Moons of Pluto

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Styx (moon).jpg Kerberos (moon).jpg
Nix best view.jpg Hydra Enhanced Color (cropped).jpg
Charon in True Color - High-Res.jpg

(Images not to scale)

The dwarf planet Pluto has five natural satellites. [1] In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, and Hydra. [2] Charon, the largest, is mutually tidally locked with Pluto, and is massive enough that Pluto–Charon is sometimes considered a double dwarf planet. [3]



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 these 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. [4] Kerberos, announced on 20 July 2011, was discovered while searching for Plutonian rings. Styx, announced on 7 July 2012, was discovered while looking for potential hazards for New Horizons. [5]

The small moons to approximate scale, compared to Charon. Nh-pluto moons family portrait.png
The small moons to approximate scale, compared to Charon.


Pluto and Charon, to scale. Photo taken by New Horizons on approach. PIA19856-PlutoCharon-NewHorizons-Color-20150714.jpg
Pluto and Charon, to scale. Photo taken by New Horizons on approach.

Charon is about half the diameter of Pluto and is massive enough (nearly one eighth of the mass of Pluto) that the system's barycenter lies between them, approximately 960 km above Pluto's surface. [6] [lower-alpha 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. [7] It is not clear if Charon is in hydrostatic equilibrium, which the definition of "dwarf planet" would require, though it is a perfect sphere to within current measurement uncertainty. [8]

Small moons

Animation of moons of Pluto around the barycenter of Pluto - Ecliptic plane
Animation of moons of Pluto - Front view.gif
Front view
Animation of moons of Pluto - Side view.gif
Side view
   Pluto ·   Charon ·   Styx ·   Nix ·   Kerberos ·   Hydra
The Hubble discovery image of Nix and Hydra Pluto system 2006.jpg
The Hubble discovery image of Nix and Hydra
Discovery image of Styx, overlaid with orbits of the satellite system Pluto moon P5 discovery with moons' orbits.jpg
Discovery image of Styx, overlaid with orbits of the satellite system

Pluto's four small circumbinary 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, [9] and Styx and Kerberos are 7 and 12 kilometers respectively. [10] [11] All four are irregularly shaped.


The relative masses of Pluto's moons. Charon dominates the system. Nix and Hydra are barely visible and Styx and Kerberos are invisible at this scale. Masses of Plutonian moons.png
The relative masses of Pluto's 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. Pluto-Charon system-new.gif
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 Pluto system is highly compact and largely empty: Prograde moons could stably orbit Pluto out to 53% of the Hill radius (the gravitational zone of Pluto's influence) of 6 million km, or out to 69% for retrograde moons. [12] However, only the inner 3% of the region where prograde orbits would be stable is occupied by satellites, [13] and the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits within this region. [14] An intense search conducted by New Horizons confirmed that no moons larger than 4.5 km in diameter exist out to a distances up to 180,000 km from Pluto (6% of the stable region for prograde moons), assuming Charon-like albedoes of 0.38 (for smaller distances, this threshold is still smaller). [15]

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. [16]

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, [17] and later by New Horizons, suggest that no ring system is present.


Styx, Nix, and Hydra are thought to be in a 3-body orbital resonance with orbital periods in a ratio of 18:22:33; the respective ratio of orbits is 11:9:6. [18] [19] The ratios should be exact when orbital precession is taken into account. Hydra and Nix are in a simple 2:3 resonance. [lower-alpha 2] [18] [20] Styx and Nix are in an 11:9 resonance, while the resonance between Styx and Hydra has a ratio of 11:6. [lower-alpha 3] This means that in a recurring cycle there are 11 orbits of Styx for every 9 of Nix and 6 of Hydra. The ratios of synodic periods are then such that there are 5 Styx–Hydra conjunctions and 3 Nix–Hydra conjunctions for every 2 conjunctions of Styx and Nix. [lower-alpha 4] [18] If denotes the mean longitude and the libration angle, then the resonance can be formulated as . As with the Laplace resonance of the Galilean satellites of Jupiter, triple conjunctions never occur. librates about 180° with an amplitude of at least 10°. [18]

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%. [21] 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. [22] The existence of Kerberos and Styx may support this idea.

