Moons of Pluto

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Pluto and its satellites (2005).jpgPluto moon P5 discovery with moons' orbits.jpg
Nix released 11-09-2015.pngHydra imaged by LORRI from 231 000 kilometres.jpg
Charon by New Horizons on 13 July 2015.png

The dwarf planet Pluto has five moons down to a detection limit of about 1 km in diameter. In order of distance from Pluto they are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest of the five moons, is mutually tidally locked with Pluto, and is massive enough that Pluto–Charon is sometimes considered a double (dwarf) planet.


Relative sizes and colors of the first four components of the Plutonian system to be discovered

The innermost and largest moon, Charon, was discovered by James Christy on 22 June 1978, nearly half a century after Pluto was discovered. This led to a substantial revision in estimates of Pluto's size, which had previously assumed that the observed mass and reflected light of the system were all attributable to Pluto alone.

Two additional moons were imaged by astronomers of the Pluto Companion Search Team preparing for the New Horizons mission and working with the Hubble Space Telescope on 15 May 2005, which received the provisional designations S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially named Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on 21 June 2006.[1] Kerberos, announced on 20 July 2011, was detected using NASA's Hubble Space Telescope during a survey searching for rings around Pluto. It was first seen in an image taken with Hubble's Wide Field Camera 3 on 28 June. It was confirmed in subsequent Hubble pictures taken on 3 and 18 July.[2] Styx, announced on 7 July 2012, was discovered while looking for potential hazards for New Horizons.[3]


Pluto and Charon, to scale. Photo taken by New Horizons on approach.
Main article: Charon (moon)
An oblique view of the Pluto–Charon system showing that Pluto orbits a point outside itself. Also visible is the mutual tidal locking between the two bodies.

Charon is about half the diameter of Pluto and is so massive (nearly one eighth of the mass of Pluto) that the system's barycenter lies between them, approximately 960 km above Pluto's surface.[4][a] Charon and Pluto are also tidally locked, so that they always present the same face toward each other. The IAU General Assembly in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned.[5]

Small moons[edit]

Nix and Hydra orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Their sizes are estimated from albedos. If their albedo is similar to that of Charon (0.35), then their diameters are 46 kilometres for Nix and 61 kilometres for Hydra. Upper limits on their diameters can be estimated by using the albedo of the darkest Kuiper-belt objects (0.04); these bounds are 137 ± 11 km and 167 ± 10 km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% that of Pluto.[6]

Kerberos has an estimated diameter of 13 to 34 km and is located between the orbits of Nix and Hydra.[2] Kerberos has a much lower albedo, approximately 0.04, than the other moons of Pluto,[7] which is difficult to explain with a giant collision.[8] Styx is believed to have a diameter of between 10 and 25 km and orbits Pluto at a distance between Charon and Nix.[9]


The relative masses of the Plutonian moons. Charon dominates the system; Nix and Hydra are barely visible at this scale.
In relation to Charon (orbit in green), Pluto (red) orbits a point outside itself.

The Plutonian system is highly compact and largely empty.[10] Moons could potentially orbit Pluto at up to 53% (or 69%, if retrograde) of the Hill radius, the stable gravitational zone of Pluto's influence. For example, Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the region where prograde orbits would be stable is occupied by satellites,[10] and the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits.[11] An intense search conducted by New Horizons to 1 July 2015 confirmed that no moons up to approximately 15 times fainter than Styx (or about 1 km in diameter) exist outside of Charon's orbit.[citation needed] Further information is expected as more sensitive data from the New Horizons flyby is sent back to Earth.

The orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005. As seen from Earth, these circular orbits appear foreshortened into ellipses depending on Pluto's position.[12]

The presence of Nix and Hydra indicates that Pluto could have a variable ring system. Small-body impacts can create debris that can form into a ring system. However, data from a deep-optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope, by occultation studies,[13] and later by New Horizons suggest that no ring system is present.


