Learn / Asteroids, comets and the far edge
The Oort cloud
Far beyond the planets, a vast round shell of icy bodies is thought to surround the Sun. Nobody has seen it. Comets are the evidence.
First look
The idea, in plain words
Every so often a comet turns up that nobody was expecting. It falls in from the dark, swings once round the Sun and heads back out, not to return for hundreds of thousands of years.
These comets come from every direction: from above the planets, from below them and from the sides. So wherever they start, it cannot be a flat ring like the planets’ orbits. It has to be all around us.
Astronomers think the Sun is wrapped in a huge, round shell of icy lumps, like the skin of a ball with the planets as a speck in the middle. It is called the Oort cloud, after the Dutch astronomer Jan Oort, who worked this out.
It is a very long way off. Sunlight reaches the Earth in about eight minutes. It takes between 10 and 28 days to reach the inside of the Oort cloud, and perhaps a year and a half to come out of the far side.
Nobody has ever seen the cloud itself. Each lump is small, dark and much too far away for any telescope. We know it only from the comets that fall out of it, which is why the shell you see here is marked as a model.
How it works
Cause and effect, with real numbers
The Oort cloud is an idea that fits the evidence, not a place anyone has photographed. Everything below is worked out from the comets that arrive from it and from calculations of how the young Solar System behaved. The figures are wide ranges because that is honestly how well they are known.
How far. Distances this large are measured in astronomical units (au). One au is the distance from the Earth to the Sun, about 150 million km.
| Place | Distance from the Sun | Time for sunlight to get there |
|---|---|---|
| Earth | 1 au | A little over 8 minutes |
| Neptune | About 30 au | A little over 4 hours |
| Heliopause, where the Sun’s wind ends | Beyond the Kuiper belt | About 17 hours |
| Inner edge of the Oort cloud | 2,000 to 5,000 au | 10 to 28 days |
| Outer edge of the Oort cloud | 10,000 to 100,000 au | Perhaps a year and a half |
If the outer edge is at 100,000 au, that is about 1.6 light years, more than a third of the way to the nearest star. Voyager 1, the most distant spacecraft, covers about a million miles (1.6 million km) a day. At that speed it will not reach the inner edge for about 300 years, and may need 30,000 years to come out of the far side.
Why a cloud and not a belt. The planets, the asteroid belt and most of the Kuiper belt go round the Sun the same way in roughly the same flat plane. Oort cloud objects are not thought to. Their orbits can be tilted at any angle, passing over, under and around the Sun, so together they fill a thick round shell.
How it got there. The leading idea is that these bodies did not form where they are now. When the planets formed 4.6 billion years ago, a great many leftover lumps called planetesimals remained among them. The gravity of the planets, Jupiter most of all, flung these lumps outwards. Some were thrown out of the Solar System for good. Others ended up on long, stretched orbits, still held by the Sun but so far out that the gravity of the rest of the galaxy tugged on them too. That steady pull from the galaxy is thought to have lifted their orbits clear of the planets, where they have stayed ever since. The cloud may also have picked up some bodies that formed round other stars.
How many. There may be hundreds of billions of icy bodies in the cloud, perhaps trillions. Spread through so much space, they are still very far apart.
The comets it sends us. Now and then something disturbs one of these bodies and it starts a long fall towards the Sun. It arrives as a long-period comet. Two recent visitors:
| Comet | What happened |
|---|---|
| C/2012 S1 (ISON) | Passed too close to the Sun and broke apart |
| C/2013 A1 (Siding Spring) | Passed very close to Mars in 2014 and survived. It will not be back for about 740,000 years |
Most long-period comets have been seen only once in recorded history. Some have orbits so long that our species did not exist the last time they came by.
What would settle it. No spacecraft will reach the cloud in any of our lifetimes, and its members are too faint to see where they are. For now the evidence stays indirect: the paths of the comets, traced backwards, and the calculations that explain how they got out there.
In the 3D view
The view pulls back until the planets' orbits shrink to a dot, and a faint round shell appears all around them. The shell is a model: nothing in it has ever been seen.
Not built yet. This lesson will be able to fly you there.
Words used here
- Oort cloud
- A vast, thin shell of icy bodies thought to surround the Solar System out to tens of thousands of au.
- Comet
- A small body of ice, dust and rock that grows a cloud of gas and a tail when it comes near the Sun.
- Kuiper belt
- A wide ring of icy bodies beyond Neptune, from about 30 to about 50 au from the Sun.
- Astronomical unit
- The average distance between the Earth and the Sun, used as the ruler inside the Solar System.
- Light year
- The distance light travels in one year, about 9.46 million million km.
- Planetesimal
- One of the bodies, from about a kilometre to hundreds of kilometres across, that planets were built from.
- Heliopause
- The edge of the heliosphere, where the solar wind is stopped by the gas between the stars.
- Orbit
- The path one body follows around another, held there by gravity.
Where this comes from
The words are written for this site. Sums in the text are worked from the figures in these sources.