Kuiper Belt


The Kuiper Belt is located in the outer areas of our solar system, just past Neptune’s orbit, and is believed to be materials that were left over from the formation of the planets.

Although the official name is the Kuiper-Edgeworth Belt, most people just call it the Kuiper Belt. It has a disk-shape that extends from the orbit of Neptune all the way out to almost 50 AU (astronomical units) from the sun.
Kuiper Belt

The Kuiper Belt is believed to be home to millions of icy, small bodies that are mostly made up of nitrogen, methane, ammonia, and water. The Kuiper Belt is nearing 20-200 times bigger than the asteroid belt and about 20 times wider.

Basic Kuiper Belt Information:

The Kuiper Belt takes up an enormous volume within our planetary system. It is a huge region in the cold, outer regions of our solar system that are often called the “third zone.”

Astronomers believe that millions of small to large, icy objects exist in this area including hundreds of thousands that are bigger than 60 mi/100 km wide. The dwarf planet Pluto is in the Kuiper Belt and is part of those objects that are over 600 mi/1,000 km wide.

Besides water ice and rock, Kuiper Belt objects also contain many other compounds that are frozen such as methane and ammonia.

Some researchers refer to the area as the Edgeworth-Kuiper Belt, while others call it the Trans-Neptunian Region.

They refer to the objects as KBOs (for Kuiper Belt objects) or TNOs (for Trans-Neptunian objects).

History of the Name:

The astronomer, Kenneth Edgewood presented a 1943 proposal to speculate that large celestial objects and comets might exist in larger numbers just outside of the planet Neptune.

Although his proposal was thought to just be a theory, the Dutch astronomer, Jan Oort, proposed that the comets that we see in our solar system had their origins in the outskirts of the system. Oort’s 1950 theory included the fact that he thought this was a vast unexplored area.


When scientists confirmed his observation, they named the Oort Cloud after him. By 1951, an additional prediction was made by Gerard Kuiper regarding the huge area outside of Neptune that contained asteroids and other bodies.

Since both Kuiper and Edgewood had made close predictions, the scientific community named the area of discovery after both astronomers as the Edgewood-Kuiper Belt.


Scientists believe that the Kuiper Belt contains the leftover debris from the solar system formation. It is similar to the relationship between Jupiter and the asteroid belt in that it’s an area of objects that could have formed a planet if Neptune hadn’t been there.

The amount of debris and material that’s in the Kuiper Belt today may be only a small percent of what was there when it first formed.

One theory that is well-supported indicates that as the orbits of the four giant planets of Jupiter, Saturn, Uranus, and Neptune shifted, it caused a majority of the original material to be lost.

TheKuiperBelt PowerLaw2

The theory is that this was 7-10 times the mass of our Earth. The theory is based on the idea that early in our solar system’s history, Neptune and Uranus were pushed out to orbit farther from the sun due to other shifts occurring by Saturn and Jupiter.

As the planets continued to drift further out they passed through a dense disk of icy smaller bodies that were left over after the giant planets had formed. Since Neptune’s orbit is the furthest out, its gravity began to bend the pats of the icy bodies inward to head toward the other giant planets.

Jupiter’s gravity is the strongest and as they neared Jupiter it created a “slingshot” effect for most of the icy bodies to either head to orbits of extreme distances, such as the Oort Cloud or completely outside of the solar system.

As Neptune pushed icy objects toward the sun this created a condition where its own orbit started to drift even further out, with its gravity forcing the rest of the icy objects that remained into the area that we find them today in the Kuiper Belt.

The objects that are in the Kuiper Belt occasionally hit each other, making the objects smaller and more fragmented. This has caused the Kuiper Belt to slowly erode away.

Smaller objects may be created such as comets and the tiniest dust is blown out of the solar system with solar winds.

Structure and Surface:

The Kuiper Belt is a huge donut-shaped area of space in our outer solar system. There are many icy bodies in the area that we refer to as KBOs (Kuiper Belt Objects) or TNOs (trans-Neptunian objects).

All of these are a variety of shapes, colors, and sizes, and they aren’t distributed very evenly in space. Astronomers were surprised to find that KBOs were often grouped together based on their shapes and sizes of their orbits.

This led to the understanding that there were several very distinct and different groups and the orbits might offer a clue for this origins and history. Scientists discovered that the category an object belongs to has a lot to do with its interaction over time with Neptune’s gravity. A majority of the Kuiper Belt objects are located in the main belt or in the scattered disk.

Classical KBOs

A big portion of the KBOs orbit the sun in what is referred to as the classical Kuiper Belt. The word “classical” refers to the belief that these KBOs have orbits that are the most similar to their original or “class” idea of what the Kuiper Belt was anticipated to be like prior to astronomers finding objects.

Astronomers of the past expected that objects beyond Neptune would have rather circular orbits that weren’t tilted too far away from the planetary plane. However, many KBOs were instead found to have elliptical as well as tilted orbits.

The two main groups within classical Kuiper Belt are called “hot” and “cold.” Instead of referring to temperature, these actually describe the object’s orbits as well as the amount of gravitational influence that Neptune has on them.

All of the classical KBOs share the similarity in the average distance from the sun of around 40-50 AU. Cold classical KBOs have more of a circular orbit that isn’t tiled that much from the plane of planets, while hot classical KBOs have a more elliptical and tilted orbit that astronomers call eccentric or inclined, respectively.

What this means is that cold classical KBOs spend a majority of their time at around the same distance from the sun, while hot KBOs may wander all over in wider ranges of distances from the sun. The orbits of hot KBOs can take them close to the sun and then farther away.

It seems that Neptune is the main influence that causes the differences between the cold and hot classical KBOs. Cold classical KBOs have orbits that never get near to Neptune and they are therefore “cool” and unbothered by Neptune’s gravity.

They have orbits that have probably been unchanged for billions of years. Hot classical KBOs have had interactions with Neptune in the past and they are affected by its gravity.

The interactions cause energy to be pumped into their orbit causing their shape to be stretched to be elliptical and tilts them out of the plane of planets.

Resonant KBOs:

There are some KBOs whose orbits are completely controlled by Neptune. Their orbit is called being in “resonance” with the giant planet, and it means that their orbits are stable and repeat with Neptune’s.

The resonant KBOs have a specific number of orbits in the same amount of time that it takes Neptune to complete a specific number of orbits.

There are a few resonant KBOs that have the same relationship with the dwarf planet Pluto and they have been assigned their own category within resonant KBOs as the plutinos.

Scattered Disk:

Beyond the main part of the Kuiper Belt is a region called the scattered disk. It’s where objects that were scattered by Neptune have been thrown and their orbits are very elliptical and highly inclined to the plane of the planets.

Some of these objects have orbits that head out as far as hundreds of AU from the sun and well above the plane of planets, and then return back to a closer area near Neptune’s orbit.

Their orbits are ever changing and evolving and are unlike the classical Kuiper Belt that has stable orbits. The scattered disk is what gives the Kuiper Belt its wide, donut-shape.

Other family objects in the Kuiper Belt:

While a majority of the objects in the Kuiper Belt are located in the main belt part or scattered disk, there are a few other “families” of objects that do orbit the sun inside and outside of the belt.

These objects were more than likely originally from the Kuiper Belt but were then pulled away by gravity from Neptune or one of the other big planets.

Detached objects:

These are objects within the Kuiper Belt that never come closer to the sun than around 40 AU. They are different from almost all other KBOs that spent at least a portion of their orbits closer to the sun that 40 AU.

Neptune’s distance from the sun is (~30 AU), and since detached objects don’t come close to Neptune’s distance scientists don’t think they were pulled out of the Kuiper Belt through interactions with Neptune.

They do believe that there is more than likely another force that is responsible for their orbits such as an undiscovered giant planet in a distant orbit, the gravity of passing stars, or even gravitational alterations that happened in the formation of the Kuiper Belt. An example of a detached KBO is the dwarf planet Sedna.


Centaurs are the objects that have orbits that are traveling through the space between the orbits of Neptune and Jupiter.

During their orbits they interact with the giant planet’s gravity, and because the gravities are so strong, they will eventually either be pushed into the inner solar system to become comets or crash into planets or the sun, or be completely ejected from the solar system entirely.

The ultimate fate of the Centaurs is constantly happening and can take tens of millions of years. Scientists believe that the evidence that there are still Centaurs remaining today could be due to the fact that they are being supplied or replenished from another location.

One explanation is that they have escaped the Kuiper Belt. Centaurs are considered to be scattered objects like those in the scattered disk, but unlike those the Centaurs are being scattered closer to the sun by the interaction with Neptune rather than farther away.

Space Visits:

  • New Horizon: launched 2006: New Horizon traveled through the asteroid belt and the Kuiper Belt on its way to visit and study the dwarf-planet Pluto and its moons.

Facts about Kuiper Belt for Kids:

  • Many of the short-period comets have been tracks by astronomers from their Kuiper Belt origins to follow their orbital periods of 200 years or less.
  • Objects in the Kuiper Belt range in size with the largest known objects being Pluto, Quaoar, Makemake, Haumea, Ixion, and Varuna; which are also often called TNOs (Trans-Neptunian Objects).
  • In exploring other galaxies, scientists have so far found that structures that are similar to our Kuiper Belt exist around at least nine other stars.
  • The study of the ice that is in Kuiper Belt objects show that the ice dates back as far as the solar system formation.
  • Kuiper Belt research helps scientists to understand the conditions of our early solar system nebula.
  • Scientists have a rough estimate that the Kuiper Belt could contain nearing 35,000 objects that have diameters larger than 100 mm.
  • The results of the New Horizons NASA mission flyby of Pluto brought more data and images of Pluto and its moon Charon and other moons within the Kuiper Belt. Before this mission, images were only grey and blurry.
  • Scientists think that Neptune’s moon, Triton, was once in the Kuiper Belt and then captured by Neptune’s gravitational pull.