Many of our known comets are periodic, or regular, visitors because they have orbits within the Solar System. Halley’s Comet is a famous example. But other comets, sometimes spectacular, are making possibly their first appearance as they sweep in to round the Sun.
So where exactly do they come from? The answer, it seems, is from a vast reservoir of icy bodies, numbered in their billions, that make up a ghostly shell around the whole of the Solar System.
Though it has never actually been observed, this spherical region is named the Oort Cloud after Jan Oort, the Dutch astronomer who independently suggested its existence in 1950. He did so 18 years after a similar idea had been proposed by Estonian astronomer Ernst Öpik, to explain the appearance in our skies of comets with orbital periods of many thousands of years.
It is not the only icy zone that provides comets. A growing number of frozen leftovers from the Solar System are being discovered in a much closer region beyond Neptune called the Kuiper Belt where Pluto also lurks. This provides short-period comets. And beyond that is the so-called scattered disc of similar objects whose orbits have been disturbed by forces such as the gravitational pull of Neptune.
It is suggested that the Oort Cloud begins at around a light-year’s distance from the Sun and may stretch as far as a third of the way to the nearest star. It is a region filled with perhaps a trillion frozen fragments of water, ammonia and methane left over from the formation of the Solar System.
Though the fragments are individually fairly insubstantial, together they could add up to several times the mass of the Earth.
Some of the most impressive comets seen are so-called long-period objects and it is thought many of these might be on their first trips into the inner Solar System after being dislodged by some event.
No observations have confirmed the cloud’s existence and it is too far to study with space probes – Voyager 1 which was said to have left the Solar System this summer after it crossed the heliopause, will not reach it for hundreds of years by which time it will long have been dead.
Even New Horizons, which flew past Pluto and is now deep in the the Kuiper Belt, will be unable to help us. But the theory that the cloud exists neatly explains the mysterious cometary visitors that occasionally grace our skies.