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 and Charon are sometimes considered a binary 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. The discovery of Styx was announced on 7 July 2012 while looking for potential hazards for New Horizons. [5]
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] Like Pluto, Charon is a perfect sphere to within measurement uncertainty. [8]
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 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 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 could eject debris off of the small moons which 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 Laplace orbital resonance with orbital periods in a ratio of 18:22:33. [18] [19] The ratios should be exact when orbital precession is taken into account. Nix and Hydra are in a simple 2:3 resonance. [lower-alpha 2] [18] [20] Styx and Nix are in an 9:11 resonance, while the resonance between Styx and Hydra has a ratio of 6:11. [lower-alpha 3] The Laplace resonance also means that 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.[ clarification needed ][ citation needed ]
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]
It is suspected that Pluto's satellite system was created by a massive collision, similar to the Theia impact 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]
| Label [lower-alpha 5] | Name (pronunciation) | Named after [34] | Image | Diameter (km) | Mass (×1019 kg) [35] | Semi-major axis (km) | Orbital period (days) | Orbital resonance (relative to Charon) | Eccentricity | Inclination (°) (to Pluto's equator) | Visual magnitude (mean) | Discovery year | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pluto | /ˈpluːtoʊ/ | Pluto, Roman god of the underworld |  | 2376.6±3.2 | 1305±7 | 2035 [32] | 6.38723 | 1 : 1 | 0.0022 [lower-alpha 6] | 0.001 | 15.1 | 1930 | |
| I | Charon | /ˈʃærən/ , [lower-alpha 7] /ˈkɛərən/ | Charon, ferryman of the underworld in Greek mythology |  | 1212±1 | 158.7±1.5 | 17536±3 | 6.38723 | 1 : 1 | 0.0022 [lower-alpha 6] | 0.080 | 16.8 | 1978 |
| V | Styx | /ˈstɪks/ | The mythical river Styx and its eponymous goddess |  | 16 × 9 × 8 [36] | 0.00075 | 42656±78 | 20.16155 | 1 : 3.16 | 0.00579 | 0.81±0.16 | 27 | 2012 |
| II | Nix | /ˈnɪks/ | Egyptian spelling of Nyx, goddess of the night in Greek mythology |  | 49.8 × 33.2 × 31.1 [37] | 0.005±0.004 | 48694±3 | 24.85463 | 1 : 3.89 | 0.00204 | 0.133±0.008 | 23.7 | 2005 |
| IV | Kerberos | /ˈkɜːrbərəs,-ɒs/ | Greek spelling of Cerberus, the many-headed dog who guards the Greek underworld |  | 19 × 10 × 9 [36] | 0.0016±0.0009 | 57783±19 | 32.16756 | 1 : 5.04 | 0.00328 | 0.389±0.037 | 26 | 2011 |
| III | Hydra | /ˈhaɪdrə/ | The Hydra, the many-headed serpent who guards the Greek underworld |  | 50.9 × 36.1 × 30.9 [37] | 0.005±0.004 | 64738±3 | 38.20177 | 1 : 5.98 | 0.00586 | 0.242±0.005 | 23.3 | 2005 |
 | This section needs additional citations for verification .(July 2021) |
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. By comparison, the Moon as viewed from Earth has an angular diameter of only 31 minutes of arc, or just over half a degree of arc. Therefore, Charon would appear to have eight times the diameter, or 25 times the area of the Moon; this is due to Charon's proximity to Pluto rather than size, as despite having just over one-third of a Lunar radius, Earth's Moon is 20 times more distant from Earth's surface as Charon is from Pluto's. This 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. [38] [39]
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. [40]
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.
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 Neptune and Pluto. 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 planetary 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 is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume, by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has only one sixth the mass of Earth's moon, and one third its volume.
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).
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object 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 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, as well as for Eris and Dysnomia. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking.
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.
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of January 2022, there are 457 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 into their physical properties that are generally not otherwise accessible.
Orcus is a large trans-Neptunian object and likely dwarf planet located in the Kuiper belt, with one large moon, Vanth. It has an estimated diameter of 870 to 960 km, comparable to the Inner Solar System dwarf planet Ceres. Orcus had been accepted by many astronomers as a dwarf planet, though as of 2024 that classification remains somewhat controversial. 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.
38628 Huya ( hoo-YAH; provisional designation 2000 EB173) is a binary trans-Neptunian object located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Huya is classified as a plutino, a dynamical class of trans-Neptunian objects with orbits in a 3:2 orbital resonance with Neptune. It was discovered by the Quasar Equatorial Survey Team and was identified by Venezuelan astronomer Ignacio Ferrín in March 2000. It is named after Juyá, the mythological rain god of the Wayuu people native to South America.
Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and is the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in September 2005 by Mike Brown and the Laser Guide Star Adaptive Optics (LGSAO) team at the W. M. Keck Observatory. It carried the provisional designation of S/2005 (2003 UB313) 1 until it was officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) in September 2006, after the daughter of the Greek goddess Eris.
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 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 (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 most populous class of resonant TNOs. It was named after Arawn, the ruler of the underworld in Welsh mythology, and was discovered on 12 May 1994, by astronomers Michael Irwin and Anna Żytkow with the 2.5-metre Isaac Newton Telescope at Roque de los Muchachos Observatory in the Canary Islands, Spain.
Vanth is a natural satellite or moon of the large trans-Neptunian object 90482 Orcus. It was discovered by Michael Brown and Terry-Ann Suer using images taken by the Hubble Space Telescope on 13 November 2005. The moon has a diameter of 443 km (275 mi), making it about half the size of Orcus and the third-largest moon of a trans-Neptunian object. Vanth is massive enough that it shifts the barycenter of the Orcus–Vanth system outside of Orcus, forming a binary system in which the two bodies revolve around the barycenter, much like the Pluto–Charon system. It is hypothesized that both systems formed similarly, most likely by a giant impact early in the Solar System's history. In contrast to Orcus, Vanth has a darker and slightly redder surface that apparently lacks exposed water ice, resembling primordial Kuiper belt objects.
Kerberos is a small natural satellite of Pluto, about 19 km (12 mi) in its longest dimension. Kerberos is also the second-smallest moon of Pluto, after Styx. 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 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, and is the smallest of the five known moons 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.
385446 Manwë, or (385446) Manwë–Thorondor, is a binary resonant Kuiper belt object in a 4:7 mean-motion resonance with Neptune. It was discovered on 25 August 2003, by American astronomer Marc Buie at Cerro Tololo Observatory in northern Chile. A study of Manwë's light curve in 2019 suggests that it may be a contact binary object.
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.
The geology of Charon encompasses the characteristics of the surface, crust, and interior of Pluto's moon Charon. Like the geology of Pluto, almost nothing was known of Charon's geology until the New Horizons of the Pluto system on 14 July 2015. Charon's diameter is 1,208 km (751 mi)—just over half that of Pluto. Charon is sufficiently massive to have collapsed into a spheroid under its own gravity.
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