Where do comets come from?

Comets race past the Sun, usually following highly elliptical orbits. We see them when they enter the inner Solar System, but where do they spend the rest of their time? Here are four examples of comets taking different routes through the Solar System.

ESA’s Rosetta mission spent two years accompanying Comet 67P/Churyumov–Gerasimenko as it approached the Sun and departed again. This comet gets as close as 1.2 Earth-Sun distances from the Sun before flying out to beyond the orbit of Jupiter. This makes it a Jupiter-family comet.

Comet 1P/Halley is a another short-period comet and has been marvelled at since prehistoric times. With an orbital period of 76 years, it flies out all the way to the so-called Kuiper Belt. This is a doughnut-shaped ring of objects orbiting the Sun beyond the orbit of Neptune, billions of kilometres from the Sun.

With an estimated orbital period of millions of years, we were lucky to have seen Comet Siding-Spring (C/2013 A1) passing through the inner Solar System. Long-period comets like these are thought to come from the Oort Cloud, a large spherical shell of objects orbiting hundreds of billions to trillions of kilometres from the Sun.

Finally, some comets don’t orbit the Sun at all. Exocomets like 2I/Borisov are visitors from interstellar space. This exocomet probably formed around another star before being flung out across the Universe.