Definition · Plain-language
Asteroid
An asteroid is a small, rocky or metallic body that orbits the Sun, left over from the formation of the Solar System.
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Rocky leftovers of the Solar System
An asteroid is a small body of rock, metal, or a mixture of both, that orbits the Sun. Asteroids are sometimes called minor planets, but they are far too small to count as planets and are not massive enough to pull themselves into a fully round shape; many are irregular, lumpy and cratered. They are leftover material from the birth of the Solar System about four and a half billion years ago — building blocks that never came together into a planet. Because they have changed little since, asteroids preserve clues about the conditions in which the planets formed.
The asteroid belt and beyond
Most known asteroids orbit in the main asteroid belt, a broad ring between the orbits of Mars and Jupiter. Despite their numbers, the asteroids are spread across such a vast volume that they are typically very far apart — nothing like the crowded fields shown in films. Jupiter’s strong gravity is thought to have prevented this material from ever forming a planet. Not all asteroids stay in the belt: some, called near-Earth asteroids, follow orbits that bring them close to Earth, which is why agencies track them as part of planetary defence.
Asteroids, comets and dwarf planets
Asteroids are distinguished from comets by composition and behaviour: asteroids are rocky or metallic and stay solid, whereas comets are icy and grow glowing tails near the Sun. The largest object in the asteroid belt, Ceres, is big enough to be round and is classified as a dwarf planet, showing that the categories grade into one another. Fragments knocked off asteroids can fall to Earth as meteorites, and a large asteroid impact is widely thought to have contributed to the extinction of the dinosaurs, underlining why understanding asteroids matters.
Key facts
At a glance
- Definition: a small rocky or metallic body orbiting the Sun
- Also called: minor planets
- Mostly found: in the asteroid belt between Mars and Jupiter
- Origin: leftover building blocks from the early Solar System
- Versus comets: rocky not icy; no tail
- Largest: Ceres, big enough to be classed as a dwarf planet
Common misconceptions
What people often get wrong
Often heard: The asteroid belt is a dense, crowded field of rocks.
Actually: Asteroids are spread across an enormous volume of space, so they are usually millions of kilometres apart. Spacecraft cross the belt easily without dodging rocks, unlike the crowded scenes in films.
Often heard: Asteroids and comets are basically the same kind of object.
Actually: Asteroids are rocky or metallic and stay solid; comets are icy and grow a coma and tail near the Sun. Their different composition and origin set them clearly apart.
Often heard: Asteroids are large enough to be round like small planets.
Actually: Most asteroids are too small for their gravity to pull them into a sphere, so they are irregular and lumpy. Only the very largest, like Ceres, are round.
Going deeper
