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Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits


The Kuiper belt1 extends from the orbit of Neptune at 30 au to an abrupt outer edge about 50 au from the Sun2. Beyond the edge is a sparse population of objects with large orbital eccentricities3,4. Neptune shapes the dynamics of most Kuiper belt objects, but the recently discovered planet 2003 VB12 (Sedna5) has an eccentric orbit with a perihelion distance of 70 au, far beyond Neptune's gravitational influence6,7,8. Although influences from passing stars could have created the Kuiper belt's outer edge and could have scattered objects into large, eccentric orbits9,10, no model currently explains the properties of Sedna. Here we show that a passing star probably scattered Sedna from the Kuiper belt into its observed orbit. The likelihood that a planet at 60–80 au can be scattered into Sedna's orbit is about 50 per cent; this estimate depends critically on the geometry of the fly-by. Even more interesting is the 10 per cent chance that Sedna was captured from the outer disk of the passing star. Most captures have very high inclination orbits; detection of such objects would confirm the presence of extrasolar planets in our own Solar System.

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Figure 1: Stirring of the eccentricities of planets by the close pass of a Sun-like star.
Figure 2: Orbital elements of planets, formed in situ in the planetary disk, after the fly-by of a one-solar-mass star as a function of final heliocentric distance.
Figure 3: Orbital elements of planets, initially in a ‘scattered disk’, after a fly-by, as described in Fig. 2.

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We acknowledge a generous allotment (of about 3,000 c.p.u. days) of computer time at the supercomputing centre at the Jet Propulsion Laboratory through funding from the NASA Offices of Mission to Planet Earth. Aeronautics and Space Science. Comments from M. Geller improved our paper. The NASA Astrophysics Theory Program supported part of this project.

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Correspondence to Scott J. Kenyon.

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Kenyon, S., Bromley, B. Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits. Nature 432, 598–602 (2004).

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