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A Uranian Trojan and the Frequency of Temporary Giant-Planet Co-Orbitals

Science  30 Aug 2013:
Vol. 341, Issue 6149, pp. 994-997
DOI: 10.1126/science.1238072

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A Uranian Trojan

Bodies that share their orbit with that of a planet and that trail or lead the planet by 60° are called Trojans. Based on data from the Canada-France-Hawaii Telescope, Alexandersen et al. (p. 994) have found an object shadowing Uranus that is predicted to remain a Trojan for at least 700,000 years and to stay in co-orbital motion for around one million years before escaping.

Abstract

Trojan objects share a planet’s orbit, never straying far from the triangular Lagrangian points, 60° ahead of (L4) or behind (L5) the planet. We report the detection of a Uranian Trojan; in our numerical integrations, 2011 QF99 oscillates around the Uranian L4 Lagrange point for >70,000 years and remains co-orbital for ∼1 million years before becoming a Centaur. We constructed a Centaur model, supplied from the transneptunian region, to estimate temporary co-orbital capture frequency and duration (to a factor of 2 accuracy), finding that at any time 0.4 and 2.8% of the population will be Uranian and Neptunian co-orbitals, respectively. The co-orbital fraction (∼2.4%) among Centaurs in the International Astronomical Union Minor Planet Centre database is thus as expected under transneptunian supply.

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