THE PROBLEM WITH COMETS

 

Gerardus D. Bouw, Ph.D.

 

 

          Comets are solar system objects with highly elliptical orbits, such as the one shown below for Haley’s comet.  During the Dark Ages, so called because the Roman Church refused to educate the laity, comets were regarded as evil omens, portending disasters.  The appearance of Haley’s comet in 1066 just before the death of the last Anglo-Saxon king, Harold II of England, helped perpetuate the myth.  It was also a bad omen for king Harold III of Norway, who was killed in the same battle fighting against Harold II.  One must not forget, though, that what portended disaster for the Anglo-Saxon was a good omen to the victorious Normans who thus conquered England.  Other civilizations viewed comets with mixed emotions, some for evil, others for good.  The name comet literally means hairy star, reflecting the ancient views of these mysterious objects.  Today we think of them as icy mud balls, though before probes flew by a couple of comets they were regarded as dirty snowballs.

Figure 1:  The current orbit of Halley’s comet.  Note that the orbit is retrograde (i.e., runs in the opposite direction) to the planetary orbits.

 

          Comets fall into two major categories: long period comets, and short period comets.  In the 1930s, astronomers recognized three categories, based on the distance from the sun of their aphelion points.[1]  These had their aphelia clustered near Saturn, near Uranus, and near Neptune respectively.  Halley’s comet belonged to the Neptune group and today falls into the short period comet category.  In the early twentieth century these were called “Periodic comets.”  At the time, what we now call long period comets were thought to be from beyond the solar system.  This was a reasonable conclusion for all the long period comets discovered in the nineteenth century had periods of tens of thousands of years or longer—if they were periodic at all.  Today, not only are the comets classed into long and short period categories, but at least the short period comets are subdivided into two major subgroups.

 

Long period comets

 

          Long period comets have periods ranging from as short as 200 years to more than 10 million years.  Such periods mean that these comets have very elongated elliptical orbits, ranging out from 34 astronomical units[2] up to 100,000 a.u., almost two light years out or half-way to the nearest star.

          In 1950, Dutch astronomer Jan Oort noticed three things about the long period comets:

 

1. no comet has been observed that definitely came from outside the solar system;
2. long period comets can come from any and all directions,
3. and that there is a strong tendency for the aphelia of long-period comets to lie at a distance some 50,000 times as far away as the earth is from the sun.[3] 

 

From those three observations, Oort concluded that long-period comets originate from a shell centered on the sun and populated by as many as a trillion (10¹²) comets.  Modern astronomers believe that the innermost part of the shell starts at 20,000 a.u. and extends out to 100,000 a.u. 

          Evolutionist astronomers believe that the Oort cloud, as the proposed shell of comets is called, is left over from the formation of the solar system.  The outer regions are postulated to have been too cold to be evaporated and blown away by the young sun’s heat as it formed.  Yet no such cloud of comets has been observed at such great distances.  It turns out that evolutionary ages of billions of years require such a cloud, nevertheless.

          The Oort cloud postulate offered a solution for a rather vexing problem for evolutionary ages.  Astronomers compute that it takes about a hundred orbital passes near the sun before a comet’s gas and dust evaporate, leaving just the rocky core.  Now even if one imagines comets with periods of 15 million years, in the 4.5 billion years that the sun is alleged to have been shining, such comets would have made as many as 300 passes near the sun, so at best they should be dark bodies of rock and sandy debris, and we should rarely, if ever, see a comet today.  Nevertheless, a handful are discovered every year, though most are faint.  So, neither the long orbital period, nor the tremendous distance from the sun (where a comet spends most of its time) solves the age problem for the evolutionists.  Another postulate is needed to save the 4.5-billion years from extinction.

