Young Earth Evidence - Short-period Comets

How Good Are Those Young-Earth Arguments?
A Close Look at Dr. Hovind's List of Young-Earth Arguments and Other Claims
by Dave E. Matson
Copyright © 1994-2002

Young-earth "proof" #3: The existence of short-period comets means that the universe is less than 10,000 years old. Comets and meteoroids only last from 10,000-15,000 years before they are blown apart by the solar wind.

In his debate with Dr. Hilpman, Dr. Hovind stated that comets lasted 10,000-15,000 years before being blown apart by the solar wind! Really! Any high school kid with a keen interest in astronomy will tell you that it is the heat of the sun which is a comet's undoing. Each time a comet, which is akin to a dirty snowball, passes near the sun it loses tons of material to vaporization. Thus, the number of orbits such a comet can make before being reduced to a swarm of gravel is limited. The solar wind along with the heat and light of the inner solar system are responsible for a comet's magnificent tail. Thus, comets brighten up as they near the sun, their tails pointing away from the sun. A few comets occasionally crash into one of the planets, especially Jupiter, or into the sun itself. Others are thrown out of the solar system forever.

In passing, let me point out that the projected life span of one short-period comet, that of Halley's comet, is 40,000 years (Chaisson and McMillan, 1993, p.339). Thus, we can forget about Dr. Hovind's 10,000-year figure! A comet's actual life span depends on its size.

Short-period comets can be used to support a young solar system, hence a young earth, only if they have no reasonable source of replenishment. By definition, they orbit the sun at least once every 200 years. Since they lose material each time they pass near the sun, they soon burn out and must constantly be replaced over billions of years. To destroy the creationist argument, we need only throw reasonable doubt on their claim that short-period comets are not replaced. If that point is in doubt, then the whole argument crumbles away.

Creationism's main argument seems to be that we don't have close-up photos of the Oort Cloud and, therefore, cannot be 100% certain that it really exists! Sorry fellas, but if you want to use this comet argument it is up to you to prove, beyond a reasonable doubt, that the Oort Cloud and other sources don't exist! (The Oort Cloud, named after Jan Hendrik Oort, is a calculated accumulation of comets and cometary material occupying the fringes of the solar system at a distance of roughly 50,000 to 100,000 AU. One AU is the average distance of the earth from the sun, i.e., 93 million miles. Various computer studies of cometary orbital data in conjunction with other evidence strongly supports the existence of the Oort Cloud.)

Let's briefly summarize what science knows about comets. In 1950, based on a study of the orbits of several long-period comets, the Dutch astronomer Jan Oort proposed that a great spherical shell of them existed at the remote frontiers of our solar system. Better statistics in more recent years have supported the existence of the Oort Cloud and put it at a distance of 50,000 AU (1.3 light-years).

During the 1980s, astronomers realized that Oort Cloud comets may be outnumbered by an inner cloud that begins about 3,000 AU from the Sun and continues to the edge of the classical Oort Cloud at 20,000 AU. Most estimates place the population of the inner Oort Cloud at about five to ten times that of the outer cloud -- say, 20 trillion or so -- although the number could be ten times greater than that. The innermost portion of the inner Oort Cloud is relatively flattened, with comets extending a few degrees above and below the ecliptic. But the cloud rapidly expands, forming a complete sphere by the time it reaches several thousand AU.

(Benningfield, 1990, p.33)

This inner cloud of comets is called the Hills Cloud. Originally, it was thought that short-period comets were merely long-period comets from the Oort Cloud which had been converted by close encounters with Jupiter or the other large outer planets. That may well be true for some of them, but modern studies of short-period comets have identified their probable origin in a region of space now named the Kuiper Belt, which resembles a flattened ring just beyond the orbit of Neptune. Computer simulations show that such a source would account beautifully for the low-inclination, short-period, prograde orbits, and other features associated with short-period comets. The Kuiper Belt probably has anywhere from 100 million to several billion comets, which probably formed there when the planets formed. The gradual pull of the giant gas planets over time continually send a few of those comets towards the sun. Thus, the short-period comets are replenished from the Kuiper Belt. The Kuiper Belt is no longer "just" a theoretical construct. As of 1998, more than 60 of the larger objects in the Kuiper Belt have been directly observed! That translates to some 70,000 objects out there whose diameter exceeds a whopping 100 kilometers—not to mention countless numbers of normal-sized comets. Jim Foley was kind enough to pass along an Internet site for those of you who may be interested in these new discoveries. The Kuiper Belt web page ( is maintained by David Jewitt, who personally discovered many of these objects.

Thanks to the Hubble Space Telescope, astronomers have finally proven that short-period comets come from a vast region of space beyond Neptune. This is the realm of the Kuiper disk — an enormous population of shadowy mini-ice worlds that slowly orbit the Sun in near total darkness.

