Instead, these skeptics argue that the present density of such comets is an indicator of a young solar system and Earth. All agree that comets have a finite lifetime. After a limited number of orbits, they are either dissipated by their interactions with the Sun, with some eventually breaking apart, or they are gravitationally ejected from the solar system, or they strike one of the planets and are destroyed. So, unless there is some mechanism for their replenishment, astronomers should not expect to see short-period comets if the solar system is old. Since scientists do see these comets, there must be a mechanism, or the solar system must be young.
Astronomers have proposed that comets come from two belts of debris in the outer extremities of the solar system left over from its formation. The nearer of the two is called the Kuiper belt. It lies just beyond the orbit of Neptune and is believed to be the source of most short-period comets. The more distant belt is called the Oort cloud and is believed to be the source for most long-period comets. Objects are dislodged from these belts into an orbit that passes closer to the Sun either by internal collisions or by gravitational interactions from planets (in the case of the Kuiper belt) or from nearby stars (in the case of the Oort cloud).
More than one thousand objects have been discovered and catalogued in the region of the Kuiper belt (Kuiper belt objects, or KBOs), providing direct evidence for its existence. The planetoid Pluto is one of the largest of these objects. Astronomers speculate that there are at least 70,000 KBOs greater than 100 km in diameter. Evidence for these has recently come from occultation observations reported previously here (9/28/06).
While there has been no direct evidence for the Oort cloud, that scenario is beginning to change as a result of two different lines of research. First, a growing number of observations, especially in the infrared part of the light spectrum, show evidence for extended clouds of material, not unlike the Oort cloud, in the outer regions surrounding certain stars under investigation for new planets and solar systems. An example is discussed here.
The second line of research appears in a study performed by astronomers, J. T. Wickramasinghe and W. M. Napier, published in a recent issue of Monthly Notices of the Royal Astronomical Society. They modeled the vertical movement of the Sun through the plane of the Galaxy that occurs with a period of about 25-35 million years. The astronomers then calculated the expected flux of comets that would come from an Oort cloud, if present, due to galactic tides and encounters with molecular clouds as a consequence of this movement. They established agreement with the ages of well-dated terrestrial impact craters in the past 250 million years.
Further research stands to confirm or negate both lines of evidence. RTB expects that such testing will lay to rest the issue of cometary replenishment, thereby affirming the ancient age of the solar system and Earth.