Shop Windows to the Universe

Hands On Mineral Identification helps you to identify over 14,500 minerals! By M. Darby Dyar, Ph.D. See our DVD collection.
The tiny blue and purple dots in this picture show where Kuiper Belt Objects are. See how they are out past Neptune, near Pluto?
Click on image for full size
Image courtesy NASA/JPL-Caltech.

The Kuiper Belt

The outer edge of our Solar System is not empty. There are many, many huge spheres of ice and rock out near Pluto's orbit. Astronomers call this huge group of planetoids "Kuiper Belt Objects", or "KBOs" for short. The Kuiper Belt is a bit like the asteroid belt, but much farther from the Sun.

Scientists think there are many thousands of KBOs. Astronomers have discovered several hundred so far. KBOs are gigantic balls of ice and rock. Some are small, some are tens of km across, and some are as big as the planet Pluto, and maybe larger! They orbit the Sun on the edge of the Solar System, near Pluto. They orbit between 30 to 50 AU (1 AU = Earth to Sun distance) from the Sun. Some astronomers think the Kuiper Belt goes out to 100 AU. KBOs take 200 years or longer to orbit the Sun!

Some astronomers had a theory about the Kuiper Belt in the early 1900s. One of them was Gerard Kuiper. In 1951, Kuiper said that some kinds of comets might come from the Kuiper Belt. The Kuiper Belt was named after Gerard Kuiper. The first KBO was discovered in 1992. It was given the odd name "1992 QB1".

The planet Pluto is also a Kuiper Belt Object. There are probably a bunch of other KBOs as big as Pluto or bigger that we haven't found yet. This is why astronomers are having a hard time deciding what a planet is. Is Pluto a planet? Are any of the other KBOs? Astronomers have found one object, called 2003 UB313 for now, that looks like it is bigger than Pluto. Some people are calling it the "tenth planet".

There are a couple of different kinds of KBOs. The different kinds have different orbits. Some have orbits like Pluto's. They are called "plutinos" (mini-Plutos). Some have orbits that are more like circles. They are called "cubewanos".

There are other objects besides KBOs out on the edge of the Solar System. The Oort Cloud is much, much further out than the Kuiper Belt. All of the objects on the frozen edge of the Solar System can be put in one big group. Astronomers call that group "Trans-Neptunian Objects" (TNOs) because they orbit further from the Sun than Neptune. KBOs and objects in the Oort Cloud are all Trans-Neptunian Objects. So are some other odd misfits that are in-between the Kuiper Belt and the Oort Cloud, including Sedna and 2003 UB313.

Some of the best known KBOs are Pluto, 1992 QB1, Orcus, Quaoar, Ixion, and Varuna. Strange names for strange objects!

Last modified January 31, 2006 by Randy Russell.

Shop Windows to the Universe Science Store!

Our online store includes books on science education, ranging from evolution, classroom research, and the need for science and math literacy!

Windows to the Universe Community



You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more


Pluto is a frigid ball of ice and rock that orbits far from the Sun on the frozen fringes of our Solar System. Considered a planet, though a rather odd one, from its discovery in 1930 until 2006, it was...more

What is a planet?

Do you know what a planet is? If so, you are doing better than professional astronomers! Right now astronomers aren't quite sure how to define a planet. Maybe you've heard that some astronomers think Pluto...more

Eris - a dwarf planet

Eris is a dwarf planet in our Solar System. Eris was one of the first three objects classified as a dwarf planet, along with Pluto and Ceres. Eris was first spotted in January 2005. Eris is a large sphere...more

Elliptical Orbits

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched...more

Eccentricity of an Orbit

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched...more

Trans-Neptunian Objects (TNO)

The outer edge of our Solar System is not empty. There are many huge spheres of rock and ice out near Pluto's orbit and beyond. Astronomers have a name for everything further out than the planet Neptune....more

New planetoid named Sedna discovered

Astronomers have discovered a new planetoid at the far edge of our Solar System. The new object, named Sedna, is probably almost as big as the smallest planet, Pluto. Sedna is very, very far away. It is...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF