Comets may be spawned when mum breaks up [circling the wagons cont'd]

Are comets born in great swarms? The puzzling abundance of comets in short solar orbits has led a pair of astronomers to suggest that they are fragments of larger bodies that crumbled as they entered the inner solar system.

Short-period comets take less than 200 years to circle the sun and are thought to originate in the Kuiper belt of icy objects beyond Neptune. Some Kuiper-belt objects (KBOs) are in vulnerable orbits that allow the gravity of the outer planets to tug them inwards, where the sun's heat turns them into comets.

However, there seem to be too few KBOs in these vulnerable orbits to maintain the 10,000-strong population of short-period comets, which is continually thinned as they burn out, fall into the sun or get ejected from the solar system. When Kathryn Volk and Renu Malhotra at the University of Arizona in Tucson calculated the rate at which Kuiper-belt objects should be drawn inwards, their result was just 1/500th of the rate needed (The Astrophysical Journal, in press).

This disparity would disappear if some of the largest KBOs crumble into 100 to 1000 pieces as they are drawn into the inner solar system. The idea is supported by the fact that comets have been seen to break up from the stress of encounters with planets. "Comets are thought generally to be pretty fragile," Malhotra says.

Alternatively, small KBOs in more stable orbits may be so numerous that they contribute the lion's share of comets, even though a smaller fraction of them are nudged our way. Luke Dones of the Southwest Research Institute in Boulder, Colorado, favours this theory. "I know of no evidence that comets can break into 100 or 1000 pieces," he says.

arXiv:0802.3913

Title: The Scattered Disk as the source of the Jupiter Family comets

Authors: Kathryn Volk, Renu Malhotra (Lunar and Planetary Laboratory, University of Arizona)

(Submitted on 26 Feb 2008 (v1), last revised 11 Jul 2008 (this version, v2))

Abstract: The short period Jupiter family comets (JFCs) are thought to originate in the Kuiper Belt; specifically, a dynamical subclass of the Kuiper Belt known as the `scattered disk' is argued to be the dominant source of JFCs. However, the best estimates from observational surveys indicate that this source may fall short by more than two orders of magnitude the estimates obtained from theoretical models of the dynamical evolution of Kuiper belt objects into JFCs. We re-examine the scattered disk as a source of the JFCs and make a rigorous estimate of the discrepancy. We find that the uncertainties in the dynamical models combined with a change in the size distribution function of the scattered disk at faint magnitudes (small sizes) beyond the current observational limit offer a possible but problematic resolution to the discrepancy. We discuss several other possibilities: that the present population of JFCs is a large fluctuation above their long term average, that larger scattered disk objects tidally break-up into multiple fragments during close planetary encounters as their orbits evolve from the trans-Neptune zone to near Jupiter, or that there are alternative source populations that contribute significantly to the JFCs. Well-characterized observational investigations of the Centaurs, objects that are transitioning between the trans-Neptune Kuiper belt region and the inner solar system, can test the predictions of the non-steady state and the tidal break-up hypotheses. The classical and resonant classes of the Kuiper belt are worth re-consideration as significant additional or alternate sources of the JFCs.

Comments: 33 pages, 6 figures. Revised Content. To be published in The Astrophysical Journal
Subjects: Astrophysics (astro-ph)
CiteÂ as: arXiv:0802.3913v2 [astro-ph]

Submission history
From: Kathryn Volk [view email]
[v1] Tue, 26 Feb 2008 21:30:44 GMT (33kb)
[v2] Fri, 11 Jul 2008 17:39:44 GMT (36kb)

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