Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Discovery of two distant irregular moons of Uranus


The systems of satellites and rings surrounding the giant planets in the Solar System have remarkably similar architectures1. Closest to each planet are rings with associated moonlets, then larger ‘regular’ satellites on nearly circular orbits close to the planet's equatorial plane, and finally one or more distant, small ‘irregular’ satellites on highly elliptical or inclined orbits. Hitherto, the only departure from this broad classification scheme was the satellite system around Uranus, in which no irregular satellites had been found2. Here we report the discovery of two satellites orbiting Uranus at distances of several hundred planetary radii. These satellites have inclined, retrograde orbits of moderate eccentricity that clearly identify them as irregular. The satellites are extremely faint (apparent red magnitudes mR = 20.4 and 21.9), with estimated radii of only 60 and 30 km. Both moons are unusually red in colour, suggesting a link between these objects—which were presumably captured by Uranus early in the Solar System's history—and other recently discovered bodies3 orbiting in the outer Solar System.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Observed positions of the newly discovered moons relative to Uranus.

Similar content being viewed by others


  1. Burns, J. A. in Satellites (eds Burns, J. A. & Matthews, M. S.) 1–38 (Univ. Arizona Press, Tucson, 1986).

    Google Scholar 

  2. Pollack, J. B., Lunine, J. & Tittemore, W. C. in Uranus (eds Bergstrahl, J. T., Miner, E. D. & Matthews, M. S.) 469–512 (Univ. Arizona Press, Tucson, 1991).

    Google Scholar 

  3. Jewitt, D., Luu, J. & Chen, J. The Mauna Kea-Cerro Tololo Kuiper belt and Centaur survey. Astron. J. 112, 1225–1238 (1996).

    Article  ADS  Google Scholar 

  4. Kowal, C. T., Aksnes, K., Marsden, B. G. & Roemer, E. Thirteenth satellite of Jupiter. Astron. J. 80, 460–464 (1975).

    Article  ADS  Google Scholar 

  5. Pickering, W. H. Ann. Harvard Coll. Obs. 53, 45–73, 85–142 (1905).

    Google Scholar 

  6. Kuiper, G. The second satellite of Neptune. Publ. Astron. Soc. Pacif. 61, 175–176 (1949).

    Article  ADS  Google Scholar 

  7. Pollack, J. B., Burns, J. A. & Tauber, M. E. Gas drag in primordial circumplanetary envelopes: A mechanism for satellite capture. Icarus 37, 587–611 (1979).

    Article  ADS  Google Scholar 

  8. Kuiper, G. in Planets and Satellites (eds Kuiper, G. & Middlehurst, B. M.) 575–591 (Univ. Chicago Press, Chicago, 1961).

    MATH  Google Scholar 

  9. Owen, W. M. J & Synnott, S. P. Orbits of the ten small satellites of Uranus. Astron. J. 93, 1268–1271 (1987).

    Article  ADS  Google Scholar 

  10. Christie, W. Report on a search for further satellites of the planets Uranus and Neptune. Publ. Astron. Soc. Pacif. 42, 253 (1930).

    Google Scholar 

  11. Smith, B. in Uranus and Neptune (ed. Bergstralh, J. T.) 213–223 (CCP-2330, NASA, Washington DC, 1984).

    Google Scholar 

  12. Cruikshank, D. P. & Brown, R. H. in Satellites (eds Burns, J. A. & Matthews, M. S.) 836–873 (Univ. Arizona Press, Tucson, 1986).

    Google Scholar 

  13. Żytkow, A., Irwin, M., Webster, R. & Tremaine, S. Aphotographic search for satellites of Uranus. Icarus 102, 298–306 (1993).

    Article  ADS  Google Scholar 

  14. Greenberg, R., Bottke, W. F., Carusi, A. & Valsecchi, G. B. Planetary accretion rates: Analytical derivation. Icarus 94, 98–111 (1991).

