Posted by: paulgarner | April 6, 2009

Jupiter family comets and the age of the solar system

Train of fragments of Jupiter family comet Shoemaker-Levy 9. Credit: H.A. Weaver, T. E. Smith (Space Telescope Science Institute), and NASA.

Train of fragments of Jupiter family comet Shoemaker-Levy 9. Credit: H.A. Weaver, T. E. Smith (Space Telescope Science Institute), and NASA.

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.

Reference

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.

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Responses

  1. If short-period comets are shown to be young (which I don’t think they are …), then that does not mean the whole solar system is young ! That is jumping conclusions in a major way.

    • Comets are conventionally thought to represent debris left over from the condensation of the solar nebula, and are therefore regarded as equivalent in age to the solar system itself. Most creationists don’t accept the solar nebula hypothesis, but nevertheless also think that comets originated at essentially the same time as the solar system. Unless the origin of comets is radically re-envisaged, it seems logical to infer that young comets are evidence of a young solar system.

  2. No, because there easily can be a huge range of ages in comets: the ones mentioned here are a subclass of comets only! So their age, if indeed young (which they probably are not: they are short-period comets, not young comets !) is only a *lower* limit to the age of the solar system.

    And then there is all the other evidence for the ages of the earth, the sun, etc., which you cannot disregard.

    • Okay, I see what you’re saying. Although the precursor population may have been primordial, different classes of objects derived from that population may have had very different histories. Accepted, and in my reply I should perhaps have been more careful to state the case in those terms. Nevertheless, Jupiter-family comet lifetimes as comets are only of the order of 10 ka (Hughes 2003; Levison and Duncan 1997) and the analysis by Volk and Malhotra does seem to indicate that the currently-estimated Kuiper Belt population could not have sustained the Jupiter family of comets for billions of years. (Discussions of other evidence will have to await other posts!)

      References

      Hughes D. W. 2003. The variation of short-period comet size and decay rate with perihelion distance. Monthly Notices of the Royal Astronomical Society 346:584-592.

      Levison H. F. and Duncan M. J. 1997. From the Kuiper Belt to Jupiter-family comets: the spatial distribution of ecliptic comets. Icarus 127:13-32.

  3. Well, yes, the currently-estimated Kuiper Belt population. Which is highly uncertain.

    Again, the short-period comets are just that, they have a short period. But they do not need to be young at all: they can be as old as the rest of the solar system, i.e. up to 4.5 billion years. And objects with that age have been found. The age of the sun is also estimated to be around 4.5 Gyr (Gigayear), and the Earth is easily over 4 Gyr.

    How young do you estimate the solar system to be ? Again, whatever you say about short-period comets, this can only provide a lower limit to its age !

    • I think we’re in danger of going around in circles here. The JFC data impose an upper limit, not a lower limit, on the age of the solar system. To sum up the argument: the JFC population cannot be sustained for billions of years given present best estimates for (1) the precursor population and (2) the rate of escape, since the precursor population is too small (by at least two orders of magnitude), assuming that (a) the JFC population has remained roughly constant for much of the lifetime of the solar system, and (b) the history of the JFCs is closely integrated with the history of the solar system as a whole. Both factors are, of course, subject to considerable uncertainties, but the conclusion stands even with favourable assumptions from the conventional point of view. Volk and Malhotra looked at various ways of resolving the discrepancy and concluded that all of them were implausible. More information is available in this paper by Bill Worraker, in which the argument is developed at greater length.


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