Do Short-Period Comets Still Reveal Recent Creation?

The Source of Comets’ Brightness

The source of comets’ brightness is their nuclei, which are only a few miles across. Comet nuclei consist of tiny dust particles held together by ice. Most of the ice is water, but there are other ices made of what are usually gases, such as carbon dioxide and methane.

The orbits of comets are very elliptical, meaning that the orbits are like long, stretched out circles. One end of a comet’s orbit brings it closest to the sun, a point that we call perihelion. Comets move most quickly at perihelion, so comets spend most of the time far from the sun, so their ice remains frozen most of the time.

But when a comet briefly passes near perihelion, the sun heats the ice, turning it into gas, and the gas quickly expands from the nucleus, carrying dust particles with it and forming the coma, which is the brightest part of a comet. Light from the sun and the solar wind push the dust particles and gases away from the sun, forming a long tail. The gas around the nucleus and the tail brighten tremendously when near perihelion, which is when a comet can briefly put on quite a show. But after perihelion, a comet rapidly fades as it moves away from the sun and soon is no longer visible. After one orbital period, the comet will return and once again briefly brighten.

Comet Tsuchinshan-ATLAS
Taken October 2024, courtesy Dr. Danny Faulkner.

Two Types of Comets

Astronomers recognize two types of comets, long-period comets and short-period comets.

Long-period comets typically have orbital periods greater than 200 years, while short-period comets having periods less than 200 years. This is not an arbitrary distinction because the orbits of the two groups of comets are very different. The orbits of the planets lie in nearly the same plane, with the planets’ orbital planes tilted, or inclined, to one another by only a few degrees. Astronomers use the ecliptic, the earth’s orbital plane, as the reference plane. Short-period comet orbits usually are inclined to the ecliptic by less than 30 degrees. On the other hand, long-period comets have much greater inclinations, all the way to 90 degrees.

Another difference between long-period comets and short-period comets is that short-period comets move in their orbits the same direction that the planets orbit the sun—counterclockwise as viewed from above the earth’s North Pole. But long-period comets are evenly split between orbiting the same direction of the planets (prograde) and opposite the direction the planets orbit (retrograde). There are a few comets that defy this classification. A notable example is Halley’s Comet, which has a period of only 76 years but has a retrograde orbit.

Halley’s Comet
“Comet Halley-exp-14a-300dpi-8x10-scan4” by Dennis Harper, CC BY-ND 2.0

Evolutionists Have a Problem

If no new short-period comets are created, then within 20,000 years, there should be no more short-period comets.

Since comet nuclei are so small, the ice available to power comets’ bright display when near perihelion is limited. So how many times can a comet make it to perihelion and still be seen? Estimates vary, but it would be optimistic to think a comet could survive 100 trips around the sun. Taking the most optimistic estimate and multiplying by 200 years, the maximum orbital period for short-period comets, we arrive at the figure of 20,000 years. That is, if no new short-period comets are created, then within 20,000 years, there should be no more short-period comets.

Young-earth creationists believe that the solar system is only a little more than 6,000 years old, so the existence of short-period comets is not a problem within a recent-creation framework. But this is a serious problem for those who believe the solar system is billions of years old. And even long-period comets could not exist for billions of years either, so they also present a problem for those who believe that the solar system has existed for billions of years.

Evolutionists’ Explanation Attempts

Of course, astronomers committed to billions of years have long known about this problem. In 1950, the Dutch astronomer Jan Oort suggested that far from the sun there is a roughly spherical distribution of comet nuclei orbiting the sun. This Oort cloud, as this has come to be called, supposedly is leftover material from the naturalistic formation of the solar system. Being so far from the sun, the comet nuclei would be very cold and hence would keep their ices. However, a passing star might rob these comet nuclei of orbital energy, causing them to fall into the inner solar system, making them long-period comets.

This might explain the existence of long-period comets, but what about short-period comets? For years, it was thought that the gravitational forces of the planets (mostly Jupiter) on long-period comets as they passed by on their way to and from perihelion would shorten some comets’ orbits, transforming them into short-period comets. But in the 1980s, computer simulations showed that this mechanism was too inefficient to work—comets would lose all their ices necessary to brighten them long before they would become short-period comets.

The Proposed Oort Cloud and Kuiper Belt
via NASA

In 1951, the astronomer Gerard Kuiper proposed that early in the solar system, there was a belt of icy asteroids beyond the orbit of Neptune. This would be the leftover material from the solar system’s formation. Kuiper thought that his Kuiper Belt no longer existed, as the gravitational tugs of the outer planets would have dispersed them to higher orbits, forming the Oort cloud. Once astronomers realized that they needed a source for short-period comets too, they began to think that the Kuiper Belt might still exist after all.

In the 1990s, technology had advanced to the point that looking for small icy bodies (comet nuclei) beyond the orbit of Neptune might reveal some of the larger members of the Kuiper Belt. Indeed, in the 1990s, the first trans-Neptunian objects (TNOs) began to be discovered. Some TNOs are quite large. Eris, discovered in 2005, is nearly the same size as Pluto. Speaking of Pluto, astronomers now understand that Pluto, discovered in 1930, was just the first (and so far, largest) TNO known. TNOs make up a second asteroid belt beyond the orbit of Neptune. The asteroids in the classic asteroid belt between the orbits of Mars and Jupiter are rocky, but the TNOs probably are more icy than rocky.

Sometimes TNOs are called KBOs (Kuiper Belt objects). What is the difference? The term TNO is more descriptive without any suggestion about origin or age, while KBO is a more loaded term, revealing a bias toward billions of years and naturalistic origin. It is interesting that over the past 30 years, in the technical astronomy literature, I have noticed a shift away from using the term KBO in favor of the term TNO.

Should Creationists Drop the Short-Period Comet Argument?

So are the TNOs the source of short-period comets, with the result that the existence of short-period comets is no longer a good argument for recent creationists? Not necessarily.

There are differences in composition between the two types of comets. Evolutionary astronomers explain this by different development over time—though both groups of comets ultimately came from the same source. TNOs, being closer to the sun, may have undergone changes that the objects in the Oort cloud did not.

More problematic is the fact that many TNOs are quite large. For instance, Pluto and Eris are more than 1,400 miles across. That is 100 times larger than what are usually considered to be large comet nuclei. It is almost unimaginable how bright a comet with a nucleus the size of Pluto or Eris would be. Throughout history, there has never been an observed comet of this magnitude. If TNOs are the source of short-period comets (and ultimately all comets), why has mankind never seen the monster comets that must exist?

Conclusion

Like so many other things, the Kuiper Bbelt relies upon the changing ideas of man.

Astronomers think that they have found the Kuiper Belt, the source of short-period comets (and within their evolutionary theories, ultimately all comets), but problems remain. But like so many other things, the Kuiper Belt relies upon the changing ideas of man (keep in mind that for many years, astronomers didn’t think the Kuiper Belt existed anymore). At Answers in Genesis, we are committed to the reliability of Scripture, and the Bible clearly teaches that God created all things in heaven and earth in six normal days (Exodus 20:11), only thousands of years ago. We believe that not only do the heavens declare God’s glory (Psalm 19:1), but they also reveal his creation and that the creation did not take billions of years.