Styx, Nix, and Hydra are in a 3-body orbital resonance with orbital periods in a ratio of 18:22:33.[14] The ratios are exact when orbital precession is taken into account. This means that in a recurring cycle there are 11 orbits of Styx for every 9 of Nix and 6 of Hydra. Nix and Hydra are in a simple 2:3 resonance.[14][15] The ratios of synodic periods are such that there are 5 Styx–Hydra conjunctions and 3 Nix–Hydra conjunctions for every 2 conjunctions of Styx and Nix.[14] As with the Laplace resonance of the Galilean satellites of Jupiter, triple conjunctions never occur.

All of the outer circumbinary moons are also close to mean motion resonance with the Charon–Pluto orbital period. Styx, Nix, Kerberos, and Hydra are in a 1:3:4:5:6 sequence of near resonances, with Styx approximately 5.4% from its resonance, Nix approximately 2.7%, Kerberos approximately 0.6%, and Hydra approximately 0.3%.[16] It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current synchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. The Pluto–Charon pair creates strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak.[citation needed] However, it was calculated that a resonance with Charon could boost either Nix or Hydra into its current orbit, but not both: boosting Hydra would have required a near-zero Charonian eccentricity of 0.024, whereas boosting Nix would have required a larger eccentricity of at least 0.05. This suggests that Nix and Hydra were instead captured material, formed around Pluto–Charon, and migrated inward until they were trapped in resonance with Charon.[17] The existence of Kerberos and Styx may support this idea.

Configurations of Hydra (blue), Nix (red) and Styx (black) over one quarter of the cycle of their mutual orbital resonance. Movements are counterclockwise and orbits completed are tallied at upper right of diagrams (click on image to see the whole cycle).


Nix, Hydra, and possibly Styx and Kerberos rotate chaotically. According to Mark R. Showalter, author of a recent study,[14] "Nix can flip its entire pole. It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north. It is almost random-looking in the way it rotates."[18] This is because they are in a dynamically changing gravitational field caused by Pluto and Charon orbiting each other. The variable gravitational field creates torques that make Nix and Hydra tumble. The torques are increased because the moons are elongated and not spherical.[19][20][21][14] Only one other moon, Saturn's moon Hyperion, is known to tumble.[21]

Changing orientation of Nix as it tumbles unpredictably, based on computer modeling.[18] This tumbling is chaotic. (Video).


Formation of Pluto's moons. 1: a Kuiper belt object approaches Pluto; 2: it impacts Pluto; 3: a dust ring forms around Pluto; 4: the debris aggregates to form Charon; 5: Pluto and Charon relax into spherical bodies.

It is suspected that the Plutonian satellite system was created by a massive collision, similar to the "big whack" believed to have created the Moon.[22][23] In both cases, it may be that the high angular momenta of the moons can only be explained by such a scenario. The nearly circular orbits of the smaller moons suggests that they were also formed in this collision, rather than being captured Kuiper belt objects. This and their near orbital resonances with Charon (see below) suggest that they formed even closer to Pluto than they are at present and that they migrated outward as Charon got into its current orbit. The color of each is grey,[no, they differ] like Charon,[24] which is consistent with a common origin. Their difference in color from Pluto, one of the reddest bodies in the Solar System, is thought to be due to the effects of sunlight on the nitrogen and methane ices of its surface, may be due to a loss of such volatiles during the impact or subsequent coalescence, leaving the surfaces of Pluto's moons dominated by water ice. Such an impact would be expected to create additional debris (more moons), but these must be relatively small to have avoided detection by Hubble. It is possible that there are also undiscovered irregular satellites, which are captured Kuiper belt objects.


The Plutonian moons are listed here by orbital period, from shortest to longest. Charon, which is massive enough to have collapsed into a spheroid, is highlighted in light purple. Pluto has been added for comparison.[14][25]

