Scattered disc

The scattered disc (or scattered disk) is a distant area in our Solar System filled with icy objects. These objects, called scattered-disc objects (SDOs), have very stretched-out paths, or orbits, around the Sun. Their orbits can be tilted and stretched so much that they sometimes come close to the Sun and sometimes move very far away.

Although the closest scattered-disc objects come near the Sun at about 30 to 35 times the distance from the Earth to the Sun, their orbits can stretch well beyond 100 times that distance. This makes these objects some of the coldest and farthest known things in our Solar System. The inner part of the scattered disc overlaps with a ring-shaped area called the Kuiper belt, but the scattered disc stretches much farther out and above and below the usual plane of the Solar System.

Astronomers think the scattered disc is where most periodic comets in our Solar System come from. These comets pass by the inner Solar System regularly. Before becoming comets, these icy objects pass through a stage called centaurs, which are found between the planets Jupiter and Neptune. Over time, the gravity of the giant planets pushes these objects toward the Sun, turning them into comets. Many objects that might be in the far-off Oort cloud are also believed to have started in the scattered disc. Some special objects, like Sedna, are sometimes thought to belong to this group as well.

Discovery

See also: History of the Kuiper belt

In the past, astronomers used special tools to find objects in space by watching how they moved between two pictures. This was slow because it needed old cameras called photographic plates. In the 1980s, new cameras called CCDs were used on telescopes. These cameras could take pictures much faster, helping scientists find many new objects.

The first object known as a scattered-disc object was found in 1996. Since then, more than 200 of these distant objects have been discovered, including some very interesting ones like Eris and Sedna. Even though there might be as many scattered-disc objects as objects in the Kuiper belt, we haven't seen as many because they are much farther away.

Subdivisions of trans-Neptunian space

Main article: Trans-Neptunian object

We know objects that are beyond the planet Neptune and we group them into different areas. One area is called the Kuiper belt. It is a ring of space that is about 30 to 50 times farther from the Earth than the Sun is.

Another area is the scattered disc. Objects here have orbits that are stretched out and tilted. Some scientists think there might be a third area, called the Oort cloud, even farther out, but we have not seen it directly.

The scattered disc is different from the Kuiper belt. Objects in the scattered disc can come close enough to Neptune to feel its gravity. This changes their paths. These objects can travel very far from the Sun, much farther than objects in the Kuiper belt. Some objects, called centaurs, might be scattered disc objects that Neptune has pulled closer to the inner Solar System.

Orbits

The scattered disc is a busy area where objects move in ways that change because of the pull from the planet Neptune. These objects are called scattered-disc objects (SDOs). They can swing far out from the Sun or come closer to it, sometimes becoming comets near Jupiter. Unlike objects in a place called the Kuiper belt, SDOs can tilt up to 40° compared to the plane where most planets orbit, called the ecliptic.

SDOs usually have paths that stretch far from the Sun, with a semi-major axis longer than 50 AU, but they come close enough to Neptune, around 30 AU, for Neptune to affect them. This is different from classical objects, like cubewanos, which mostly stay in more circular paths with smaller tilts. Even though SDOs move in many directions, they often end up in paths that match Neptune’s orbit for a while.

Formation

See also: Formation and evolution of the Solar System

The scattered disc was formed when objects from the Kuiper belt were pushed into unusual orbits by the gravity of Neptune and other outer planets. This happened either over a very long time or more quickly when Neptune moved to its current position.

Scientists use computer programs to study how this might have happened. Some think Neptune’s movement caused many objects to end up in the scattered disc. This helped shape the orbits of the outer planets.

Composition

Scattered objects, like other trans-Neptunian objects, are made mostly of frozen materials such as water and methane. They are not very heavy for their size.

Scientists once thought these objects would look red. But scattered objects often look white or grey. One idea is that impacts have shown brighter layers underneath. Another idea is that because scattered objects are far from the Sun, their methane freezes and creates a bright ice layer. This makes them look paler.

Comets

Main article: Comet § Short period

Early ideas thought that a place called the Kuiper belt gave us many comets in our Solar System. But studies since 1992 show these comets really come from the scattered disc, where their paths can change more easily.

Comets are mainly of two kinds: short-period and long-period. Short-period comets include Jupiter-family comets and Halley-type comets. Jupiter-family comets likely begin in the scattered disc, while Halley-type comets, like famous Halley's Comet, come from a faraway area called the Oort cloud. There are also objects called centaurs, which are thought to be a middle step between the scattered disc and Jupiter-family comets.