By Sabine Stanley, Ph.D., Johns Hopkins University

The Kuiper Belt was discovered with the help of the clues provided by the comets observed from the Earth. It is considered to be a kind of reservoir for short-period comets. So what exactly does this Belt compose of? Read on to know more about this fascinating part of space.

Kuiper Belt

Short-period comets come from the Kuiper Belt. What is this region of space? Comets provided the earliest clues. The fact that we see comet tails means that the comets are somewhat fresh. If they had their current orbits throughout the lifetime of the solar system, they would have lost their icy components long ago, and we wouldn’t see tails.

So there must be a source region for comets from which new comets come from. But this region began to be considered more seriously after Pluto’s discovery in 1930. Finding Pluto led scientists to hypothesize that other small icy bodies are likely to exist in the far reaches of the solar system, too. But it wasn’t until 1978 that Charon was discovered. And it wasn’t until 1992 that a third object was discovered, now known as Albion. Albion has an elliptical orbit taking it from its closest distance from the Sun at 41 astronomical units (AU) out to its furthest distance at 47 AU.

By 2018, over 2000 Kuiper Belt objects (or KBOs for short) had been discovered. Estimates suggest there are trillions of objects in the Kuiper Belt. Of those, there may be over 100,000 KBOs with diameters larger than 100 kilometers.

Components of Kuiper Belt

The Kuiper Belt has different components. The whole Belt extends from about 30 to 55 AU from the Sun. But most discovered KBOs lie between orbital distances of 42 to 48 AU, a region known as the Classical Belt. These distances mark resonances that KBOs have with Neptune’s orbit that keep the KBOs in stable orbits. Neptune’s orbit is roughly 30 AU, and a 2:3 orbit resonance with Neptune happens at 42 AU. Here, KBOs called plutinos, for their similarity to Pluto’s orbit, can maintain stable orbits for long periods of time, and we find a large number of objects at this distance. The outer edge of the Classical Belt at 48 AU marks a 1:2 orbit resonance with Neptune, which again provides stability to KBO orbits. The objects at 48 AU are called two-tinos because of this 2:1 resonance.

There are also so-called scattered disk objects, with larger ellipticities and inclinations. They can be found anywhere from about 30 AU to over 100 AU. The KBOs in the scattered disk have likely been strongly perturbed by gravitational interactions with the gas giants, putting them in these scattered orbits. The scattered disk objects are believed to be the source for short-period comets since KBOs in the Classical Belt have very stable orbits and are unlikely to be perturbed into comet like orbits.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Dwarf Planets

Pluto and Charon are some of the largest known KBOs. But there are other Kuiper Belt Objects similar in size to Pluto. There are also three KBOs known to be large enough to be spherical, and hence are dwarf planets like Pluto.

One of the dwarf planets is Eris, the 9th most massive planetary object known in our solar system. It’s about 25% more massive than Pluto, although its diameter of 2326 kilometers makes it about 50 kilometers smaller in diameter than Pluto. Eris even has a small moon, 350 kilometers in diameter, named Dysnomia. Eris is part of the scattered disk, the source region for short-period comets. It orbits the Sun with an inclination about 44° from the plane of the solar system, much bigger than the 17° of Pluto. Eris’s orbit is also more elliptical than that of Pluto, taking Eris from within the orbit of Pluto at its closest orbital distance of 38 AU (most recently in the year 1699) to a farthest distance of almost 100 AU (most recently in 1977). So while Pluto has been at its closest to the Sun—and to the Earth—in recent decades, Eris has been at its farthest and coldest. But Eris will return; an Eris year lasts “only” 560 Earth-years.

Makemake, named after a fertility god on Easter Island, is a dwarf planet about the size of Pluto. But unlike the plutino resonance with Neptune, Makemake’s orbit takes it far enough from Neptune to remain stable, yet not too far, putting it in the classical and very stable portion of the Kuiper Belt. Makemake travels between a closest point at 38 AU and a farthest point at 53 AU. Makemake is probably the largest object in the Classical Belt, and is almost as bright as Pluto. It also has a moon, estimated to be about 100 miles across. Makemake will be at its maximum distance from the Sun in 2033.

Perhaps the most fascinating dwarf planet is Haumea, which is about the size of Pluto. It’s shaped more like a football—it’s almost twice as long as it is wide. That’s because it has a very large equatorial bulge caused by its spin. Haumea is the fastest spinning object we know of in the solar system, larger than 100 km in diameter, with a rotation time of only 3.9 hours! Haumea appears to be denser than Pluto, and perhaps as dense as Earth’s Moon. That means it’s much more rock-rich than Pluto and probably only has a thin outer ice layer.

Haumea also has 2 moons named Namaka and Hi’iaka, as well as a ring. Haumea’s ring was discovered in 2017. The ring has a radius of about 2300 kilometers and a width of about 70 kilometers. Haumea’s rapid rotation, ring, and even the tiny moons suggest that the Haumea observed now resulted from at least one, and possibly two collisions.

Although Haumea, Makemake, Eris, and Pluto have been the only certified dwarf planets in the outer solar system, there are many other known KBOs that are likely to be dwarf planets. There is just not enough data yet to constrain their masses and confirm that they are indeed spherical.

Learn more about the dwarf planet Pluto and its moon Charon.

Other Famous KBOs

Besides Pluto and Charon, the only Kuiper Belt Object to have been visited by spacecraft is a flattened, peanut-shaped object numbered 2014 MU69. After the New Horizons spacecraft flew by Pluto in 2015, it travelled another billion miles outward to the newly discovered object. It orbits between 42 and 46 AU, in an orbit that takes almost 300 years. When New Horizons flew by on New Year’s Day of 2019, it determined that MU69 is a contact binary, like comet 67P. But while 67P is only about 4 kilometers long, this peanut is about 30 kilometers long. It became the most distant object ever studied by a spacecraft to that time.

Of course, there are also far more distant objects. For example, Sedna is a large object that may be a dwarf planet, but it’s not sure yet if it’s round. However, it is known that the diameter is roughly 1000 kilometers across. Sedna has an extremely elliptical orbit around the Sun that takes it from about 3 times Neptune’s distance at 76 AU to over 900 AU!

Sometimes, the presence of a large planetary object can be inferred even if it can’t be seen. That’s how Neptune was discovered, based on Uranus’s orbit. It turns out that orbits of objects in the farthest part of the Kuiper Belt also hint that there may be another large object out there, although it hasn’t been found yet. It is known as “Planet 9”, and it is estimated to be about 10 Earth masses and several Earth diameters. It would therefore be only a bit smaller than the ice giants Uranus and Neptune. This planet would help explain the high ellipticity and extreme inclination of the orbits of some extreme objects beyond the Kuiper Belt, like Sedna.

This hypothetical Planet 9’s orbit is also believed to be highly elliptical, taking it from about 200 AU to as far as 1200 AU! And one orbit takes tens of thousands of years. The hunt is on for the hypothetical Planet 9 as scientists further refine what its orbit must be like to explain the perturbed orbits of KBOs.

The Kuiper Belt contains the farthest solar system objects that have ever been discovered, but we know that there must be objects much farther because of long-period comets.

Learn more about how the solar system family is organized.

Common Questions about Kuiper Belt

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