Where do comets come from?

Astronomers can identify 4,695 comets but there are an estimated trillion out there. Most come from the Oort cloud at the edge of the solar system.

A cross-section of Oort cloud, a giant sphere of material that surrounds our solar system, which lies right at centre of the illustration. – SPL Creative/Getty Images

“Comets are like cats. They have tails and do exactly what they want,” says David Levy, co-discoverer of Shoemaker-Levy 9, the comet that collided with Jupiter in 1994. It is a neat analogy. A comet’s tail is certainly a striking feature and unlike planets, the orbits of which are known precisely, most comets appear once in our skies and then vanish. In short, they come and go at will. But how do comets acquire these feline features?

Astronomers know a total of 4,694 comets, although this figure represents only a tiny fraction of our Sun’s comet population, which is estimated to be around a trillion. These are divided into two populations: short-period comets that take less than 200 years to orbit the Sun and long-period comets which take more than 200 years.

As a comet approaches the Sun, its ice vaporises, carrying grains of dust with it.

All long-period and a few short period comets come from the Oort cloud (named after Dutch astronomer Jan Oort), a vast sphere of material that hangs at the edge of the solar system – the last remains of the ancient dust cloud from which our solar system first formed.

The remaining short-period comets come from the Kuiper belt (named after Dutch-American astronomer Gerard Kuiper) just beyond the orbit of Neptune, five to eight billion kilometres from the Sun. Occasionally a comet is disturbed in its orbit, perhaps given a gravitational nudge by a passing star, and is sent spiralling inward toward the Sun, passing across our skies never to be seen again.

A few get trapped permanently in close solar orbits, such as Comet 67P/Churyumov-Gerasimenko, which takes a six and a half years to orbit the Sun.

And as for the tail? As a comet approaches the Sun, its ice – which also contains carbon monoxide and carbon dioxide – vaporises, carrying grains of dust with it. These stream out behind the comet, pushed by the sun’s “radiation pressure”, to produce a fuzzy, curved tail. Some of the vaporised material is also ionised and caught up in the solar wind, a stream of energetic protons, to create a second sharp, straight tail like that of Comet Hale-Bopp, which lit up night skies in 1997.

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