Rubble and ice: the small bodies

The Sun has eight planets. It also has millions of smaller things going round it.

They are the leftovers. When the planets were being built, 4.6 billion years ago, not every piece was used. The spare pieces are still out there.

Close to the Sun the leftovers are made of rock and metal. These are the asteroids, and most of them circle in a wide belt between Mars and Jupiter.

Far from the Sun, where it is very cold, the leftovers are made mostly of ice. Beyond Neptune there is a second, much bigger belt of them. Pluto is one.

Now and then one of the icy ones falls in towards the Sun, warms up and grows a tail. Then we call it a comet.

Why there are two kinds. The young Sun was surrounded by a disc of gas and dust. Close in it was too warm for water to freeze, so only rock and metal could clump together. Further out, beyond what is called the frost line, ice could form as well. That one difference still shows: rocky bodies inside, icy bodies outside.

The asteroid belt. Most known asteroids go round the Sun between Mars and Jupiter, from about 2 to 3.3 times the Earth’s distance. More than 1.4 million have been found. Jupiter’s pull kept stirring this region and stopped the pieces from ever gathering into a planet.

The belt is nothing like the crowded rock fields of films. Put all of it together and it would have only about 3 per cent of the mass of the Moon, and more than a third of that is in one body, the dwarf planet Ceres, 940 km across. The asteroids are typically hundreds of thousands of kilometres apart or more. Every spacecraft sent through the belt has crossed it without trouble.

Trojans. Two swarms of asteroids share Jupiter’s own orbit, one a sixth of a lap ahead of the planet and one a sixth of a lap behind. The pulls of Jupiter and the Sun combine to hold them there. NASA’s Lucy spacecraft, launched in 2021, is due to reach the first of them in 2027.

Near-Earth asteroids. Some asteroids have been nudged onto paths that come close to ours. More than 35,000 are known and they are watched carefully. The lesson on falling rocks says more.

The Kuiper Belt. Beyond Neptune, from about 30 to 50 times the Earth’s distance from the Sun, lies a broad ring of icy bodies. Pluto was the first to be found, in 1930, and the second was not found until 1992. Thousands are known now, and there are thought to be hundreds of thousands more than 100 km across. Four of the five named dwarf planets live out here: Pluto, Haumea, Makemake and Eris. The New Horizons spacecraft flew past Pluto in 2015 and a small Kuiper Belt body, Arrokoth, in 2019.

The Oort Cloud. Further still there is thought to be a vast, thin shell of icy bodies all around the Solar System, reaching perhaps a third of the way to the nearest star. Nobody has ever seen it. It is worked out from comets that arrive from every direction on orbits thousands of years long, which must come from somewhere. On this site it is drawn as a faint haze and labelled as theory.

Where	Distance from the Sun (Earth = 1)	Made of	Seen directly?
Asteroid belt	2 to 3.3	Rock and metal	Yes
Jupiter’s Trojans	5.2	Dark rock, perhaps ice inside	Yes
Kuiper Belt	30 to 50	Ice and rock	Yes
Oort Cloud	Thousands to perhaps 100,000	Ice	No

Why they matter. Planets have melted, weathered and remade their rocks many times over. Small bodies have mostly sat unchanged in the cold. They are the original ingredients, and that is why spacecraft have gone to the trouble of bringing pieces of asteroids home.