Scattered Disc

The Scattered Disc is a disc of planetesimals, dust and stuff in the outer solar system beyond the orbit of Neptune, and beyond the edge of the Kuiper Belt. Whereas KBOs generally enjoy stable orbits, those in the scattered disc are wild and random.

This photograph of Arrokoth was taken from Nasa’s New Horizons spacecraft 1st January 2019 [Nasa/JHUAPL/SwRI]; and Thomas Appéré for colour processing.
A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept “infinitesimal”, which indicates an object too small to see or measure. Planetesimals can be anywhere in size from several metres to hundreds of kilometres.

Scattered Disc Objects (SDOs) are among the coldest and most distant objects in the solar system. They reside beyond the Kuiper Belt in a vague sparsely populated band between 4.8 billion kilometres and 150 billion kilometres from the Sun. SDO clumps may overlap with the outer reaches of the Kuiper. There are no SDOs closer to the Sun than about 5.2 billion kilometres. Below that range they would fall into the gravitational influence of Neptune and begin to normalise their orbits.

Astronomers believe that SDOs originated from the Kuiper Belt having been dislodged by the gravitational forces of Neptune. They also believe that most periodic comets in the Solar System emanate from the scattered disc region.

Typically, SDOs are made up of water ice or frozen methane. They are cold volatile objects with extreme and unstable orbits that bring them near the Sun in cycles that range from decades to thousands of years. Unlike most other objects in the solar system, such as the planets and most asteroids, these objects have highly inclined and eccentric orbits, with circular orbits being the exception rather than the rule. Their random orbital paths range from around 4 480 million kilometres at perihelion to as far as 15 billion kilometres at aphelion or more. Furthermore, their orbits are way off the planetary plane, diving into the solar system like Kamikazi pilots. Some orbits are as crazy as 55° to the plane of the ecliptic.

Eris is the largest member of the SDO area, discovered in 2005, and now classified as a dwarf planet. It is ranked second in size behind Pluto.

SDOs are a subset of the broader family of trans-Neptunian objects.

Trans-Neptunian Object – Arrokoth

A trans-Neptunian object (TNO), also written “trans-Neptunian object”, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune. Beyond the orbit of Neptune, thousands of TNOs are contained within a thick band called the Kuiper Belt. Objects in the Kuiper Belt are thought to be remnants from the formation of the solar system, having failed to coalesce into a fully-fledged planet.

Arrokoth is a particularly interesting TNO. It was discovered on 26th June 2014 by NASA’s New Horizons science team, using the Hubble Space Telescope. Arrokoth is about 6.6 billion kilometres from Earth and is the most distant and most primitive object ever explored by a spacecraft. Technically Arrokoth is a contact binary 36 kilometres long, composed of two planetesimals 21 kilometres and 15 kilometres across. Why it is interesting is because the boffins think it demonstrates a more robust theory of how planets formed. It was previously thought they formed from collisions. (I find this puzzling because I would have thought a collision would shatter rather than join). Anyway that was the perceived theory until the Arrokoth findings. Now they think that planets form from gradual accretion. (Again I would have thought this was obvious!).

The theory of how Arrokoth formed is now as follows:

  • A rotating cloud of small icy bodies start to coalesce in the outer solar system;
  • Eventually two larger bodies remain;
  • These two bodies slowly spiral closer until they touch, forming the “bi-lobed” object we see today.

This is now the perceived process for all planetary formations. Nasa explains further that Arrokoth is actually two objects that gently merged into one. End to end, it measures about 35 kilometres long. It’s about 20 kilometres wide and 10 kilometres thick.

Arrokoth is in a region of space beyond Neptune called the Kuiper Belt that is swarming with small, icy and ancient objects. Because they are so far from the Sun, Kuiper Belt objects have only been slightly heated since forming, and are thought to be well-preserved, frozen samples of what the outer solar system was like after its birth more than 4.5 billion years ago. It takes Arrokoth about 293 Earth years to make one trip around the Sun.

New Horizons flew by Arrokoth on 1st January 2019 at a distance of 3 538 kilometres, snapping images that showed a double-lobed object that looked like a partially flattened snowman. It’s also very red (loaded with iron, perhaps?).

Arrokoth means “sky” in the Powhatan/Algonquian language. Maybe scientists should create a new instrument called a “sizemometer”. Its job would be to monitor celestial objects to record their growth through accretion.


One SDO candidate is Sedna. But, as it orbits much further out from the sun, along the outer reaches of the scattered disc, some observers refer to it as  a “detached inner Oort Cloud Object.”

There are three known members of the Sednoids group, Sedna being the big chief. They are described as “detached objects” because, although they orbit the sun, they are unaffected by any interplanetary gravitational influences. For example, such objects are detached as their orbital paths are not influenced by Neptune or any of the other planets. They have highly elliptical orbits around the sun and could be said to be on their own mission.

The orbital range of the sednoids is estimated at 11.4 billion kilometres at perihelion and as much as 148 billion kilometres at aphelion. As this takes them to the outer edges of the solar system they are also referred to as “inner Oort Cloud objects”. Sedna was discovered in 2004 and is the furthest known object in the solar system.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar

Last updated: Wednesday, 1st April 2020