The Oort Cloud is a distant shell of icy bodies surrounding the whole solar system.

The longer version

Icy debris marks the outermost region of the solar system, a roughly spherical shell extending far beyond the planets. Its inner edge lies far beyond the orbit of Neptune. Objects there are leftovers from planet formation that were scattered outward by the gravity of the giant planets billions of years ago.

Because they sit so far away, they are only weakly bound to the Sun, and passing stars or galactic tides can gently redirect them inward. Those that fall toward the inner system become long period comets. No spacecraft has reached the region, so its structure is inferred mostly from the comets it sends.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.