Cosmos

Comets, Asteroids and Pluto

Between and beyond the planets drift millions of leftover worlds: rocky asteroids, icy comets and dwarf planets like Pluto. They are the solar system's original building material.

By The Cosmos EditorsUpdated 1 November 2026 4 min read
Pluto photographed by New Horizons in 2015, with its bright heart-shaped nitrogen ice plain, Sputnik Planitia.
Pluto photographed by New Horizons in 2015, with its bright heart-shaped nitrogen ice plain, Sputnik Planitia. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker, Public domain via Wikimedia Commons
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The planets did not use up all the material in the cloud that formed the solar system. Billions of smaller bodies were left over, too few or too scattered to gather into a planet, or kept apart by Jupiter's disruptive gravity. They range from pebbles to worlds a couple of thousand kilometres across. Most of them are in two great reservoirs: a belt of rock between Mars and Jupiter, and a belt of ice beyond Neptune. Together they hold a record of how the solar system was built.

The asteroid belt

Between the orbits of Mars and Jupiter circle well over a million known asteroids, with countless more too small to catalogue. Films show spaceships dodging through dense fields of tumbling rock, but the belt is mostly empty space; asteroids are typically millions of kilometres apart, and spacecraft fly through without special precautions. Add every one together and they would weigh less than four percent of the Moon.

Asteroids are made of rock and metal, the stuff of the inner solar system. Some are rubble piles, loose collections of fragments held together by weak gravity. The largest, Ceres, is 940 kilometres across, round, and large enough to count as a dwarf planet; NASA's Dawn spacecraft found that it has a salty, muddy interior and bright patches of salt where briny water once seeped to the surface. Vesta, the second largest, has a crust, mantle and core like a miniature planet.

Jupiter is the reason the belt never became a planet. Its gravity stirred the region so violently that bodies collided and shattered rather than merging.

Comets

Comets are the ice of the outer solar system: dirty snowballs a few kilometres across, made of frozen water, carbon dioxide, ammonia and dust, left untouched in the cold for 4.6 billion years. Most spend their lives far beyond the planets, invisible. Occasionally one is nudged onto an orbit that brings it toward the Sun. As it warms, its ices turn to gas and burst from the surface, carrying dust with them, and the comet grows a glowing cloud and a tail that can stretch for over a hundred million kilometres. The tail always points away from the Sun, pushed by sunlight and the solar wind, so a comet leaving the Sun travels tail-first.

The European Rosetta spacecraft orbited Comet 67P for two years and dropped a lander on it, revealing a black, dusty, cliff-faced world shaped like a rubber duck, with jets of gas erupting as the Sun rose on each part of its surface. Comets delivered water and carbon-rich molecules to the young Earth, and may have helped stock the oceans and the chemistry that led to life.

The Kuiper belt and beyond

Past Neptune lies the Kuiper belt, a vast doughnut of icy bodies at 30 to 50 times Earth's distance from the Sun. It is like the asteroid belt but far larger and made of ice rather than rock. Pluto is its most famous member. Discovered in 1930 and counted as the ninth planet for 76 years, it was reclassified as a dwarf planet in 2006 after astronomers found other bodies of similar size nearby, including Eris, which is slightly heavier.

Whatever its label, Pluto turned out to be extraordinary. When New Horizons flew past in 2015, it found mountains of water ice as tall as the Rockies, a vast plain of nitrogen ice churning in slow convection cells, glaciers, possible ice volcanoes and a thin blue atmosphere, on a world at minus 230 degrees. Its surface is young in places, meaning something still drives activity inside. Its moon Charon is so large that the two orbit a point in the space between them.

Far beyond even the Kuiper belt, astronomers infer the Oort cloud: a spherical shell of perhaps a trillion comets, extending halfway to the nearest star, the source of comets that visit the inner solar system only once in millions of years.

Visitors and threats

Every day about a hundred tonnes of this debris falls onto Earth, nearly all as dust and sand-grain meteors that burn up harmlessly as shooting stars. Larger objects are rarer but real. A ten-kilometre asteroid struck Mexico 66 million years ago and ended the age of dinosaurs. In 2013 a 20-metre rock exploded over Chelyabinsk in Russia, shattering windows across the city. Surveys now track more than 30,000 near-Earth asteroids, and in 2022 NASA's DART spacecraft deliberately crashed into a small asteroid and measurably changed its orbit, the first test of a planetary defence.

Why it matters

Asteroids and comets are the most primitive material in the solar system, unaltered since before the planets formed. Samples returned from asteroids Ryugu and Bennu contain water and organic molecules, the same ingredients that seeded Earth. These small bodies are both the leftover bricks of our world and, occasionally, the greatest natural hazard it faces.

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Images

Comet Hale-Bopp in 1997, showing its white dust tail and blue gas tail, both pointing away from the Sun.
Comet Hale-Bopp in 1997, showing its white dust tail and blue gas tail, both pointing away from the Sun. E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (https://sternwarte.at), CC BY-SA 3.0 via Wikimedia Commons
The nucleus of Comet 67P, visited by the Rosetta spacecraft: a dark, dusty, two-lobed body four kilometres long.
The nucleus of Comet 67P, visited by the Rosetta spacecraft: a dark, dusty, two-lobed body four kilometres long. ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0, CC BY-SA 3.0 igo via Wikimedia Commons
Ceres, the largest object in the asteroid belt and a dwarf planet in its own right, photographed by the Dawn spacecraft.
Ceres, the largest object in the asteroid belt and a dwarf planet in its own right, photographed by the Dawn spacecraft. Justin Cowart, Public domain via Wikimedia Commons

Test yourself

4 questions · answers explained at the end

  1. 1.Where is the main asteroid belt?

  2. 2.Why does a comet's tail point away from the Sun?

  3. 3.Why was Pluto reclassified as a dwarf planet?

  4. 4.What ended the age of the dinosaurs 66 million years ago?

Answer every question to check.
Topics:asteroidcometPlutodwarf planetKuiper beltasteroid beltOort cloudCeresNew Horizons