1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
2. National Research Council of Canada, 5071 West Saanich Road, Victoria, British Columbia V9E ZE7, Canada
3. University of Oslo, Institute of Theoretical Astrophysics, Postbox 1029 Blindern, 0315 Oslo, Norway
4. Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
5. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
6. Department of Astronomy, Cornell University, Ithaca, New York 14853, USA
7. Obervatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
8. Jet Propulsion Laboratory, MS 301-150, 4800 Oak Grove Drive, Pasadena, California 91109, USA

Correspondence to: Matthew J. Holman¹ Correspondence and requests for materials should be addressed to M.J.H. (Email: mholman@cfa.harvard.edu).

Abstract

Each giant planet of the Solar System has two main types of moons. 'Regular' moons are typically larger satellites with prograde, nearly circular orbits in the equatorial plane of their host planets at distances of several to tens of planetary radii. The 'irregular' satellites (which are typically smaller) have larger orbits with significant eccentricities and inclinations. Despite these common features, Neptune's irregular satellite system, hitherto thought to consist of Triton and Nereid, has appeared unusual. Triton is as large as Pluto and is postulated to have been captured from heliocentric orbit; it traces a circular but retrograde orbit at 14 planetary radii from Neptune. Nereid, which exhibits one of the largest satellite eccentricities, is believed to have been scattered from a regular satellite orbit to its present orbit during Triton's capture^{1, 2}. Here we report the discovery of five irregular moons of Neptune, two with prograde and three with retrograde orbits. These exceedingly faint (apparent red magnitude m_R = 24.2–25.4) moons, with diameters of 30 to 50 km, were presumably captured by Neptune.

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