Hydra, Nix, Styx orbital resonance cycle.png
Hydra, Nix, Styx orbital resonance cycle.png
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).


Rotations of the small moons of Pluto
(animation; 01:00; released 10 November 2015)

Prior to the New Horizons mission, Nix, Hydra, Styx, and Kerberos were predicted to rotate chaotically or tumble. [18] [23]

However, New Horizons imaging found that they had not tidally spun down to near a spin synchronous state where chaotic rotation or tumbling would be expected. [24] [25] New Horizons imaging found that all 4 moons were at high obliquity. [24] Either they were born that way, or they were tipped by a spin precession resonance. [25] Styx may be experiencing intermittent and chaotic obliquity variations.

Mark R. Showalter had speculated that, "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." [26] Only one other moon, Saturn's moon Hyperion, is known to tumble, [27] though it is likely that Haumea's moons do so as well. [28]


Formation of Pluto's moons. 1: a Kuiper belt object approaches Pluto; 2: it collides with Pluto; 3: a dust ring forms around Pluto; 4: the debris aggregates to form Charon; 5: Pluto and Charon relax into spherical bodies. Creation of the moons of Pluto.jpg
Formation of Pluto's moons. 1: a Kuiper belt object approaches Pluto; 2: it collides with 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 Pluto's satellite system was created by a massive collision, similar to the "big whack" thought to have created the Moon. [29] [30] In both cases, 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 closer to Pluto than they are at present and migrated outward as Charon reached its current orbit. Their grey color is different from that of Pluto, one of the reddest bodies in the Solar System. This is thought to be due to a loss of volatiles during the impact or subsequent coalescence, leaving the surfaces of the moons dominated by water ice. However, such an impact should have created additional debris (more moons), yet no moons or rings were discovered by New Horizons, ruling out any more moons of significant size orbiting Pluto. [1]


Pluto's 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. [18] [31] All elements are with respect to the Pluto-Charon barycenter. [18] The mean separation distance between the centers of Pluto and Charon is 19,596 km. [32]

Mass (×1019 kg) [33] Semi-major
axis (km)
Orbital period
Orbital resonance
(relative to Charon)
EccentricityInclination (°)
(to Pluto's equator)
magnitude (mean)
Pluto /ˈplt/
Pluto in True Color - High-Res (cropped).jpg
2,376.6±3.21305±72,035 [32] 6.387231 : 10.0022 [lower-alpha 5] 0.00115.11930
Pluto I Charon /ˈʃærən/ , [lower-alpha 6]
Charon in True Color - High-Res.jpg
1,212±1158.7±1.517,536±36.387231 : 10.0022 [lower-alpha 5] 0.00116.81978
Pluto V Styx /ˈstɪks/ Styx (moon).jpg 16 × 9 × 8 [34] 0.0007542,656±7820.161551 : 3.160.005790.81±0.16272012
Pluto II Nix /ˈnɪks/ Nix best view.jpg 49.8 × 33.2 × 31.1 [35] 0.005±0.00448,694±324.854631 : 3.890.002040.133±0.00823.72005
Pluto IV Kerberos /ˈkɜːrbərəs,-ɒs/ Kerberos (moon).jpg 19 × 10 × 9 [34] 0.0016±0.000957,783±1932.167561 : 5.040.003280.389±0.037262011
Pluto III Hydra /ˈhdrə/
Hydra Enhanced Color.jpg
50.9 × 36.1 × 30.9 [35] 0.005±0.00464,738±338.201771 : 5.980.005860.242±0.00523.32005

Scale model of the Pluto system

Mutual events

Simulated view of Charon transiting Pluto on 25 February 1989. Charon Eclipses Pluto on 25 February 1989.jpg
Simulated view of Charon transiting Pluto on 25 February 1989.

