Beyond Neptune

Far out of the solar system lies the Kuiper Belt and Oort cloud: mostly undiscovered area rich in asteroids and comets!

Kuiper Belt Objects (KBOs) and the Oort Cloud are rarely talked about. The planets tend to hog the spotlight, but today is the day we shine the light in the furthest part of the solar system.

A gorgeous artist’s impression, but I cannot find a name to thank!

The Kuiper Belt is home to hundreds of thousands of icy and rocky worlds, and makes up a huge amount of the solar system’s volume. It is estimated that the Kuiper belt is about 20 Astronomical Units wide which is about 2’992’000’000 km, making it nearly 20 x wider than the Asteroid Belt!

Temperature in this region approaches absolute zero, meaning most materials here will be frozen or solid. Asteroids are mostly rock and a mixture of ices: ammonia ice, water ice, and methane ice.

Most of these KBOs are minuscule compared to the planets; in 2009, Hubble had found the smallest Kuiper Belt object yet so far: a 980m asteroid!

Image credit: NASA SSE

The Kuiper Belt is not just a torus of asteroids. It has structure and can be split into 2 main parts: the classical belt and the Scattered disk.

The inner region is called the classical belt because KBOs within it orbit peacefully around the sun; their orbits are unaltered by Neptune’s gravity, nor are they heavily tilted or eccentric. Everything our classical model of the Kuiper belt described.

These KBOs are “cold”, because they haven’t interacted with Neptune. However, within the classical belt are also some “hot” objects, where Neptune has had some effect on the asteroid, tilting and squashing its orbit.

Pluto and Charon. Credits: NASA/Johns Hopkins University.

If the KBO’s orbit and Neptune’s orbit can be represented as an exact ratio, then the object’s orbit is in resonance with Neptune. Objects that are resonant are controlled by Neptune’s gravitational force. The most common resonance ratio is 2:3, where for every 2 orbits a KBO makes, Neptune makes exactly 3. Objects with this resonance are called plutinos (and yes, it includes Pluto).


Overlapping the classical belt is the scattered disk: a barren area, home to unstable, tilted, and eccentric objects. These most likely have been flung into the disk by Neptune. There are a few objects in the scattered disk with a stable orbit; Eris, a dwarf planet and the largest scattered disk object, has a stable but highly eccentric orbit (sometimes it’s closer to the sun than Pluto).

Artist’s impression of the Kuiper Belt. Credit: ESO/M. Kornmesser

The Kuiper belt is a well-known source of comets. When KBOs collide they create fragments. Some of these become comets and are pulled by Neptune’s gravity towards the sun (with the help of Jupiter).

Once they reach the sun, the ices on them deplete and they become “dead” comets, but total depletion doesn’t usually happen in one go. If the comet doesn’t crash into the sun, Jupiter’s gravity may enclose the comet into a small loop or launch the comet back out into the Kuiper belt.

The Oort Cloud

The other main source of comets, specifically long-period comets, is the Oort Cloud.

The size of the Oort cloud vs everything else. Credit: ESO/L. Calçada

The Oort cloud completely surrounds the solar system like a giant bubble and is approximately 87’000 AU (1.4 light years) thick!

An impression of the Oort cloud – comic book style! By Don Dixon

The rock and ice that make the cloud most likely were flung out forcefully by planetary gravity, settling far away from the planets but close enough to still be under the sun’s influence. The debris doesn’t orbit like asteroids in a belt, but rather in multiple directions like a swarm of bees.


Very little is known about comet behaviour in the Oort cloud. To begin its journey towards the sun, a comet must be disturbed, most likely by a collision with another piece of debris. Just like Kuiper Belt comets, the Oort cloud comet will either crash into something or exhaust most or all its ices and get flung out by Jupiter and Neptune.

Sliding Spring, a comet from the Oort cloud that passed close to Mars, survived its journey to the sun in 2014 and is still on its way back home.

Sliding Spring in 2014 Credit: NASA, ESA, J.-Y. Li (PSI), C.M. Lisse (JHU/APL), and the Hubble Heritage Team (STScI/AURA)

A Kuiper Belt asteroid that’s recently been in the new is Ultima Thule. It is actually composed of two asteroids that have collided. Researchers are still trying to figure out how both the asteroids joined, but they believe both objects most likely came into contact very slowly (or they’ll obliterate each other) and were spinning rapidly at one point which flattened them out.

NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory

It’s been a pleasure writing about the amazing edge of the solar system. To end this post, I recommend you find some red and blue cellophane, and look at this artist’s asteroid rendering in 3D!


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