How big is Oort Cloud? | Scale of the Universe

The Oort Cloud is a vast, roughly round shell of icy objects that astronomers think surrounds the Solar System, far beyond the planets. Its outer edge is thought to lie somewhere between 10,000 and 100,000 astronomical units (au) from the Sun, where 1 au is the average distance from Earth to the Sun. The size given here is about 18 trillion kilometers, or 1.9 light-years, across; a light-year is the distance light travels in one year. That puts the edge about 60,000 au out, a value within that range but not a measurement, because the cloud is known only indirectly, from comets.

Size

No one has measured the size of the Oort Cloud. It is so far away that NASA has no direct images of it. Much of what astronomers know about the cloud comes from studying comets that pass near Earth.

NASA puts the cloud's inner edge between 2,000 and 5,000 au from the Sun, and its outer edge between 10,000 and 100,000 au. When the Dutch astronomer Jan Oort proposed the cloud in 1950, he pictured it reaching out to about 100,000 au. An edge that far out would make the cloud about 200,000 au, or 3.2 light-years, across.

Size comparisons

Neptune orbits about 30 au from the Sun. At the width given here, the Oort Cloud is about 2,000 times as wide as the orbit of Neptune.
If the distance from Earth to the Sun were shrunk to 1 centimeter, Neptune would orbit 30 centimeters (12 inches) from the Sun. The Oort Cloud would begin 20–50 meters away and end somewhere between 100 meters and 1 kilometer (0.6 miles) away.
An edge 60,000 au from the Sun is about a fifth of the distance from the Sun to Proxima Centauri, 4.2 light-years. Even an edge at 100,000 au would be less than two-fifths of the way.
The Boomerang Nebula, a cloud of gas around a dying star, is about 2 light-years long, close to the width given here for the Oort Cloud.
The Voyager 1 spacecraft travels about 1.6 million kilometers (1 million miles) a day. NASA estimates it will take about 300 years to reach the Oort Cloud and perhaps 30,000 years to pass through it.

Why astronomers think it is there

Oort proposed the cloud to explain where long-period comets come from. These comets take so long to orbit the Sun that most have been seen only once in recorded history. They also arrive from every direction, not along the flat plane that the planets share.

The objects in the cloud circle the Sun at many different angles, forming a thick bubble rather than a flat belt, which is why it is called a cloud. There may be hundreds of billions, or even trillions, of icy bodies in it.

Now and then something disturbs one of them, and it begins a long fall toward the Sun. The comet Siding Spring made a very close pass by Mars on one such trip, and will not return for about 740,000 years.

How it formed

The leading idea is that the Oort Cloud's objects did not form where they are now. After the planets formed, 4.6 billion years ago, many leftover chunks called planetesimals, made of the same material as the planets, remained among them. The gravity of the planets, mainly Jupiter, flung these chunks in every direction. Some left the Solar System entirely.

Others were thrown into long orbits that carried them far enough out for the gravity of the Milky Way galaxy to tug on them. The galaxy's pull likely moved them into the outer edges of the Solar System, where the planets could no longer disturb them. One review estimates that about 5% of the material the giant planets scattered ended up in the Oort Cloud. The cloud may also have captured objects that did not form in the Solar System.