Answer:
An atmosphere is the layer of gases that surrounds a planet.
Step-by-step explanation:
The definition above is exactly what an atmosphere is.
All planets have atmospheres, of varying compositions depending on the location of the planet, the distance from the star they go round, the composition of the planet itself, and the presence or not of other celestial bodies nearby like moons or large asteroids.
Mars for example, has a very thin atmosphere, something that makes it hard for life to prosper in the red planet (so far, we have no evidence of life in Mars), and also makes it very cold, because atmospheres also act as mitigators of planetary climates.