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 called blink comparators to find objects in space by looking at how they moved between two pictures. This was slow because it needed photographic plates or films. In the 1980s, new cameras called CCDs were used on telescopes. These cameras could take electronic pictures much faster and better, allowing scientists to discover 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

Known objects beyond the planet Neptune are grouped into different areas. One group is called the Kuiper belt, which is a ring of space from about 30 to 50 times the distance from the Earth to the Sun. Another group is the scattered disc, which has objects with more stretched-out and tilted orbits. 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 because its objects can come close enough to the planet Neptune to feel its gravity, which 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 have been pulled closer to the inner Solar System by Neptune.

Orbits

The scattered disc is a busy area where objects can have orbits that change due to the pull of the planet Neptune. These objects, called scattered-disc objects (SDOs), can swing far out or come closer to the Sun, sometimes becoming comets near Jupiter. Unlike other objects in the area known as 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, such as ratios like 1:3 or 2:7.

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 process helped shape the orbits of the outer planets as well.

Composition

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

Originally, scientists thought all these objects would look red because of chemical changes on their surfaces. However, scattered objects often appear white or grey. One idea is that impacts have exposed brighter layers beneath. Another idea is that because scattered objects are farther from the Sun, their methane freezes and creates a layer of bright ice, making them look paler.

Comets

Main article: Comet § Short period

Early ideas suggested that a region called the Kuiper belt was the source of many comets in our Solar System. But research since 1992 shows that these comets actually come from the scattered disc, where their paths 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 start in the scattered disc, while Halley-type comets, like famous Halley's Comet, come from a distant area called the Oort cloud. There are also objects called centaurs, which are thought to be a middle stage between the scattered disc and Jupiter-family comets.