The Kuiper Belt and Plutoids

Across more than three billion kilometers past Neptune exists a region of our Solar System filled with billions of rocky and icy objects.

A Visualization of the Kuiper Belt beyond Neptune: image credit NASA Discovery Program


Our Solar System contains a central star (that we call the Sun) and eight large planets happily circling around it. It is the immense gravity of the Sun that has kept our planets in their stable orbits for billions of years.

But the influence of the Sun’s gravity extends far past Neptune, the Solar System’s last planet. Across more than three billion kilometers past Neptune exists a region of space filled with billions, if not trillions of rocky and icy objects that also circle the Sun. This massive body of objects is collectively known as the Kuiper Belt.

The Kuiper Belt is named in honor of Gerard Kuiper, a Dutch-American astronomer who was renowned for planetary sciences in the 20th century. He believed that small left-over objects from the formation of the planets must have at one time encircled our Sun. However Kuiper also mistakenly believed that any such objects would have floated off into space over the billions of years since our Solar System was born.

Largest Known Kuiper Belt Objects
Some of the largest known objects in the Kuiper Belt in comparison to Earth (Credit: NASA Planetary Science Division)

The first objects inside the Kuiper Belt were not directly observed until 1992 by Professor Dave Jewitt and Dr. Jane Lu from the University of Hawaii. Over the next 25 years after their discovery, we have still have only directly observed about a dozen other Kuiper Belt objects. These dwarf-planets have a relatively small size of a few hundred of kilometers to a few feet in diameter, making direct observation almost impossible even with today’s massive ground-based telescopes.

A large majority of the objects in the Kuiper Belt are in fact giant snowballs. They are made out of frozen gases and liquids such as methane, ammonia and water. As these icy objects get jostled and knocked around by their neighbors, they are eventually pulled towards the center of the Solar System to meet a fiery death as they reach the Sun. We call these objects comets, and most comets that travel through our Solar System originated from the Kuiper Belt.

Unfortunately with the discovery of the Kuiper Belt came the demotion of Pluto from planet to dwarf-planet. Pluto exists well within the Kuiper Belt region and once we started finding other objects similar to Pluto’s size, the scientific community felt that Pluto should be re-classified as a dwarf planet.

But at 2370 kilometers in diameter, Pluto is still king of the dwarf planets! In fact, there was such an outcry when Pluto was removed as a planet in 2006 that the International Astronomical Union (IAU) decided a few years later that all dwarf planets found past Neptune would be given a new classification called “Plutoid”.

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