Kuiper Belt — The Ring of Asteroids Beyond Neptune

by Carson

You have probably heard that Pluto was the ninth planet, but do you know that it’s just a member of a large reservoir of asteroids, comets, and dwarf planets? That region is the Kuiper Belt. Let’s explore it in this article.

Where is the Kuiper Belt?

The Kuiper Belt is a region beyond the orbit of Neptune, about 30 to 50 astronomical units from the Sun. Therefore, objects there are Trans-Neptunian Objects (TNOs) because they orbit the Sun outside Neptune. Moreover, it looks like a donut instead of a sphere because most materials are in low-inclination orbits.

How Many Objects Are There in the Kuiper Belt?

Despite being farther out, the Kuiper Belt is much larger and more massive than the asteroid belt. Scientists estimate that there are a few hundred thousand objects larger than 100 km across in the Kuiper Belt. That dwarfs the asteroid belt, which only has a few hundred such objects. In fact, some estimates show that the number of dwarf planets in this region could be among the tens or even the hundreds. That is a few hundred objects hundreds of kilometers wide, compared to just a few dwarf planet candidates in the asteroid belt. There could even be billions, if not trillions, of smaller objects inside this region!

But there are only a few thousand trans-Neptunian objects ever discovered. That’s because this area is so far away. As such, asteroids no larger than a few kilometers wide may be too faint for telescopes to detect. Moreover, the orbital speeds of these objects are so slow that it is not easy to detect movement from Earth-based photos. That’s why some dwarf planets there, like Haumea, Makemake, and Eris, weren’t found until the 21st century! The same reason is also why the Oort Cloud remains hypothetical — there are no direct observations yet.

However, the Kuiper Belt doesn’t look massive compared to the planets in our Solar System. It probably weighs no more than 10% of Earth’s mass. However, that’s still enormous relative to the asteroid belt, which weighs just a few percent of the Moon’s mass.

What Are the Objects in the Kuiper Belt Made Of?

Objects in the Kuiper Belt tend to be icy. That means they are rich in volatiles such as water, ammonia, and carbon dioxide. These materials should also be present in the inner Solar System, so why aren’t they as prominent? That’s because these volatiles evaporate when they encounter high temperatures near the Sun. However, at lower temperatures beyond the outer planets, these chemicals are preserved in a solid state inside the objects, making them particularly icy.

How Did the Kuiper Belt Form?

The Kuiper Belt formed similarly to the asteroid belt. It started no different from any other region in the Solar System. In the nebula where the Sun formed, leftover material becomes countless planetesimals that would coalesce to form planets. However, in the end, this area did not create any planets, similar to the case in the asteroid belt. So what is keeping these objects separate and intact?

The mechanism of keeping the Kuiper Belt is the same as that maintaining the asteroid belt. It is the gravitational pull of nearby planets. For example, Jupiter’s gravity perturbs asteroids between Mars and Jupiter so that they do not collide at low speeds and accrete into planets. Likewise, Neptune’s gravity affects the objects in the Kuiper Belt so that they remain separate objects instead of combining into one. Therefore, Neptune moderates the Kuiper Belt like Jupiter moderates the asteroid belt.

That’s why these objects are remnants of the early Solar System. This makes them intriguing targets for spacecraft missions to study. However, due to the vast distances from Earth to this region, only one mission, New Horizons, has visited any objects there.

How to classify Kuiper Belt Objects?

Kuiper Belt objects are classified by their orbital characteristics. In the first layer, they consist of two separate groups: Resonant Kuiper Belt objects and classical Kuiper Belt objects.

Resonant Kuiper Belt objects are in orbital resonances with Neptune, which means that there is a simple integer ratio between the object’s orbital period and Neptune’s. For instance, Pluto orbits the Sun in about 248 years, and it turns out that many other minor planets are co-orbital with it, with similar orbital periods. That’s not because Pluto’s gravity is trapping these things into resonances. Instead, they are all in the same 2:3 resonance with the planet Neptune. As a result, whenever they orbit the Sun twice, Neptune goes around our star three times. Hundreds of known objects are in this group, named “plutinos” after the dwarf planet Pluto. Likewise, objects in the 1:2 resonance group orbit the Sun once when Neptune runs two laps around the Sun, and are called the “twotinos”.

Neptune vs plutinos vs twotinos (distances to scale)
The innermost circle in blue denotes the orbit of Neptune
The intermediate circle in red represents the average orbit of plutinos from the Sun
The outermost circle in green represents the average orbit of twotinos from the Sun
Note that the orbits of these objects are not perfectly circular in reality

On the other hand, classical Kuiper Belt objects, or cubewanos, are not resonant with Neptune. It is further split into two categories: “cold” and “hot” objects. Despite the terminology, this has nothing to do with the temperature of these objects or how far they are from the Sun. Instead, they describe the volatility of their orbits. “Cold” objects have more stable and unperturbed orbits that are more circular and closer to the ecliptic. On the other hand, “hot” objects have gravitationally interacted with Neptune so that they are in more eccentric and inclined orbits.

Cold vs hot Kuiper Belt objects
The two orbits are shown in white. The less inclined orbit belongs to 15760 Albion, which is a “cold” object; the more inclined orbit is the orbit of 136472 Makemake, which is a “hot” object
Image credit: JPL Solar System Dynamics


In this article, we have introduced:

  1. The nature and location of the Kuiper Belt
  2. The formation of the Kuiper Belt
  3. The icy composition of Kuiper Belt objects
  4. The dynamical classifications of Kuiper Belt objects

If you want to learn more about this mysterious part of the outer Solar System, feel free to visit the websites in the references below.


  1. (2021, November 17.). In Depth | Kuiper Belt – NASA Solar System Exploration. Retrieved November 25, 2022, from https://solarsystem.nasa.gov/solar-system/kuiper-belt/in-depth/
  2. (n.d.). The Kuiper Belt. Retrieved November 25, 2022, from http://pluto.jhuapl.edu/Arrokoth/About-the-Kuiper-Belt.php
  3. Tillman, N. T. (2019, April 30). The Kuiper Belt: Objects at the Edge of the Solar System. Retrieved November 25, 2022, from https://www.space.com/16144-kuiper-belt-objects.html

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