The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur

W. M. Grundy, J. A. Stansberry, K. S. Noll, D. C. Stephens, D. E. Trilling, S. D. Kern, J. R. Spencer, D. P. Cruikshank, H. F. Levison

Research output: Contribution to journalArticlepeer-review

54 Scopus citations


Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a close binary system. The mutual orbit has a period of 9.554 ± 0.011 days and a semimajor axis of 1840 ± 48   km. These values enable computation of a system mass of (5.41 ± 0.42) × 1018   kg. Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the system's effective radius (109-11+10   km) and geometric albedo (0.084-0.014+0.021). We estimate the average bulk density to be 1.37-0.32+0.66   g cm-3, consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ≤0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System.

Original languageEnglish (US)
Pages (from-to)286-297
Number of pages12
Issue number1
StatePublished - Nov 2007


  • Centaurs
  • Kuiper belt
  • Satellites
  • Trans-neptunian objects

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur'. Together they form a unique fingerprint.

Cite this