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Moons of Pluto

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Title: Moons of Pluto  
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Subject: Charon (moon), Styx (moon), Hydra (moon), Pluto, Natural satellite
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Moons of Pluto

(Images not to scale)

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.


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.

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][1] 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

The Hubble discovery image of Nix and Hydra
Discovery image of Styx, overlaid with orbits of the satellite system

Pluto's four small moons orbit Pluto at two to four times the distance of Charon, ranging from Styx at 42,700 kilometres to Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon.

All are much smaller than Charon. Nix and Hydra, the two larger, are roughly 42 and 55 kilometers on their longest axis respectively,[6] and Styx and Kerberos are 7 and 12 kilometers respectively.[7][8] All four are irregularly shaped.


The relative masses of the Plutonian moons. Charon dominates the system. Nix and Hydra are barely visible and Styx and Kerberos are invisible at this scale.
An oblique schematic 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.

The Plutonian system is highly compact and largely empty.[9] 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,[9] and the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits.[10] 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. 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.[11]

The discovery of Nix and Hydra suggested that Pluto could have a 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,[12] 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.[13] 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.[13][14] 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.[13] 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%.[15] 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.

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.[16] 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 complete cycle).


Nix, Hydra, and possibly Styx and Kerberos rotate chaotically. According to Mark R. Showalter, author of a recent study,[13] "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."[17] 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.[18][19][20][13] Only one other moon, Saturn's moon Hyperion, is known to tumble,[20] though it is likely that Haumea's moons do so as well.


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.[21][22] 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, like Charon,[23] 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 at some point in its history, is highlighted in light purple. Pluto has been added for comparison.[13][24]

Image Diameter
Mass (×1019 kg) Semi-major
axis (km)
Orbital period
Orbital period
(relative to Charon)
Eccentricity Inclination (°)
(to Pluto's equator)
Magnitude (mean) Discovery
2372±4 1305±7 2035 6.387230 1 : 1 0.0022[2] 0.001 15.1 1930/02/18
Pluto I Charon ,[3]
1208±3 158.7±1.5 17536±3* 6.387230 1 : 1 0.0022[2] 0.001 16.8 1978/06/22
Pluto V Styx 7 × 5 ± ?[7] ? 42656±78 20.16155±0.00027 1 : 3.16 0.0058 ± 0.0011 0.81 ± 0.16 27 2012/06/26
Pluto II Nix 51 × 41 × 36[6] 0.005 ± 0.004 48694±3 24.85463±0.00003 1 : 3.89 0.002036 ± 0.000050 0.133 ± 0.008 23.7 2005/06/15
Pluto IV Kerberos 12 × 4.5 ±?[8] ? 57783±19 32.16756±0.00014 1 : 5.04 0.00328 ± 0.00020 0.389 ± 0.037 26 2011/06/28
Pluto III Hydra
55 × 40 ± ?[6] 0.005 ± 0.004 64738±3 38.20177±0.00003 1 : 5.98 0.005862 ± 0.000025 0.242 ± 0.005 23.3 2005/06/15

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

Scale model of the Pluto system

The small moons to approximate scale, compared to Charon


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 and up to 780 meters per pixel of Hydra. Poorer resolution images were returned of Styx and Kerberos.[26]


  1. ^ See 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 , but both are acceptable.


  1. ^
  2. ^
  3. ^
  4. ^
  5. ^
  6. ^ a b c
  7. ^ a b [1]
  8. ^ a b [2]
  9. ^ a b
  10. ^
  11. ^
  12. ^
  13. ^ a b c d e f
  14. ^
  15. ^
  16. ^
  17. ^
  18. ^
  19. ^
  20. ^ a b
  21. ^
  22. ^
  23. ^
  24. ^ Orbital elements of small satellites from Showalter and Hamilton, 2015; mass and magnitude from Buie & Grundy, 2006
  25. ^ Pluto data from .
  26. ^ 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

  • Animation of the Plutonian system
  • Hubble Spots Possible New Moons Around Pluto (NASA)
  • Two More Moons Discovered Orbiting Pluto (
  • New Horizons Mission Site
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