At the Edge of the Solar System


Comets must have formed and remained at low temperatures. That is why they still have a high content of solid but volatile substances such as water and carbon monoxide. They must therefore have been made in the outer regions of the nebula disc that formed around the newborn Sun, and which produced the Solar System. The outer regions of this zone are far from the warmth of the Sun, so comets that formed there remained icy.

Some comets are orbiting very far away, perhaps still in the place that they formed. Others may have formed somewhere further in towards the Sun, but still cold, say somewhere near Uranus and Neptune, and have been disturbed by the movements of the planets and ejected outwards, beyond Neptune to where they are now. Many comets would have been kicked out of the Solar System entirely. But those that just failed to break free now orbit in a cloud around the fringes of the Solar System called the Oort Cloud, after the Dutch astronomer who first suggested it, Jan Hendrik Oort, (161) Oort.

Oort inferred the existence of the Oort Cloud, the source of the long- period comets, by looking at how far away comets come from and the way their orbits are oriented. Some comets have long, thin orbits. They appear, as if from nowhere, getting close enough to see as they track in through the outermost planets. They must have originated from somewhere further away than that, and eventually they swing around the Sun and go back there. Their periods are very long; in fact many have such long periods that astronomers have seen them only once in centuries. This again indicates how far away they live. Their orbits point out from the Sun in all directions so the region where they live must be spherical, centered on the Sun. Oort’s vision was that there was a swarm of slowly moving, cold comets in a far- flung cloud on the perimeter of the Solar System. If a comet was disturbed from its orbit there by the passing of a star or a massive cloud of gas in its journey around the Milky Way Galaxy, past the Sun, the comet would plunge down directly towards the Sun on a long, thin, highly elongated elliptical orbit, oriented at a direction inwards from the spherical cloud.

The Oort Cloud is hypothetical. It has never been seen.

Other comets have short periods (less than or about 30 years) and only ever come from directions that lie in the plane of the Solar System. They come from somewhere nearer than the Oort Cloud, a disc-like zone that extends outwards from the orbit of Neptune. They come from the Kuiper Belt, and a thicker zone that surrounds it called the scattered disc. How did the comets get there? Some comets from the Oort Cloud could be perturbed on their infall and become members of the Kuiper Belt. The minor planet 2008 KV42 is a trans-Neptunian object (TNO) moving around the Sun backwards in an orbit that is almost perpendicular to the orbits of the planets. Its odd inclination and backwards motion suggests that it made a transition from the Oort Cloud to become a TNO on its way

© Springer International Publishing Switzerland 2016

P. Murdin, Rock Legends, Springer Praxis Books, DOI 10.1007/978-3-319-31836-3_7

to being a comet. Other comets might have found their way directly into the Kuiper Belt and the scattered disc from closer in, moved there from interactions with the major planets of the Solar System, and are hanging about there like sullen teenagers with nothing to do, waiting around before they fall in towards the Sun, on their way back home.

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