Reasons to Believe

Further Comments on Comets

About a year and a half ago I wrote about the controversy between some creationists and the astronomy community over whether the short-lived nature of comets argues for a young earth or for a replenishment mechanism (and, thus, for an old earth). Summarizing what I said there, all parties agree that comets have a limited lifetime, but young-earth creationists argue that the continued presence of comets implies that the solar system must be quite young. On the other hand, astronomers who are convinced the solar system is billions of years old argue there must be an external supply for new comets. Research points to two belts of debris in the outer extremities of the solar system left over from its formation.

The nearest is the Kuiper belt, a little farther from the Sun than the planet Neptune. The more distant is the Oort cloud, which is many times farther out from the Sun than the Kuiper belt. Those who object to the astronomical explanation argue that little or no direct evidence demonstrates the existence of the Oort cloud and that all the objects discovered in the Kuiper belt are too large to qualify as an adequate supply of comets.

In the previous post, I discussed two lines of evidence that may overturn this argument for the Oort cloud. First, astronomers have discovered Oort-like clouds surrounding other stars that have planets orbiting them. Second, a more recent study related the age of terrestrial impact craters with the motion of our solar system through the plane of our galaxy. Now we have a new discovery that will vindicate the Kuiper belt.

A recent study has confirmed the existence of a small kilometer-sized object in the Kuiper belt (see here for a summary of this report). Objects this small and dim cannot be observed directly. However, the authors of this study were able to detect it using data from the guidance sensors of the Hubble Space Telescope. The guidance sensors provide high-precision navigational information for the telescope’s control system by looking at guide stars for pointing. These sufficiently sensitive sensors can record the effects of a small object passing in front of the guide star.

By combing through thousands of hours of guide data taken in the direction of the Kuiper belt, the researchers detected an occultation event that fit what would be expected from such an object. They were able to calculate its distance and size, confirming the existence of Kuiper belt objects in the appropriate size range for comets.

This evidence provides an example of the scientific process, where an explanation for some phenomenon is tested over time and is either confirmed or rejected on the basis of new discoveries. We expect that with further observation, the young-earth explanation for comets will lose favor while the replenishment mechanism––consistent with an old-earth model––gains momentum.

Subjects: Solar System Design

Dr. David Rogstad

Dr. Dave Rogstad received his PhD in physics from Caltech and worked over 30 years for NASA’s Jet Propulsion Laboratory. Though now retired, Dave continues to serve as an RTB board member and participates regularly in several RTB podcasts.