The Oort Cloud
The Oort Cloud is a gigantic invisible bubble of frozen icy bodies that surrounds the entire Solar System. It is so far out that nothing humans have ever made has reached anywhere near it. Scientists think it contains trillions of icy chunks, all left over from when the planets were forming. Every now and then, one of them is bumped onto a new orbit that sends it falling towards the Sun, and we see a long-period comet.
- What is it?A bubble of icy bodiesSurrounding the Solar System
- How far away?2,000 to 100,000 AUUp to 1,000 times further than Pluto
- How many objects?TrillionsMostly cometary nuclei
- Discovered?Not yetPredicted but never directly seen
- Predicted byJan OortDutch astronomer, in 1950
- Comets it sendsLong-period cometsLike Hale-Bopp (1997)
How far is the Oort Cloud?
Distances in AU. One AU is the distance from Earth to the Sun. (one AU is roughly 150 million km)
The outer edge of the Oort Cloud may be a quarter of the way to the next star. Even Voyager 1, our fastest spacecraft, would take 30,000 years to reach it.
What is the Oort Cloud?
The Oort Cloud is a spherical shell of icy bodies surrounding the Sun and all of its planets. Unlike the Asteroid Belt and the Kuiper Belt, which are flat rings, the Oort Cloud completely envelops the Solar System like a giant bubble. Its inner edge is thought to be around 2,000 AU from the Sun (approx. 40 times further than Pluto) and its outer edge stretches out to perhaps 100,000 AU, almost a quarter of the distance to the nearest star.
Has anyone ever seen it?
No. The Oort Cloud has never been observed directly. Even the biggest objects in it would be incredibly faint at that distance, far too dim for any telescope to spot. We know it must exist because of the comets that arrive from it. Long-period comets come from every direction in the sky, which means their home must be a sphere rather than a flat ring. They take thousands or even millions of years to orbit the Sun, which means their starting points must be enormously far out.
The cloud was first predicted by Dutch astronomer Jan Oort in 1950, who calculated where long-period comets must originate. The Oort Cloud is named after him.
What knocks comets out of the cloud?
The icy bodies in the Oort Cloud orbit the Sun very slowly, taking millions of years to complete one loop. They are so far from the Sun that even small forces can change their orbits.
The main disturbers are passing stars. Every few hundred thousand years, a star drifts close enough to our Solar System to nudge some of the Oort Cloud objects. Some of them get knocked onto orbits that bring them falling all the way back into the inner Solar System. The gravity of the Milky Way galaxy as a whole also gently pulls on the Oort Cloud and can dislodge objects over very long timescales.
When an Oort Cloud object falls towards the Sun, it heats up, its ice starts to melt, and we see it as a long-period comet. Hale-Bopp, which dazzled the night sky in 1997, was an Oort Cloud comet.
Deeper dive: the inner Oort, the Hills cloud and stellar encounters
The Oort Cloud is now thought to have two distinct regions. The outer Oort Cloud (roughly 20,000 to 100,000 AU) is the classical spherical reservoir Jan Oort first proposed. The inner Oort Cloud or Hills cloud (named after astronomer Jack Hills, 1981) lies closer in, roughly 2,000 to 20,000 AU, in a more disc-shaped distribution. The Hills cloud is thought to contain more bodies than the outer Oort, but they are bound more tightly to the Sun and only occasionally feed the outer cloud.
The Oort Cloud objects probably did not form where they are now. The early Solar System was much more compact, and most of the icy planetesimals formed between the orbits of the giant planets. Gravitational scattering by Jupiter, Saturn, Uranus and Neptune flung huge numbers of these objects outward to vast distances. Some were ejected entirely into interstellar space, some fell back into the inner Solar System, and some ended up on stable but very distant orbits. The latter group became the Oort Cloud.
Stellar encounters are the main long-term disturbing influence. Every passing star (roughly one every 100,000 years comes within 1 light-year of the Sun) gives the cloud a small gravitational kick. Galactic tides (the differential pull of the Milky Way) also stir it over million-year timescales. Both effects continuously feed new long-period comets into the inner Solar System. The rate of long-period comets we see is consistent with calculations of these stirring processes.
Some recent theoretical work suggests a possible hypothetical "Planet Nine" in the inner Oort Cloud region could explain unusual orbits of certain distant trans-Neptunian objects (Sedna, 2012 VP113). The search continues but no such planet has been confirmed yet.
For where short-period comets come from, see the Kuiper Belt. For more on icy visitors from these regions, see what is a comet.