One of the most popular creationist arguments for a young solar system is the rapidity with which short-period comet populations are being depleted. Although appeals are made to the Kuiper Belt as a source of new short-period comets, there appears to be too few objects in this region to maintain the numbers over billions of years (Worraker 2004). A recently published paper in The Astrophysical Journal indicates that this problem for conventional timescales remains unresolved.
Volk and Malhotra (2008) studied a subclass of Kuiper Belt Objects (KBOs) known as “the scattered disk” that are thought to be the main source of new Jupiter family comets (JFCs). (Jupiter family comets have orbital periods of less than 20 years, in distinction to Halley family comets which have orbital periods of between 20 and 200 years.) Their theoretical calculations indicate that a population of at least (0.8-1.7) x 108 comet-sized (>1 km diameter) scattered disk objects within 30-50 AU of the sun is necessary to supply the observed population of JFCs in steady state on billion-year timescales. However, the best-fit observational number (3 x 105) falls short of the theoretical requirement by more than two orders of magnitude and the number based on the 95% confidence limit (2 x 108) is only marginally consistent with the theoretical prediction.
The authors discuss a number of possible reasons for the discrepancy, including (1) incompleteness of the observed JFC population, (2) uncertainty in the conversion of magnitude to size when comparing population estimates, (3) a possible upturn of the size distribution of the scattered disk objects beyond current observational limits, (4) questioning the assumption that the JFC population is in steady state with its source, (5) postulating a different or additional source for the JFCs and (6) the breakup of the scattered disk objects into multiple fragments during their transport from the trans-Neptune region into the inner solar system. However, revisions to the first two factors would tend to make the discrepancy worse, while the others are regarded as implausible or problematic given what we currently know.
As I concluded in The New Creationism (p.107), “The simplest explanation as to why we can still see short-period comets is that the solar system is young.” Volk and Malhotra’s recent analysis strengthens my confidence in that conclusion.
Volk K. and Malhotra R. 2008. The scattered disk as the source of the Jupiter family comets. Astrophysical Journal 687:714-725. [Click here for the preprint]
Worraker B. 2004. Missing: a source of short-period comets. TJ 18(2):121-127.