What is the Oort cloud?

Picture the Sun with its eight planets packed close around it. Now picture, very far beyond them, a huge round shell of frozen lumps of ice and dust. That shell is the Oort cloud.

It is not a cloud you could see, like one in the sky. The bodies in it are spread so thin that a spacecraft passing through would almost certainly never meet one. There may be hundreds of billions of them, perhaps trillions.

They are leftovers. When the planets formed, the giant planets flung many small icy bodies far outwards. Some escaped. The rest are still held, loosely, by the Sun. They are some of the oldest, least changed material in our neighbourhood.

Nothing in the Oort cloud has ever been seen out there. The bodies are too small, too dark and too far away. So how can we say the cloud exists?

The clues are comets. Of the 4,083 comets in JPL's database, some take thousands or millions of years to go round the Sun once. They arrive from every direction, tilted at any angle, not along the flat plane of the planets. In 1950 the Dutch astronomer Jan Oort worked backwards from their orbits. He proposed that they come from a vast, distant, round shell of icy bodies. Now and then a passing star or the pull of the galaxy nudges one inwards, and it becomes a comet.

Not every comet comes from there. Short-period comets, the kind that return within decades, mostly come from much nearer, beyond Neptune. Halley's Comet is a middle case, with a steep orbit that brings it back every few generations.

The edge of the Solar System depends on what you mean by edge. By gravity, the Oort cloud is the farthest part.

Its inner edge lies thousands of AU from the Sun. One AU is the distance from Earth to the Sun. Its outer edge may lie anywhere between between 10,000 and 100,000 AU. These are estimates from models, and astronomers do not agree on them. NASA says sunlight would take perhaps a year and a half to cross it.

Nearer in, beyond Neptune, lies the Kuiper belt. It is a flat ring of icy worlds, Pluto among them, that thins out around 50 AU. The Kuiper belt is like a nearby lake. The Oort cloud is a mist around the whole countryside. The far edge of the cloud may be 2,000 times as far out as the edge of the belt.

The two get mixed up, so here is the difference in plain words.

The Kuiper belt is flat, like the planets' orbits. It begins just past Neptune. Telescopes have found thousands of its members, and New Horizons flew past two of them. It is the home of most short-period comets.

The Oort cloud is round. It surrounds the Sun on all sides. It starts far beyond the Kuiper belt and the Sun's wind. Not one of its members has been seen in place. It is the home of long-period comets, the ones that visit once and may not return for a million years.

Voyager 1 is the farthest thing we have built. Right now it is 25.8 billion kilometres from the Sun, moving out at 16.92 kilometres per second. It has left the Sun's wind behind, but not the Sun's gravity.

NASA's Voyager team says it will take about 300 years to reach the inner edge of the Oort cloud. That is an estimate, and it depends on where you put the edge: if the edge is as far as some NASA pages say, the trip could take several times longer. Passing through the whole cloud may take about 30,000 years.

Beyond the Oort cloud lies the space between the stars, and then other stars. The nearest is Proxima Centauri, part of the Alpha Centauri system, 4.2465 light-years away. That is only about 2.7 times the widest estimate for our own cloud.

Astronomers expect other stars to have clouds like ours. If Alpha Centauri has one as large, the two may reach a good part of the way towards each other, with only a gap of a few tens of thousands of AU between them. Nobody knows, because neither has been seen. But the idea stands: our Solar System does not end at a wall. It fades, and somewhere out there it meets the reach of our neighbours.