    Article  ADS  Google Scholar 

  15. Hamilton, D. P. & Krivov, A. V. Dynamics of distant moons of asteroids. Icarus 128, 241–249 (1997).

    Article  ADS  Google Scholar 

  16. Goldreich, P. Inclination of satellite orbits about an oblate precessing planet. Astron. J. 70, 5–9 (1966).

    Article  ADS  Google Scholar 

  17. Gladman, B. & Kavelaars, J Kuiper belt searches from the Palomar 5-m telescope. Astron. Astrophys. 317, 35–38 (1997).

    ADS  Google Scholar 

  18. Luu, J. Twice in a blue moon. Nature 390, 441–443 (1997).

    Article  ADS  CAS  Google Scholar 

  19. Gladman, B. et al. IAU Circ. No. 6764 ((1997).

    Google Scholar 

  20. Fitzsimmons, A., Fletcher, M., Irwin, M., Offut, W. & Hergenrother, C. IAU Circ. No. 6771 ((1997).

    Google Scholar 

  21. Marsden, B. & Williams, G. V. IAU Circ. No. 6780 ((1997).

    Google Scholar 

  22. Marsden, B. & Williams, G. V. IAU Circ. No. 6765 ((1997).

    Google Scholar 

  23. Thomas, F. & Morbidelli, A. The Kozai resonance in the outer solar system and the dynamics of long-period comets. Celest. Mech. Dyn. Astron. 64, 209–229 (1996).

    Article  ADS  Google Scholar 

  24. Thomas, P. C., Weitz, C. & Veverka, J. Small satellites of Uranus: Disk-integrated photometry and estimated radii. Icarus 81, 92–101 (1989).

    Article  ADS  Google Scholar 

  25. Thomas, P. C., Veverka, J. & Helfenstein, P. Voyager observations of Nereid. J. Geophys. Res. 96, 19253–19259 (1991).

    Article  ADS  Google Scholar 

  26. Veverka, J., Brown, R. H. & Bell, J. F. in Uranus (eds Bergstrahl, J. T., Miner, E. D. & Matthews, M. S.) 528–560 (Univ. Arizona Press, Tucson, 1991).

    Google Scholar 

  27. Cruikshank, D. P. Near-infrared studies of the satellites of Saturn and Uranus. Icarus 41, 246–253 (1980).

    Article  ADS  CAS  Google Scholar 

  28. Tholen, D. J. & Zellner, B. Eight-color photometry of Hyperion, Iapetus and Phoebe. Icarus 53, 341–347 (1983).

    Article  ADS  Google Scholar 

  29. Tholen, D. J. & Zellner, B. Multicolor photometry of outer jovian satellites. Icarus 58, 246–253 (1984).

    Article  ADS  CAS  Google Scholar 

  30. Luu, J. & Jewitt, D. Color diversity among the Centaurs and Kuiper belt objects. Astron. J. 112, 2310–2318 (1996).

    Article  ADS  Google Scholar 

  31. Green, S. F. et al. Surface reflectance properties of distant solar system bodies. Mon. Not. R. Astron. Soc. 290, 186–192 (1997).

    Article  ADS  Google Scholar 

  32. Fink, U. et al. The steep red spectrum of 1992 AD: An asteroid covered with organic material? Icarus 97, 145–149 (1992).

    Article  ADS  CAS  Google Scholar 

  33. Wilson, P. D. Models of Organic-Rich Surfaces in the Outer Solar System.Thesis, Cornell Univ.((1997).

    Google Scholar 

Download references


Observations at the Palomar Observatory were made as part of a continuing collaborative agreement between the California Institute of Technology and Cornell University. We thank the telescope operators and mountain staff at Palomar for their assistance; D. Tholen, A. Fitzsimmons, C.Hergenrother and S. Lilly for their confirming observations; and T. Yokoyama for numerical integrations of the provisional orbits. Support came from the Natural Sciences and Engineering Research Council of Canada (B.G. and JJK.), and NASA's Planetary Geology and Geophysics program (P.D.N. and J.A.B.).

Author information

Authors and Affiliations


Corresponding author

Correspondence to Brett J. Gladman.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Gladman, B., Nicholson, P., Burns, J. et al. Discovery of two distant irregular moons of Uranus. Nature 392, 897–899 (1998).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing