When people think about the solar system, they usually think about the eight planets and maybe the asteroid belt between Mars and Jupiter. But the solar system doesn't stop at Neptune. Beyond it lies a vast, dimly-lit ring of icy bodies stretching billions of kilometres into the darkness. This is the Kuiper Belt.
What's out there
The Kuiper Belt is a doughnut-shaped region of the solar system that begins just beyond Neptune's orbit (about 30 times farther from the sun than Earth) and extends to roughly 50 times Earth's distance. It's home to hundreds of thousands of icy objects — remnants left over from the formation of the solar system about 4.6 billion years ago.
These objects, called Kuiper Belt Objects or KBOs, range from small icy chunks to dwarf planets. Pluto — famously demoted from planet status in 2006 — is the most well-known Kuiper Belt Object. Others include Eris, Makemake, and Haumea.
Think of the solar system like a fried egg. The yolk is the sun, the white is the region of the planets — and the Kuiper Belt is the crispy edge you didn't notice was there. Most of the action happens in the middle, but the outer edge is enormous and contains its own strange world of frozen objects.
Where comets come from
Many of the comets that swing through the inner solar system — creating those dramatic tails of gas and dust — originated in the Kuiper Belt. Gravitational nudges from the giant planets occasionally knock a KBO out of its stable orbit, sending it on a long inward journey toward the sun. As it gets warmer, ices vaporise and stream behind it, creating the comet's tail.
New Horizons and Pluto
In 2015, NASA's New Horizons spacecraft became the first to fly past Pluto, revealing a surprisingly complex world with mountains of water ice, a heart-shaped nitrogen plain, and a thin atmosphere. In 2019 it flew past another KBO called Arrokoth — the most distant object ever visited by a spacecraft. It turned out to look like a rusty snowman.
Beyond the Kuiper Belt
Even further out, past the Kuiper Belt, astronomers believe there exists an even more distant region called the Oort Cloud — a vast spherical shell of icy objects extending perhaps a quarter of the way to the nearest star. We've never directly observed it, but we think it's where long-period comets (ones that take thousands of years to orbit the sun) come from.
Most people think the solar system is just the eight planets. Some people also know about the asteroid belt between Mars and Jupiter. But the solar system goes much further than that. Past the last planet, Neptune, there is a huge ring of icy rocks. This ring is called the Kuiper Belt.
What's out there
The Kuiper Belt is shaped like a ring doughnut. It starts just past Neptune's path around the sun. Neptune is already about 30 times further from the sun than Earth is. The Kuiper Belt stretches out to about 50 times Earth's distance from the sun. Hundreds of thousands of icy objects float around out there. They are leftovers from when the solar system formed about 4.6 billion years ago.
These objects are called Kuiper Belt Objects, or KBOs. Some are small chunks of ice. Others are much bigger and are called dwarf planets. Pluto is the most famous Kuiper Belt Object. In 2006, scientists decided Pluto was no longer a full planet. It became a dwarf planet instead. Other dwarf planets in the Kuiper Belt include Eris, Makemake, and Haumea.
Think of the solar system like a fried egg on a plate. The yolk is the sun. The egg white is where all the planets live. The Kuiper Belt is like the crispy brown edge of the egg that you might not notice at first. Most things happen near the middle. But that outer crispy edge is enormous and full of its own strange frozen worlds.
Where comets come from
Comets are big balls of ice and dust that travel through space. They have long glowing tails made of gas and dust. Many comets start their lives in the Kuiper Belt. Sometimes the gravity of the giant planets gives a KBO a push. That push knocks it out of its usual path. It then travels slowly towards the sun. As it gets closer and warmer, the ice turns to gas. The gas streams out behind it and makes the comet's tail.
New Horizons and Pluto
In 2015, a NASA spacecraft called New Horizons flew past Pluto. It was the first spacecraft ever to do this. What it found was very surprising. Pluto has mountains made of water ice. It has a large flat plain shaped like a heart, covered in nitrogen ice. It even has a thin layer of air around it. In 2019, New Horizons flew past another KBO called Arrokoth. Arrokoth is the most distant object any spacecraft has ever visited. It looked just like a snowman made of two round lumps stuck together, with a reddish colour.
Beyond the Kuiper Belt
Even further out than the Kuiper Belt, scientists believe there is another region called the Oort Cloud. Imagine a giant bubble made of billions of icy rocks surrounding the whole solar system. The Oort Cloud might stretch about a quarter of the way to the nearest star. No telescope has ever seen it directly. But scientists think it is where very long-period comets come from. These are comets that take thousands of years to travel once around the sun.