The outermost layers of a planet's or moon's atmosphere where gas molecules are so spread out they escape into space.
From Greek 'exo-' (outside) + 'sphaira' (sphere). The term emerged in 20th-century planetary science to describe the transitional region between atmosphere and space where particles have enough energy to escape gravity.
The exosphere is where the last wisps of Earth's air say goodbye to our planet—it's the fuzzy boundary where our atmosphere just...gives up trying to stay close. Satellites orbiting there are technically in space, but still occasionally collide with stray oxygen atoms!
Complete word intelligence in one call. Free tier — 50 lookups/day.