Every so often a comet arrives from nowhere: a long-period comet on an orbit so stretched that it last came near the Sun thousands or millions of years ago. In 1950 the Dutch astronomer Jan Oort noticed that these comets come from all directions and that their orbits all begin very far away. He proposed that the Sun is surrounded by an enormous reservoir of icy bodies. The Estonian astronomer Ernst Öpik had suggested something similar in 1932. Today it is called the Oort cloud.
How far it reaches
The Oort cloud is thought to begin about 2 000 times as far from the Sun as Earth, well beyond the Kuiper belt, and to extend perhaps 100 000 to 200 000 AU, more than a light-year and possibly up to 3. Its outer edge is about where the Sun’s gravity stops dominating and the pull of nearby stars and the galaxy takes over. The inner part is thought to be flatter, a thick disc; the outer part is a roughly spherical shell.
It may contain trillions of objects larger than a kilometre. Yet they are so spread out that the whole cloud might weigh only a few times as much as Earth.
Why we have never seen it
The objects of the Oort cloud are small, dark and almost unimaginably far away, lit only by a Sun that looks from there like a bright star. No telescope can see them directly. The cloud’s existence is inferred from the comets it sends inwards. Passing stars and the tide of the Milky Way’s gravity gently nudge its members, and occasionally one is knocked onto an orbit that brings it in towards the Sun.
Where it came from
The cloud’s bodies are thought to have formed much closer in, among the giant planets, and been flung outward by their gravity early in the Solar System’s history. Some may even have been captured from other stars in the cluster where the Sun was born.
NASA’s Voyager 1, the most distant spacecraft, is now beyond the heliopause, but by NASA’s estimate, which takes the inner edge to be nearer 1 000 AU, it will not reach the Oort cloud for about 300 years, and will not leave the cloud for perhaps 30 000.