The word 'exosphere is derived from the Greek word 'exo' meaning 'outside' or 'external.' The Exosphere is the very edge of our atmosphere. This layer separates the rest of the atmosphere from outer space. The top of the exosphere marks the line between the Earth’s atmosphere and interplanetary space. It starts at an altitude of about 500 km, from the top of the thermosphere, and goes out to about 10,000 km, where it merges into the solar wind.

The air in the exosphere is very thin, and almost completely ionized. It mainly consists of immensely low densities of hydrogen, helium and traces of atomic oxygen and carbon dioxide closer to the Exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. and the particles constantly escape into space. These free-moving particles may migrate in and out of the magnetosphere or the solar wind. The temperature in the exosphere wildly varies and can range from 0 to over 1700º C. It is colder at night and much hotter during the day.

Although the exosphere is the most distance layer of earth's atmosphere it is the planet's first line of defense against the powerful rays of the Sun. It is also the first layer to face and protect the earth from the unknown meteors, asteroids, and cosmic rays. The Exosphere is the ideal region for the satellites as there is very little friction and they can safely orbit without being disrupted. Most of the molecules that exist in the exosphere end up being pulled back into earth's lower atmospheric levels by gravity. However, some zoom out into space instead, due to the low level of gravity and pressure in the exosphere.

Since the Exosphere gradually fades into outer space, there is no clear upper boundary of this layer.