Transits occur when one of Pluto's moons passes between Pluto and the Sun. This occurs when one of the satellites' orbital nodes (the points where their orbits cross Pluto's ecliptic) lines up with Pluto and the Sun. This can only occur at two points in Pluto's orbit; coincidentally, these points are near Pluto's perihelion and aphelion. Occultations occur when Pluto passes in front of and blocks one of Pluto's satellites.

Charon has an angular diameter of 4 degrees of arc as seen from the surface of Pluto; the Sun appears much smaller, only 39 to 65 arcseconds. Charon's proximity further ensures that a large proportion of Pluto's surface can experience an eclipse. Because Pluto always presents the same face towards Charon due to tidal locking, only the Charon-facing hemisphere experiences solar eclipses by Charon.

The smaller moons can cast shadows elsewhere. The angular diameters of the four smaller moons (as seen from Pluto) are uncertain. Nix's is 3–9 minutes of arc and Hydra's is 2–7 minutes. These are much larger than the Sun's angular diameter, so total solar eclipses are caused by these moons.

Eclipses by Styx and Kerberos are more difficult to estimate, as both moons are very irregular, with angular dimensions of 76.9 x 38.5 to 77.8 x 38.9 arcseconds for Styx, and 67.6 x 32.0 to 68.0 x 32.2 for Kerberos. As such, Styx has no annular eclipses, its widest axis being more than 10 arcseconds larger than the Sun at its largest. However, Kerberos, although slightly larger, cannot make total eclipses as its largest minor axis is a mere 32 arcseconds. Eclipses by Kerberos and Styx will entirely consist of partial and hybrid eclipses, with total eclipses being extremely rare.

The next period of mutual events due to Charon will begin in October 2103, peak in 2110, and end in January 2117. During this period, solar eclipses will occur once each Plutonian day, with a maximum duration of 90 minutes. [36] [37]


The Pluto system was visited by the New Horizons spacecraft in July 2015. Images with resolutions of up to 330 meters per pixel were returned of Nix and up to 1.1 kilometers per pixel of Hydra. Lower-resolution images were returned of Styx and Kerberos. [38]


  1. "P1P2_motion.avi". Archived from the original (AVI) on 4 November 2005. and barycenter for animations
  2. The ratio of 18:22:33 in the 3-body resonance corresponds to a 2-body resonance with ratio 2:3 between Hydra and Nix.
  3. The ratio of 18:22:33 in the 3-body resonance corresponds to a 2-body resonance with ratio 9:11 between Nix and Styx. In analogy, the ratio of 18:22:33 in the 3-body resonance corresponds to a 2-body resonance with ratio 6:11 between Hydra and Styx.
  4. This is calculated as follows: for every orbit of Hydra there are orbits of Nix and orbits of Styx. The conjunctions then occur at a relative rate of for Styx-Hydra, for Nix-Hydra and for Styx-Nix. Multiplying all three rates by (to make them integers) yields that there are Styx-Hydra conjunctions and Nix-Hydra conjunctions for every Styx-Nix conjunctions.
  5. 1 2 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).
  6. Many astronomers use this, Christy's pronunciation, rather than the classical /ˈkɛərɒn/ , but both are considered to be acceptable.

Related Research Articles

Double planet A binary system where two planetary-mass objects share an orbital axis external to both

In astronomy, a double planet is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies.

Orbital resonance Regular and periodic gravitational influence by two orbiting celestial bodies exerted on each other

In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large solar system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

Pluto Dwarf planet in the Kuiper belt of the Solar System

Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto was discovered in 1930, it was declared to be the ninth planet from the Sun. Beginning in the 1990s, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt and the scattered disc, including the dwarf planet Eris. This led the International Astronomical Union (IAU) in 2006 to formally define the term "planet"—excluding Pluto and reclassifying it as a dwarf planet.