          In order to keep the comets reasonably young, it is now assumed that they spend billions of years orbiting the sun in nearly circular orbits.  Occasionally a passing star perturbs the comets in the Oort cloud, like a breeze disrupts particles of smoke.  Many of the comets will be thrown out of the Oort cloud into interstellar space, but some will fall towards the sun.  As they enter the realm of the planets, the planets will further perturb the long period comets, ejecting some completely from the solar system and capturing some into shorter-period or even short period cometary orbits.  These, of course, will fade quickly from the scene in evolutionary time, but there are more on the way to replace them.  In this way, if there were indeed a trillion comets in the Oort cloud and not just a few million, evolutionists think to explain why we see long period comets today.  The most recent candidate for an Oort cloud object was the long period comet called Sedna that was discovered in 2004.[4]

          But is that reasonable?  Let us assume that the average distance between stars is 4 light years.  We observe that most of the stars in the solar neighborhood move with a speed of about 2 miles per second or 3 km/sec.  If so, we can expect a close passage roughly once every four light years of travel.  Now to travel 4 light years at 2 miles per second takes only 400,000 years, so in 4.5 billion years 11,250 stars should have passed close enough to the sun to disrupt the comets in the Oort cloud.  After so many close encounters, would any comets remain in the Oort cloud?  And how come we don’t see comets from any of the 11,250 passing stars?  Why are they all from our solar system?  Increasing the mean distance between stars to six light years reduces the figure to 7500 close encounters, still a significant number.  Increasing the speed of the stars increases the number of encounters.  For instance, stars with a speed of 20 miles per second would cover the four light years in 40,000 years, a tenth the time.

          We find then that whether or not the Oort cloud exists—and its existence really is still up in the air—the cloud may not at all satisfy the evolutionists’ need to keep the inner solar system supplied with comets for 4.5 billion years.  We conclude that even if the Oort cloud exits, it poses no significant challenge to the 6,000-year old solar system account of the Holy Bible. 

But evolutionists take heart; some astronomers are honest and smart enough to see the impossibility of insisting that the Oort cloud is a remnant of the formation of the solar system 4.5 billion years ago.  They now propose that the Oort cloud is not a leftover from the protoplanetary disk that allegedly formed the solar system.  Instead, they postulate that it consists of short period comets that have been ejected from a region in the solar system called the Edgeworth-Kuiper belt, named after Gerard Kuiper and Kenneth Edgeworth who proposed its existence in 1950. 

 

Short period comets

 

          In 1992, astronomers became aware of small bodies orbiting the sun beyond Neptune.  There are at least 70,000 “trans-Neptunians,” as these objects are called, with diameters larger than 60 miles (100 km) in a zone extending outwards from the orbit of Neptune (at 30 a.u.) to 50 a.u.  Observations show that the trans-Neptunians reside within a thick band around the ecliptic.  In other words, they form a ring, or belt, surrounding the sun.  This ring is generally referred to as the Kuiper Belt.

          Short period comets, by definition, have a period of 200 years or less.  That, in turn, means that their aphelia are under 34 a.u., 4 a.u. beyond Neptune and 6 a.u. short of Pluto’s 39.4 a.u. mean distance from the sun.  Though presently Pluto and its moon, Charon, are closer to the sun than is Neptune, Pluto spends most of its time in the Kuiper belt.  Indeed, more and more astronomers are starting to view the planet as a Kuiper belt object, though for historical reasons, Pluto is not about to lose its status as planet.

Because Pluto spends so much time in the Kuiper belt, and because so many of the known Kuiper belt objects are binaries, it behooves us to look a bit more closely at that icy object and its moon.  With a diameter of 1400 miles (2274 km), Pluto is the largest known Kuiper belt object by at least a factor of two.  Pluto’s moon, Charon, orbits 12,120 miles (19,640 km) above Pluto.  Charon’s diameter is 723 miles (1170 km).  Pluto and Charon are phase locked.  That means that just as the moon always shows the same face to the earth, so Charon always presents the same side to Pluto.  The moon’s rotation is thus phase-locked to the earth.  But the similarity does not stop there; Pluto always shows the same face to Charon, too.  That means that seen from Pluto, Charon never sets in one hemisphere and never rises in the other.  Like Uranus, Pluto’s rotational pole lies almost in its orbital plane, being inclined 124°.  (That means that Pluto’s north pole lies 34° below its orbital plane.  For comparison, Uranus’ pole points about 10° below its orbital plane, whereas every other planet’s north pole lies above its orbital plane.  The earth’s north pole is inclined 23.5° to the ecliptic, the yearly path of the sun about the earth commonly called “earth’s orbit.”)  From an evolutionary stance, it is reasonable to expect that in the tens of millions of Plutonian years that have elapsed in the alleged 4.5 billion years since the solar system was formed, Pluto and Neptune should have cleaned out much of the Kuiper
belt objects around them.  That means that the number of Kuiper belt objects should increase further out from these two planets. 

