What is a comet? The icy visitors of the solar system, explained
By the WhyCosmos team · Last updated:
A comet is a small body of ice, dust and rock that orbits the Sun. Far from the Sun it is a dark, frozen lump usually only a few kilometres across. As it comes closer, sunlight warms it, ice turns straight into gas, and it grows a glowing cloud called a coma and long tails that point away from the Sun.
Comets were once feared as omens; the Hindi word, dhoomketu, describes a smoky banner in the sky. Today we know them as the solar system's deep freeze, little changed since the planets formed. In 2025 the world watched one that had come from another star entirely. This page explains what comets are made of, why their tails do not trail behind them, and how to tell a comet from an asteroid.
How to explain it to your child
Explain it to an 8-year-old
Think of a comet as a dirty snowball travelling around the Sun. Most of the time it is far away and frozen. When it comes closer to the Sun it warms up, and gas and dust puff off it, making a bright fuzzy cloud and a long tail that can stretch across the sky.
Explain it to a 13-year-old
Comets are icy small bodies. Their solid nucleus, usually a few kilometres across, holds frozen water, carbon dioxide and other ices mixed with dust. Near the Sun the ices turn directly into gas, forming a coma that can grow larger than Earth, and two kinds of tail shaped by sunlight and the solar wind, which can stretch for millions of kilometres.
Key facts
| Made of | Ice, dust and rock |
|---|---|
| Solid centre | The nucleus, usually a few kilometres across |
| Glowing cloud | The coma |
| Tails | Usually two: a curved dust tail and a straight ion (gas) tail |
| Tail direction | Away from the Sun, whichever way the comet is moving |
| Where they come from | Mostly the Kuiper Belt (short-period comets) and the Oort Cloud (long-period comets) |
| Most famous | Halley's Comet: last bright in our sky in 1986, returning in 2061 |
| Hindi name | Dhoomketu |
Why does a comet's tail point away from the Sun?
A comet's tail does not stream behind it like hair in the wind. It is pushed outwards by the Sun, so it always points away from the Sun, even when the comet is heading back out into space tail first. Most bright comets show two tails:
| Tail | Made of | Pushed by | What it looks like |
|---|---|---|---|
| Dust tail | Tiny dust grains | The pressure of sunlight | Broad, curved and yellowish-white |
| Ion tail | Electrically charged gas | The solar wind | Narrow, straight and often bluish |
Where do comets come from?
Short-period comets, which return in less than about 200 years, mostly come from the Kuiper Belt, a ring of icy bodies beyond Neptune. Long-period comets, which can take thousands or even millions of years to complete one orbit, are thought to come from the Oort Cloud, a vast distant shell of icy bodies surrounding the solar system. The gravity of a passing star or of the giant planets can nudge a comet out of these regions and send it falling towards the Sun.
What was 3I/ATLAS, the comet from another star?
On 1 July 2025, the ATLAS survey telescope in Chile spotted a comet moving far too fast to belong to our solar system. Named 3I/ATLAS, it was only the third interstellar object ever confirmed, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019, and it was clearly an active comet with a coma and tail.
It made its closest approach to the Sun on 29 October 2025, roughly between the orbits of Mars and Earth, and its closest approach to Earth on 19 December 2025, at nearly twice Earth's distance from the Sun. It was never a danger to Earth. Because it is moving too fast for the Sun's gravity to hold it, it is now heading back out of the solar system for good.
What is the difference between a comet and an asteroid?
Traditionally, an asteroid is mostly rock or metal and looks like a point of light, while a comet contains much more ice and other substances that turn to gas easily, so it grows a coma and tails near the Sun. Asteroids are mostly found between Mars and Jupiter; comets spend most of their lives far beyond Neptune.
Nature does not always respect neat categories, though. Some objects in the asteroid belt occasionally show comet-like tails, and some old comets have lost so much ice that they now look like asteroids.
Famous comets, and when you can see one
Halley's Comet is the most famous, returning to the inner solar system roughly every 75 to 76 years. It last passed close to the Sun, and was last visible to the naked eye, in 1986, and will be back in 2061. Twice a year Earth passes through dust it left behind, which produces the Eta Aquariid meteor shower in May and the Orionids in October.
In 2014 the European Space Agency's Rosetta mission placed a lander, Philae, on comet 67P, the first landing on a comet. Bright naked-eye comets arrive unpredictably every few years: Comet NEOWISE in July 2020 and Comet Tsuchinshan-ATLAS in October 2024 could both be seen from India. A free app such as Stellarium shows where to look when one appears.
The solar system and its scale open Astronomy 101, the WhyCosmos live course for ages 8 to 16.
Questions parents and kids ask
No. A comet does not burn. It glows because sunlight lights up the gas and dust released from its icy nucleus, and some of the gas also gives off its own glow when energised by sunlight.
Halley's Comet is a famous periodic comet that returns to the inner solar system roughly every 75 to 76 years. It last passed close to the Sun in 1986 and will be back in 2061.
A comet contains a lot of ice and grows a glowing coma and tails near the Sun. An asteroid is mostly rock and metal and does not usually grow a tail.
It is possible, but large comet impacts are extremely rare, and astronomers track known comets. The most famous comet impact seen by humans was on Jupiter, when pieces of Comet Shoemaker-Levy 9 struck it in 1994.
Dhoomketu. The word describes a smoky banner, after the comet's long glowing tail.
No. At its closest, in December 2025, 3I/ATLAS was nearly twice as far from Earth as the Sun is. It was never on a path that could hit Earth, and it is now leaving the solar system.
Your child asks questions like this every week.WhyCosmos runs live online astronomy classes for ages 8–16, taught by real astrophysicists — where questions like these are the curriculum.
Related terms in this glossary
Sources and further reading
NASA: Comets · NASA: Comet 3I/ATLAS · NASA: Halley's Comet · ESA: Rosetta