Emily has a very nice blog entry on this: http://www.planetary.org/blog/article/00000551/ . Basically, Henry Hsieh and David Jewitt have not only found three comets lurking in the outer part of the Main Asteroid Belt: they've proven that they probably formed there originally! (There were suspicions about one of these -- Elst-Pizarro -- before.) These are probably the tip of (literally) an iceberg out there -- and they also turn out to be lurking in just that part of the Belt from which Jupier likely directed objects inwards to crash into the inner planets during the Solar System's earliest days, meaning that they are also promising as the source for Earth's and Mars' now-famous excess of water beyond what they probably had when they originally formed. (There was a very brief abstract several months ago on their discovery for this year's COSPAR meeting, but the details were extremely sparse then: http://www.cosis.net/abstracts/COSPAR2006/02767/COSPAR2006-A-02767.pdf?PHPSESSID=92daca68c968a79b4e2f32646b163e72 )
This, of course, meshes nicely with the recent density measurements of the Trojan asteroid Patroclus (made possible by the fact that it has a moon) suggesting that it too is an iceball: http://www.keckobservatory.org/news/science/060201_patroclus/index.html . And that, in turn, meshes nicely with the discovery by Joshua Emery and Dale Cruikshank that the surfaces of the Trojans have IR spectra similar in some ways to that of comets, which may be due to HCN polymers in them such as those comets are thought to contain (although the Trojans do not themselves have comas): http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2075.pdf
To complicate matters further, there was a dynamics study several years ago -- right now I can't remember by whom -- suggesting that several percent of Kuiper Belt and Oort Cloud objects may be originally be rocky asteroids from the Asteroid Belt (including its inner parts!) As if the definitions of "planet" and "moon" weren't intolerably fuzzy themselves now, the definitions of "comet" and "asteroid" are also being irreversibly fuzzed up -- and there's no point trying to conceal it from schoolkids.
Very intriguing ideas...
Perhaps, Oort cloud existence could become unnecessary if we find an efficient mechanism (Jupiter gravity + collisions?) able to transform these objects in long period comets!
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