# What is the Oort cloud?

> The Oort cloud in simple terms: a vast shell of icy bodies no one has seen, how far it reaches, how it differs from the Kuiper belt, and Voyager 1's trip there.

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The Oort cloud is a vast, thin shell of icy bodies that astronomers think surrounds the whole Solar System. No one has ever seen it. We know it is there because some comets fall towards the Sun from very far away, from every direction. NASA thinks it may hold between 200 billion and 2 trillion icy bodies, and its outer edge may lie between 10,000 and 100,000 AU from the Sun.

- Voyager 1's distance from the Sun: 25,769,046,578 km
- Time for Voyager 1 to reach the inner edge of the Oort cloud: 300 years
- Number of known comets: 4,083
- Distance from the Sun to the outer edge of the main Kuiper belt: 50 AU

## A shell of ice around us

The Oort cloud in simple terms

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.

## Seen by no one, known anyway

How we know it is there

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.

## How far it reaches

How far is the Oort cloud?

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.

## Kuiper belt and Oort cloud, side by side

Kuiper belt vs Oort cloud

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's long road

Is Voyager 1 past the Oort cloud?

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.

## Where the Sun's reach meets the next star's

What lies beyond the Oort cloud?

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.

## FAQ

### What is the Oort cloud in simple terms?

It is a huge, round shell of icy bodies far beyond the planets, held loosely by the Sun's gravity. NASA thinks it may hold between 200 billion and 2 trillion of them. It is the source of comets that take thousands of years or more to orbit the Sun.

### Is the Oort cloud real?

Most astronomers think so, but it has never been seen directly. It was proposed by Jan Oort in 1950 to explain long-period comets that fall in from every direction. Its size and number of bodies are model estimates.

### How far away is the Oort cloud?

Its inner edge is thousands of AU from the Sun, and its outer edge may be between 10,000 and 100,000 AU away. Astronomers do not agree on either figure.

### Is Voyager 1 past the Oort cloud?

No. Voyager 1 has left the Sun's wind, but NASA estimates it needs about 300 years just to reach the cloud's inner edge, and perhaps about 30,000 years to pass through it.

### What is the difference between the Kuiper belt and the Oort cloud?

The Kuiper belt is a flat ring just beyond Neptune, ending around 50 AU, and many of its members have been seen. The Oort cloud is a round shell much farther out, and none of its members has been seen in place.

## Related

- [Where does the Solar System end?](https://www.theuniversecounts.com/solar-system/the-edge/where-does-the-solar-system-end.md)
- [Voyager 1](https://www.theuniversecounts.com/solar-system/spacecraft/voyager-1.md)
- [The edge](https://www.theuniversecounts.com/solar-system/the-edge.md)

Every long-period comet you might see has been falling towards us for longer than people have kept records. What would you want to ask the first one we visit?

## Sources

- Distance from the Sun to the outer edge of the main Kuiper belt (Estimated): [NASA: Kuiper Belt: Facts](https://science.nasa.gov/solar-system/kuiper-belt/facts/)
- Distance from the Sun to the outer edge of the Oort cloud (Debated): [NASA: Oort Cloud: Facts](https://science.nasa.gov/solar-system/oort-cloud/facts/)
- Distance to Proxima Centauri (Measured, with error): [Astronomy & Astrophysics: Gaia Data Release 3: Summary of the content and survey properties (Gaia Collaboration, Vallenari et al. 2023)](https://doi.org/10.1051/0004-6361/202243940); [ESA: Gaia Data License](https://www.cosmos.esa.int/web/gaia-users/license)
- Earth's average distance from the Sun (Measured): [NASA NSSDCA: Earth Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
- Jupiter's average distance from the Sun (Measured): [NASA NSSDCA: Jupiter Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html)
- Neptune's average distance from the Sun (Measured): [NASA NSSDCA: Neptune Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html)
- Number of icy bodies in the Oort cloud (Debated): [NASA: Oort Cloud: Facts](https://science.nasa.gov/solar-system/oort-cloud/facts/)
- Number of known comets (Measured): [NASA JPL: JPL SBDB Query API, comets](https://ssd-api.jpl.nasa.gov/sbdb_query.api?fields=pdes&limit=1&sb-kind=c)
- Time for Voyager 1 to pass beyond the Oort cloud (Estimated): [NASA: Voyager Frequently Asked Questions](https://science.nasa.gov/mission/voyager/frequently-asked-questions/)
- Time for Voyager 1 to reach the inner edge of the Oort cloud (Estimated): [NASA: Voyager Frequently Asked Questions](https://science.nasa.gov/mission/voyager/frequently-asked-questions/)
- Voyager 1's distance from the Sun (Measured): [NASA JPL: JPL Horizons API, Voyager 1 heliocentric vectors (geometric)](https://ssd.jpl.nasa.gov/api/horizons.api?format=text&COMMAND='-31'&OBJ_DATA='NO'&MAKE_EPHEM='YES'&EPHEM_TYPE='VECTORS'&CENTER='500@10'&START_TIME='2026-10-18'&STOP_TIME='2026-10-19'&STEP_SIZE='1d'&VEC_TABLE='3'&VEC_CORR='NONE'&OUT_UNITS='KM-S'&TIME_TYPE='UT')
- Voyager 1's distance from the Sun when it crossed the heliopause (Measured): [American Geophysical Union: Voyager Observations of the Heliospheric Interface (AGU Fall Meeting 2014 abstract)](https://studio.m-anage.com/agu/fm14/webprogram/Paper4456.html); [NASA: Voyager Interstellar Mission](https://science.nasa.gov/mission/voyager/interstellar-mission/)
- Voyager 1's speed relative to the Sun (Measured): [NASA: Voyager 1: What Is a Light-Day?](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/)