Charon (moon) Largest natural satellite of Pluto

Charon, known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

Natural satellite Astronomical body that orbits a planet

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body. While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

Tidal locking Situation in which an astronomical objects orbital period matches its rotational period

Tidal locking, in the best-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon.

Barycenter Center of mass of multiple bodies orbiting each other

In astronomy, the barycenter is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important concept in fields such as astronomy and astrophysics. The distance from a body's center of mass to the barycenter can be calculated as a two-body problem.

Minor-planet moon Natural satellite of a minor planet

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of November 2021, there are 447 minor planets known or suspected to have moons. Discoveries of minor-planet moons are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.

90482 Orcus Trans-Neptunian object and dwarf planet

Orcus is a trans-Neptunian dwarf planet with a large moon, Vanth. It has a diameter of 910 km (570 mi). The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.

Dysnomia (moon) Moon of Eris

Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and likely the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of S/2005 (2003 UB313) 1 until officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) after the daughter of the Greek goddess Eris.

Nix (moon) Moon of Pluto

Nix is a natural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra on 15 May 2005 by astronomers using the Hubble Space Telescope, and was named after Nyx, the Greek goddess of the night. Nix is the third moon of Pluto by distance, orbiting between the moons Styx and Kerberos.

Hydra (moon) Moon of Pluto

Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km (32 mi) across its longest dimension. It is the second-largest moon of Pluto, being slightly larger than Nix. Hydra was discovered along with Nix by astronomers using the Hubble Space Telescope on 15 May 2005, and was named after the Hydra, the nine-headed underworld serpent in Greek mythology. By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.

15810 Arawn

15810 Arawn, provisional designation 1994 JR1, is a trans-Neptunian object (TNO) from the inner regions of the Kuiper belt, approximately 133 kilometres (83 mi) in diameter. It belongs to the plutinos, the largest class of resonant TNOs. It was named after Arawn, the ruler of the Celtic underworld, and discovered on 12 May 1994, by astronomers Michael Irwin and Anna Żytkow with the 2.5-metre Isaac Newton Telescope at La Palma Observatory in the Canary Islands, Spain.

Kerberos (moon) Small natural satellite of Pluto

Kerberos is a small natural satellite of Pluto, about 19 km (12 mi) in its longest dimension. It was the fourth moon of Pluto to be discovered and its existence was announced on 20 July 2011. It was imaged, along with Pluto and its four other moons, by the New Horizons spacecraft in July 2015. The first image of Kerberos from the flyby was released to the public on 22 October 2015.

Styx (moon) Small natural satellite of Pluto

Styx is a small natural satellite of Pluto whose discovery was announced on 11 July 2012. It was discovered by use of the Hubble Space Telescope. As of 2020, it is the smallest known moon of Pluto. It was imaged along with Pluto and Pluto's other moons by the New Horizons spacecraft in July 2015, albeit poorly with only a single image of Styx obtained.

Satellite system (astronomy) Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

Chaotic rotation involves the irregular and unpredictable rotation of an astronomical body. Unlike Earth's rotation, a chaotic rotation may not have a fixed axis or period. Because of the conservation of angular momentum, chaotic rotation is not seen in objects that are spherically symmetric or well isolated from gravitational interaction, but is the result of the interactions within a system of orbiting bodies, similar to those associated with orbital resonance.

Organa (crater)

Organa is the informal name given to a crater on Pluto's largest moon, Charon. The crater was discovered by NASA's New Horizons space probe on its flyby of Pluto. The name was chosen as a reference to Leia Organa from the Star Wars media franchise in the theme of naming Charon's craters after science fiction characters. Organa crater is rich in frozen ammonia, which suggests it was created very recently. This crater is located in the northern Pluto-facing hemisphere of Charon.


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