Where do comets come from?
Comets are primitive bodies that formed at the same time as the Sun and planets about 4.6 billion years ago. But once they come close enough to be seen, comets begin to fall apart and they must eventually vanish from sight, often in less than a million years after first sighting. So comets are very old, but once they swing near the Sun they do not last very long. This means that ancient reservoirs must be furnishing the inner solar system with new comets. They come from two reservoirs, one that is very far away, at the fringe of the outer solar system, and a nearer one at the edge of the planetary realm. These small icy worlds have been hibernating in the cold outer reaches of space ever since the formation of the solar system.
The long-period and short-period comets
Most discovered comets have arrived near the Sun from distant regions far beyond the major planets. They have very elongated trajectories that take them back to the distant regions they came from. These comets are known as the long-period comets, with orbital periods larger than 30 Earth years. The long-period comets are observed in the inner part of the solar system just once, arriving unannounced and unpredicted. As you might expect, they come from very far away, at the outer fringes of the solar system.
Some known comets, about 150 in all, have appeared more than once during the past two centuries. These are the short-period comets, which revolve around the Sun with orbital periods of less than 30 Earth years. They are sometimes distinguished by putting a number and the letter “P” before their name, with the short-period comet number corresponding to the order of recognition. The short-period comets are seen time and again, trapped in tight orbits within the planetary realm. Most of them have low orbital inclinations near the plane of the Earth’s orbit, with mean distances from the Sun of just a few times that of the Earth, or a few AU. It is these short-period comets whose regular returns we are able to predict, and which we can examine in detail with spacecraft.
The Oort cloud
The size and orientation of the trajectories of long-period comets can be explained if they come from a remote, spherical shell belonging to the outer parts of the solar system. This vast comet repository is known as the Oort cloud, named after the Dutch astronomer Jan H. Oort (1900-1992) who first postulated its existence. Since the long-period comets approach the Sun from enormous distances, of 100,000 AU or more, the Oort cloud has a diameter of up to twice this size. By way of comparison, the average distance between the Earth and the Sun is just 1 AU, about 150 billion meters. And because long-period comets enter the planetary realm at all possible angles, with every inclination to the Earth’s orbital plane, they must come from a spherical shell. This would also explain the fact that long-period comets move in all directions. Roughly half of them move along their trajectories in the retrograde direction, opposite to the orbital motion of the planets.
But how do comets fall from the Oort comet cloud to the heart of the solar system? The distant comets are only weakly bound to the solar system, and are easily perturbed by the gravitation of nearby, moving objects, which throw some of the comets back into the planetary system. The random gravitational jostling of individual stars passing nearby, for example, knocks some of the comets in the Oort cloud from their stable orbits, either injecting them into interstellar space or gradually deflecting their paths toward the Sun. Every one million years, about a dozen stars pass close enough to stir up the cometary objects, sending a steady trickle of comets into the inner solar system on very long elliptical orbits. A giant interstellar molecular cloud can also impart a gravitational tug when it moves past the comet cloud, helping to jostle some of them out of their remote resting-place. Tidal forces generated in the cloud by the disk of our Galaxy, the Milky Way, also help to feed new long-period comets into the planetary region. As time goes on, the accumulated effects of these tugs will send a few comets in toward the Sun – or outward to interstellar space. If the several hundred new comets observed during recorded history have been shuffled into view by the perturbing action of nearby stars or molecular clouds, then there are at least one hundred billion, or 1011, comets in the Oort cloud. There may be a trillion, 1012, or even ten trillion, 1013, of them. This large population of unseen comets can sustain the visible long-period comets and persist without serious depletion for many billions of years, until long after the Sun expands to consume Mercury and boil the Earth’s oceans away.
The Kuiper belt
The Oort cloud cannot easily explain the comets with the shortest periods, the so-called Jupiter-family comets with periods less than 20 Earth years. These comets have relatively small orbits tilted only slightly from the orbital plane of the Earth, and they usually move in the same prograde direction as the planets. Unlike their longer-period cousins, the motions of the Jupiter-family comets resemble those of the planets. The main source of these comets is thought to be a ring of small icy objects at the outer edge of the planetary realm, just beyond the orbit of Neptune and a thousand times closer than the Oort cloud. It is known as the Kuiper belt, named after the Dutch-American astronomer Gerard P. Kuiper (1905-1973) who predicted its existence in 1951. The name Edgeworth-Kuiper belt is used in the United Kingdom, acknowledging Kenneth E. Edgeworth’s (1880-1972) proposal of the belt’s existence in 1943.
The density in this outer region of the primeval planetary disk was so low that the small objects did not coalesce into a single larger planet. They instead formed the flattened Kuiper belt of 100 million to 10 billion, or 108 to 1010, small frozen worlds that have remained there for billions of years.
Once a comet is launched into the planetary realm, from either the Kuiper belt or the Oort cloud, it may not stay on the same trajectory. Its orbit can be transformed if it passes near Jupiter, the most massive of planets. The giant planet’s gravity can perturb the comet into a new elliptical orbit around the Sun.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University