ScienceWeek

PLANETARY SCIENCE: ON COMETS IN THE ASTEROID BELT

The following points are made by Alan Fitzsimmons (Science 2006 312:535):

1) Astronomers have known for more than two centuries that comets can be split into two groups as defined by their orbits about the Sun. Long-period comets, so named because they have orbital periods of more than 200 years, originate from the Oort Cloud of comets surrounding the Sun and stretching at least 10% of the way to the nearest star [1]. The second group are known as the Jupiter-family comets, with orbital periods near 20 years, whose dynamical evolution is controlled by gravitational encounters with the giant planet. Theoretical work pinpointed the source of this second group to a comet belt beyond the planet Neptune [2-4]; this was dramatically proven by the discovery of the first such object in 1992 [5]. Now new work [6] has issued a shock to planetary science by demonstrating the existence of a third dynamical class of comets, orbiting much closer to the Sun and lying entirely within the main asteroid belt.

2) The story starts in 1996 with the discovery that an asteroid first seen 17 years earlier was in fact a comet, henceforth named 133P/Elst-Pizzaro. Observationally, all but the largest asteroids are optically unresolved and appear as point sources, whereas active comets are recognizable when near the Sun from the surrounding atmosphere of sublimated ices and dust particles. Each year, several objects classified as asteroids but lying in elongated comet-like orbits are found to exhibit a coma and/or tail and hence are reclassified as comets. The surprising fact about 133P/Elst-Pizarro was that its orbit was unlike that of any other comet, as it lay completely within the asteroid belt between Mars and Jupiter. Another comet on a similar orbit was discovered late last year, and Hsieh and Jewitt report finding a third in a dedicated survey for such objects. All three objects are relatively stable against strong gravitational perturbations from Jupiter, which implies that they exist where they formed.

3) Hsieh and Jewitt [6] show that the detected atmosphere of dust particles cannot be caused by weak processes such as electrostatic levitation, nor can it be the debris cloud from an impact by a smaller body, and hence it must result from the steady sublimation of ices as with other comets. There is no escaping the recognition that we now have a third dynamical class of active comets identified in the solar system, which Hsieh and Jewitt [6] have labeled "main-belt comets".

4) This throws up a number of questions. Perhaps the most important is how they can exist in the first place. Comets are ephemeral bodies, as each time they pass the Sun they lose a small fraction of their mass via sublimation of the surface ices. For example, the lifetime of Mark Twain's nemesis, Halley's comet, has been estimated as less than 100,000 years [8]. The comets we see today disappear on these time scales, to be replenished by new comets from the Oort Cloud and the trans-Neptunian reservoirs. But the main-belt comets are still in their source regions, where continuous solar heating would have seen them vanish very soon after formation. Hsieh and Jewitt [6] believe that the likely answer for main-belt comets is that they have suffered a small collision in the recent past, which has exposed subsurface ices to solar heating, and that these ices may sublimate on and off for at least several years before exhaustion.

References (abridged):

1. J. H. Oort, Bull. Astron. Inst. Neth. 11, 91 (1950)

2. K. E. Edgeworth, Mon. Not. R. Astron. Soc. 109, 600 (1949)

3. G. P. Kuiper, in Astrophysics, J. A. Jynek, Ed. (McGraw-Hill, New York, 1951), p. 357

4. J. A. Fernandez, Mon. Not. R. Astron. Soc. 192, 481 (1980)

5. D. Jewitt, J. Luu, Nature 362, 730 (1993)

6. H. H. Hsieh, D. Jewitt, Science 312, 561 (2006)

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