Definition · Plain-language
Comet
A comet is an icy body that orbits the Sun and, as it draws near, releases gas and dust to form a glowing head and a long tail.
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A “dirty snowball” of ice and dust
A comet is a small body made of frozen gases, water ice, dust and rocky grains — a mixture often nicknamed a “dirty snowball” or “icy dirtball”. Its solid heart, the nucleus, is usually just a few kilometres across and very dark. For most of its orbit, far from the Sun, a comet is frozen and inactive, indistinguishable from a dark lump of ice. Comets are leftovers from the formation of the Solar System, preserved in the deep cold of its outer reaches, so they carry pristine material from over four and a half billion years ago.
Why comets grow tails
A comet comes to life as it approaches the Sun. Rising heat causes its ices to turn straight from solid to gas, a process called sublimation, releasing gas and dust that surround the nucleus in a glowing cloud known as the coma. The same outflow forms the tails. A comet typically has two: a dust tail, which curves gently along its path, and a fainter ion (gas) tail, swept straight back by the solar wind. Crucially, both tails always point away from the Sun, not behind the comet’s motion, so an outbound comet travels tail-first.
Where comets come from
Comets originate in two distant, frigid regions of the Solar System. Shorter-period comets, which return in tens to a couple of hundred years, mostly come from the Kuiper Belt, a disc of icy bodies beyond Neptune. Long-period comets, which take thousands of years or longer, are thought to come from the Oort Cloud, a vast spherical shell of icy objects surrounding the Solar System far beyond the planets. Each time a comet rounds the Sun it loses some material, so over many passes it gradually fades, and some leave behind the dust that creates meteor showers.
Key facts
At a glance
- Definition: an icy body that orbits the Sun and grows a coma and tail
- Made of: ice, frozen gases, dust and rock — a “dirty snowball”
- Nucleus: the small, dark, solid core, usually a few kilometres across
- Tails: always point away from the Sun, not behind its motion
- Origin: the Kuiper Belt (short-period) and Oort Cloud (long-period)
- Leaves behind: dust trails that cause meteor showers
Common misconceptions
What people often get wrong
Often heard: A comet’s tail trails behind it like smoke from a moving object.
Actually: A comet’s tails always point away from the Sun, pushed by sunlight and the solar wind. An outbound comet actually travels tail-first, not tail-behind.
Often heard: Comets are burning, like a fireball moving through space.
Actually: Comets are not on fire. Their glow comes from sunlight reflecting off dust and from gas energised by the Sun, as ices vaporise — there is no combustion in airless space.
Often heard: Comets and asteroids are the same kind of object.
Actually: Comets are icy and grow tails near the Sun; asteroids are rocky or metallic and do not. They also form in different regions — comets in the cold outer Solar System, asteroids mostly in the inner belt.
Going deeper