Image Mean diameter
Mass (×1019 kg) Semi-major
axis (km)
Orbital period
Orbital period
(relative to Charon)
Eccentricity Inclination (°)
(to Pluto's equator)
Magnitude (mean) Discovery
Pluto[26] /ˈplt/
Global LORRI mosaic of Pluto in true colour.jpg
2372±4 1305±7 2035 6.387230 1 : 1 0.0022[b] 0.001 15.1 1930
Pluto I Charon /ˈʃærən/,[c]
Charon by New Horizons on 13 July 2015.png
1208±3 158.7±1.5 17536±3* 6.387230 1 : 1 0.0022[b] 0.001 16.8 1978
Pluto V Styx /ˈstɪks/ 188–28[27] 0.000–0.002 42656±78 20.16155±0.00027 1 : 3.16 0.0058 ± 0.0011 0.81 ± 0.16 27 2012
Pluto II Nix /ˈnɪks/ Nix in Color Released September 11 2015.png 42 × 36 ± ?[28] 0.005 ± 0.004 48694±3 24.85463±0.00003 1 : 3.89 0.002036 ± 0.000050 0.133 ± 0.008 23.7 2005
Pluto IV Kerberos /ˈkɜrbərəs/ 31 km ± ?[19] 0.002 ± 0.001 57783±19 32.16756±0.00014 1 : 5.04 0.00328 ± 0.00020 0.389 ± 0.037 26 2011
Pluto III Hydra /ˈhdrə/
Hydra imaged by LORRI from 231 000 kilometres.jpg
55 × 40 ± ?[28] 0.005 ± 0.004 64738±3 38.20177±0.00003 1 : 5.98 0.005862 ± 0.000025 0.242 ± 0.005 23.3 2005

The maximum distance between the centers of Pluto and Charon is 19,571 ± 4 km.

Scale model of the Pluto system[edit]


The Plutonian system was visited by the New Horizons spacecraft in July 2015. Images with resolutions of up to 330 meters per pixel should be returned of Nix, up to 780 meters per pixel of Hydra, and better than 2 kilometers per pixel for Styx and Kerberos.[29]


  1. ^ See "P1P2_motion.avi" (AVI). [dead link] and barycenter for animations
  2. ^ a b Orbital eccentricity and inclination of Pluto and Charon are equal because they refer to the same two-body problem (the gravitational influence of the smaller satellites is neglected here).
  3. ^ Many astronomers use this, Christy's pronunciation, rather than the classical /ˈkɛərɒn/, but both are acceptable.