(Astronomy, October 1995, p.28)

Theoretical calculations indicate that the great bulk of comets were originally formed in the region between Uranus and Neptune. They represent planetesimals which escaped being gobbled up by the outer planets. Gravitational interactions eventually tossed them into elliptical orbits which took them thousands of astronomical units (AU) away from the sun. This region, then, is the ultimate source of those comets making up the Oort Cloud.

Oort determined that comets tossed into highly elliptical orbits by Uranus and Neptune would be nudged into more nearly circular orbits by encounters with passing stars. Stellar encounters also would scatter comets above and below the ecliptic plane, creating a sphere of comets instead of a flattened disk. After four decades of refinements to Oort's original ideas, astronomers today believe the Oort Cloud extends from about 20,000 to 100,000 AU (almost 2 light-years) from the Sun and contains as many as two trillion comets with a total mass several times Earth's.

(Benningfield, 1990, p.31)

A star passing within a few light-years would likely perturb the orbits of the comets in the Oort Cloud, sending some of them towards the sun. Statistics indicate that about 5000 stars have passed that closely during the earth's lifetime. An encounter with a giant molecular cloud, which is likely to happen every few hundred million years as our sun orbits our galaxy, would also perturb the Oort Cloud.

Another newly discovered agent for perturbing Oort Cloud comets is gravitational tides. Created by the gravitational force of material in the Galactic disk, these tides could alter the orbits of Oort Cloud comets. In fact, some astronomers estimate that as many as 80 percent of the long-period comets entering the inner solar system for the first time were shoved from their previous orbits by the gentle tug of Galactic tides.

(Benningfield, 1990, pp.32-33)

Once in a great while, perhaps 9 times during the lifetime of our Earth (Astronomy, February 1982, p.63), a star will pass so close as to stir up even the Hills Cloud of comets (the innermost Oort Cloud which is shaped mostly like a disk). A collision with a giant molecular cloud would have a similar effect.

Occasionally, though, a star or giant molecular cloud passes directly through both Oort Clouds, scattering comets like a cue ball striking the neatly racked balls on a billiard table. Such an event kicks many comets into the outer cloud, replenishing those lost to other processes.

(Benningfield, 1990, pp.33-34)

Thus, we have adequate sources for replenishing both our long-period comets and our short-period comets over a period of several billion years. In the case of the latter, we can actually see some of the larger ones lurking in the Kuiper Belt!

Granted, we don't have photos of comets in the Oort Cloud or the Hills Cloud. At those distances comets are too small to show up even in the best telescopes. The fact that the Oort and Hills Clouds are still "theoretical" does not mean that they are based on guesswork and rank speculation. Computer simulation, as already mentioned, matched the short-period comets to the Kuiper Belt. Now, we have visual confirmation. Similar studies of long-period comets, even from the 1950s, points clearly to their origin in the Oort Cloud. All in all, a great deal of computer work has been done that supports and refines the above models. The astronomical community treats the Oort Cloud, at the very least, as an excellent working hypotheses.

That there is some kind of large comet reservoir beyond the range of our telescopes follows directly from a simple observation. Astronomers detect new long-period comets at the rate of about one per month. By that rough estimate, 24,000 long-period comets have entered the inner solar system since the time of Christ! Orbital analysis show that these approaching comets generally take several million years to orbit the sun, and, as they are more or less randomly distributed in their orbits, we may deduce that the bulk of them are presently beyond the range of our telescopes. Only the exceptional comet, at any given moment, would be in that small portion of its orbit which crosses the inner solar system.

For the sake of argument, suppose that it takes each of these comets four million years to orbit the sun. In 2000 years we see only 2000/4,000,000 or 1/2000 of them. Thus, we would have about 48 million comets altogether. However, even that figure is extremely low since only the exceptional comet would have an elongated orbit which takes it anywhere near the sun. Oort showed that most of them would happily orbit the fringes of our solar system and never come near the inner regions. Obviously, as you can see from this ballpark calculation, there is an ample source of comets beyond the range of our telescopes.

This reservoir of cometary nuclei surrounding the Sun is called the Oort Cloud . . . Estimates of the number of "dirty snowballs" in the Oort Cloud range as high as 12 billion. Only such a large reservoir of cometary nuclei would explain why we see so many long-period comets, even though each one takes several million years to travel once around its orbit.

(Kaufmann, 1994, p.304)

Another simple observation applies to the short-period comets, which means that we didn't even need the visual confirmation of the Kuiper Belt to win the argument! If there were no means for replenishing comets, then all of them would have the same age. In creationist eyes, they would all be 6000 years old. Yet, observations show that short-period comets with equivalent orbits and sizes have a variety of ages. They range from gaseous "babies," which could hardly have gone around the sun more than a few times, to burnt-out gravel heaps, which have been around the sun once too often. This simple observation proves, beyond a reasonable doubt, that the short-period comets are being replaced.

Benningfield (1990, p.32) gives some interesting evidence indicating that vast comet clouds exist around certain stars, but we shall not pursue the matter further. The point has already been made. In order to win this argument, the creationist must prove that there are no reasonable sources for replenishing comets. Instead, we find compelling evidence for cometary reservoirs!

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