Floating Through a Cluster

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Science  15 Feb 2002:
Vol. 295, Issue 5558, pp. 1197
DOI: 10.1126/science.295.5558.1197b

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Candidate extrasolar planets have been found in some unexpected places, principally as isolated objects (free-floaters) in globular clusters. Free-floaters are expected to be rare because the standard planet formation model requires that planets form in disks around stars. Furthermore, the relatively delicate disk of gas and dust would have been bombarded and shredded by stellar radiation from nearby stars, which would make planetary accretion within a cluster a rare event indeed.

Using a prototype GRAPE-6 computer (capable of half a trillion floating point instructions per second or 0.5 teraflops), Hurley and Shara have performed N-body simulations of the dynamical interactions between Jupiter-mass planets placed initially in circular orbits around stars in a globular cluster. The planets become gravitationally freed from their parent star if the star traverses near the cluster core. These drifters then float radially outward from the core over a time span of billions of years and do not appear to be destroyed by the cluster environment. Their simulations show that as many as 100 planets per star may form within a globular cluster, such as M22, where the presence of a large number of candidate extrasolar planets has been suggested by observations. Thus, these free-floaters may be more common than previously thought. — LR

Astrophys. J.565, 1251 (2002).

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