 

Figure 3:  Pluto (left) and Charon photographed by the Hubble telescope in 1994.  The bright spots are reflections of the sun from their icy surfaces.

 

Most short period comets are now thought to belong to the Kuiper belt, with a small percentage being long period comets that have been perturbed into short-period orbits by encounters with the planets.  Just how the comets originate is still a matter of speculation, but the theory is that close encounters between Kuiper belt objects (KBOs) cause some to be ejected from the belt.  Some of those ejected will escape into interstellar space; others may fall into long period orbits, some of which will approach the sun.  Some of the ejected objects will head for the sun and become short period comets.  That, at least, is the theory.

 

 

          Although observations so far have confined the Kuiper belt to a region between 30 and 50 a.u., it is expected to extend to 1000 a.u. from the sun.  The perceptive reader will note that this leaves a huge empty gap between the Kuiper belt and the Oort cloud, fully 19,000 a.u. if the long period comets are thought to originate from a shell ranging from 20,000 to 100,000 a.u., as stated above.  The following table illustrates the problem.

 

Distance in a.u.	Period in years
100,000	32 million
20,000	3 million
1,000	32 thousand
30	164

 

There is no reservoir of comets with periods between 32 thousand and three million years.  The distinction made between short and long period comets suggests that such a division is real.  Indeed, the upper value of 200 years for the short period comets suggests that even among them, the majority originate not from the Kuiper belt but come from inside the Kuiper belt.  But there is more. 

 

The Kuiper belt does not help comet origin theories[5]

 

          In 2003, astronomers using NASA’s Hubble Space Telescope conducted a search and discovered three of the faintest and smallest objects ever detected in the Kuiper belt.  Each object is a lump of ice and rock—roughly the size of Philadelphia—orbiting beyond Neptune and Pluto.  The Kuiper belt is presumed by evolutionists to be the leftover building blocks, or “planetesimals,” from the solar system’s formation.

          The study’s big surprise is that so few Kuiper belt members were discovered.  With Hubble’s exquisite resolution, Gary Bernstein of the University of Pennsylvania and his co-workers expected to find at least 60 Kuiper Belt members as small as 10 miles (15 km) in diameter, but only three were discovered. 

          That there were many fewer Kuiper belt objects observed than was predicted makes it difficult for evolutionists to understand how so many comets appear near earth.  Instead of ejecting each other, the study is a sign that the smaller planetesimals have been shattered into dust by colliding with each other over the past few billion years.

          Bernstein and his colleagues used Hubble to look for planetesimals that are much smaller and fainter than can be seen from ground-based telescopes.  Hubble’s Advanced Camera for Surveys was pointed at a region in the constellation Virgo over a 15-day period in January and February 2003.  A bank of 10 computers on the ground worked for six months searching for faint-moving spots in the Hubble images.  The search netted three small objects, named 2003 BF91, 2003 BG91, and 2003 BH91, which range in size from 15-28 miles (25-45 km) across.  They are the smallest objects ever found beyond Neptune, but an icy body of their size that approached the inner regions of the solar system, can be seen from earth as a comet.

          If the Hubble telescope could search the entire sky, it would find perhaps a half million planetesimals, at least, that is the claim of the secular astronomer.  If collected into a single planet, however, the resulting object would be only a few times larger than Pluto.  Why the Kuiper Belt planetesimals did not form a larger planet, and why there are far fewer small planetesimals than expected, are questions that evolutionists confidently assert will be answered with further Kuiper Belt studies.  Their batting average is not good. 

          So it’s back to the drawing board for the world’s astronomers.  There is still no explanation for comets that fits the criterion that the universe was created 12 billion years ago, nor is there any solid evidence that the solar system is 4.5 billion years old.  The best and most consistent theory for the origin of the cosmos and the solar system is still that they were created by our omniscient, omnipotent God about 6,000 years ago.