Re-viewing an Old Comet Reservoir

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Science  04 Sep 2009:
Vol. 325, Issue 5945, pp. 1211-1212
DOI: 10.1126/science.1177312

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A few times a year, new long-period comets (LPCs) on elongated orbits come to within ∼ 1 astronomical unit (AU) of the Sun (1 AU is the distance between the Earth and the Sun), where they draw our attention by releasing majestic tails of dust and ions from their frozen surfaces. The original orbits typically trace back out to distances of at least 20,000 AU. Since Jan Oort's classic 1950 paper (1), it was believed that the reservoir for the LPCs is a roughly spherical “Oort Cloud” of approximately 1 trillion comets, which extends from 20,000 to 100,000 AU (about halfway to the nearest stars). The bodies in the Oort Cloud are thought to be the surviving population of unincorporated remnants of planetary building blocks that were gravitationally scattered outward by the growing planets. On page 1234 of this issue, Kaib and Quinn (2) present results of a simulation that suggest that a substantial fraction of LPCs (perhaps the majority) are stored in an “inner” Oort Cloud reservoir considerably closer to the Sun than expected, at distances of around 3000 to 10,000 AU.