The Origin of Comets
By Mark Bailey and Bill Napier
Today it is believed that comets arrive in the inner solar system from several reservoirs. Chief amongst these is the Oort cloud, a swarm of about 100 billion comets orbiting the Sun and extending almost half way to the nearest star. At such distances, the Sun's gravity has about a billionth of the strength it has in the neighbourhood of the Earth, and the comets are very weakly bound to the solar system. Small disturbances caused by passing stars or clouds of gas, nudge comets out of the Oort cloud, and a very few will reach the planetary system from this cloud after a journey of several million years. These are the long-period comets. They may then be thrown into chaotic orbits, and will eventually be ejected into interstellar space, fall into the Sun, or collide with a planet.
The main perturber of the Oort cloud is the Galactic tide, and this varies in a 30-45 million year cycle as the Sun oscillates up and down about the plane of the Galaxy. There are therefore high-risk and low-risk periods of possible cometary collision in the history of the Earth, depending on its position in the Galaxy (currently we are in a high-risk period). A dense inner cloud of comets may exist, but little can be said about it, as it is relatively impervious to perturbations from stars, Galaxy or planets.
Usually, it is supposed that cometary nuclei - dirty snowballs or 'icy dirtballs' as they are perhaps more aptly known - are the building blocks and leftovers from the epoch of planet formation in the solar system, more than 4.5 billion years ago.
Those that failed to accrete onto a growing planet and which avoided falling into the Sun, were mostly ejected into interstellar space by a gravitational slingshot from the growing planets. A small proportion did not quite escape and formed the Oort cloud.
However the chemistry of the recent bright comets, Hale-Bopp and Hyakutake, indicates that they have never been heated much more than a few tens of degrees above absolute zero, which puts their origin much farther out, at least on the fringes of the planetary system. An in situ origin 4.5 billion years ago, in the cold, dense nebula that also gave birth to the Sun, cannot be ruled out.A second known reservoir of comets is the Kuiper belt (or Edgeworth-Kuiper Belt), a flattish, disc-like swarm of icy bodies orbiting just beyond Neptune. Nearly four hundred such bodies have so far been found, but there may be 10 billion or so more than a kilometre (half a mile or so) across orbiting in this region; it dwarfs the asteroid belt between Mars and Jupiter. As with the asteroids in the main belt, the evolution of the Kuiper belt comets is dominated by mutual collisions, and by perturbations from the nearest giant planet, in this case Neptune. It seems most likely that this reservoir is a primordial remnant from the process that gave rise to the planetary system. As to exactly what that process was, there is now much uncertainty: recent discoveries of extrasolar planetary systems show a diversity that is hard to reconcile with standard theories in this field.