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  2. ^ a b "NASA's Hubble Discovers Another Moon Around Pluto". NASA. 20 July 2011. Retrieved 20 July 2011. 
  3. ^ "Hubble Discovers a Fifth Moon Orbiting Pluto". 29 July 2012. Retrieved 2015-07-29. 
  4. ^ Staff (30 January 2014). "Barycenter". Retrieved 4 June 2015.  External link in |work= (help)
  5. ^ "The IAU draft definition of "planet" and "plutons"". International Astronomical Union. 16 August 2006. Retrieved 4 June 2015. 
  6. ^ Weaver, Harold A., Jr.; Stern, S. Alan; Mutchler, Max J.; et al. (2006). "Discovery of two new satellites of Pluto". Nature 439 (7079): 943–945. arXiv:astro-ph/0601018. Bibcode:2006Natur.439..943W. doi:10.1038/nature04547. PMID 16495991. 
  7. ^ "Pluto's moons are even weirder than thought". Retrieved 2015-06-20. 
  8. ^ "Pluto's moons dance to a random beat". Retrieved 2015-06-20. 
  9. ^ Sanders, Ray (11 July 2012). "Hubble Space Telescope detects fifth moon of Pluto". Retrieved 11 July 2012. 
  10. ^ a b Stern, S. Alan; Weaver, Harold A., Jr.; Steffl, Andrew J.; et al. (2006). "Characteristics and Origin of the Quadruple System at Pluto". Nature 439 (7079): 946–948. arXiv:astro-ph/0512599. Bibcode:2006Natur.439..946S. doi:10.1038/nature04548. PMID 16495992. 
  11. ^ Kenyon, S. J. (2015-06-03). "Astronomy: Pluto leads the way in planet formation". Nature 522 (7554): 40–41. doi:10.1038/522040a. 
  12. ^ "Orbits of 4 Bodies in Pluto System about Barycenter as Seen from Earth". Hubblesite. Retrieved 21 June 2006. 
  13. ^ Pasachoff, Jay M.; Babcock, Bryce A.; Souza, Steven P.; et al. (2006). "A Search for Rings, Moons, or Debris in the Pluto System during the 2006 July 12 Occultation". Bulletin of the American Astronomical Society 38 (3): 523. Bibcode:2006DPS....38.2502P. 
  14. ^ a b c d e f Showalter, M. R.; Hamilton, D. P. (3 June 2015). "Resonant interactions and chaotic rotation of Pluto’s small moons". Nature 522 (7554): 45–49. doi:10.1038/nature14469. 
  15. ^ Witze, Alexandra (2015). "Pluto’s moons move in synchrony". Nature. doi:10.1038/nature.2015.17681. 
  16. ^ Matson, J. (11 July 2012). "New Moon for Pluto: Hubble Telescope Spots a 5th Plutonian Satellite". Scientific American web site. Retrieved 12 July 2012. 
  17. ^ Lithwick, Y.; Y. Wu (2008). "On the Origin of Pluto's Minor Moons, Nix and Hydra". arXiv:0802.2951 [astro-ph]. 
  18. ^ a b Chang, Kenneth (3 June 2015). "Astronomers Describe the Chaotic Dance of Pluto’s Moons". New York Times. Retrieved 4 June 2015. 
  19. ^ a b "NASA’s Hubble Finds Pluto’s Moons Tumbling in Absolute Chaos". Retrieved 3 June 2015. 
  20. ^ "Hubble Finds Two Chaotically Tumbling Pluto Moons". 3 June 2015. Retrieved 3 June 2015. 
  21. ^ a b Drake, Nadia; 03, National Geographic Published June. "Pluto’s Moons Dance to a Random Beat". National Geographic News. Retrieved 4 June 2015. 
  22. ^ Canup, R. M. (8 January 2005). "A Giant Impact Origin of Pluto-Charon". Science 307 (5709): 546–550. Bibcode:2005Sci...307..546C. doi:10.1126/science.1106818. PMID 15681378. Retrieved 2011-07-20. 
  23. ^ Stern, S. A.; Weaver, H. A.; Steff, A. J.; Mutchler, M. J.; Merline, W. J.; Buie, M. W.; Young, E. F.; Young, L. A.; Spencer, J. R. (23 February 2006). "A giant impact origin for Pluto’s small moons and satellite multiplicity in the Kuiper belt" (PDF). Nature 439 (7079): 946–948. Bibcode:2006Natur.439..946S. doi:10.1038/nature04548. PMID 16495992. Retrieved 2011-07-20. 
  24. ^ "Hubble's Latest Look at Pluto's Moons Supports a Common Birth". Hubblesite. Retrieved 21 June 2006. 
  25. ^ Orbital elements of small satellites from Showalter and Hamilton, 2015; mass and magnitude from Buie & Grundy, 2006
  26. ^ Pluto data from D. R. Williams (7 September 2006). "Pluto Fact Sheet". NASA. Retrieved 24 March 2007. .
  27. ^ The orbits and masses of satellites of Pluto
  28. ^ a b "New Horizons 'Captures' Two of Pluto's Smaller Moons". New Horizons. Retrieved 2015-07-29. 
  29. ^ New Horizons flyby timeline
  • S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, & J.R. Spencer (2006), Characteristics and Origin of the Quadruple System at Pluto, Nature, submitted (preprint)
  • Steffl A.J., Mutchler M.J., Weaver H.A., Stern S.A., Durda D.D., Terrell D., Merline W.J., Young L.A., Young E.F., Buie M.W., Spencer J.R. (2005), New Constraints on Additional Satellites of the Pluto System, Astronomical Journal, submitted (preprint)
  • Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2005), Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2, submitted (preprint)
  • Brozovic, M., Showalter, M. R., Jacobson, R. A., & Buie, M. W. (2015), The orbits and masses of satellites of Pluto, Icarus, 246, 317
  • IAU Circular No. 8625, describing the discovery of 2005 P1 and P2
  • IAU Circular No. 8686, reporting a more neutral color for 2005 P2
  • IAU Circular No. 8723 announcing the names of Nix and Hydra
  • Background Information Regarding Our Two Newly Discovered Satellites of Pluto – The website of the discoverers of Nix and Hydra

External links[edit]