View source for Moons of Pluto

{{Short description|Natural satellites orbiting Pluto}}
{{Use dmy dates|date=April 2019}}
{{Multiple image
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|total_width=300
|image1=Charon in True Color - High-Res.jpg
|image2=Hydra reprocessed.png
|image3=Nix best view.jpg
|image4=Kerberos (moon).jpg
|image5=Styx (moon).jpg
|footer=<div>
* Top: Pluto's largest moon, [[Charon (moon)|Charon]], with its dark [[Mordor Macula]]
* Middle: [[Hydra (moon)|Hydra]] ''(left)'' and [[Nix (moon)|Nix]] ''(right)''
* Bottom: [[Kerberos (moon)|Kerberos]] ''(left)'' and [[Styx (moon)|Styx]] ''(right)''
(Images not to scale)</div>
}}
The [[dwarf planet]] [[Pluto]] has five [[natural satellite]]s.<ref name="Kenyon2019">{{cite journal|title=A Pluto-Charon Sonata: The Dynamical Architecture of the Circumbinary Satellite System|first1=Scott J.|last1=Kenyon|first2=Benjamin C.|last2=Bromley|journal=The Astrophysical Journal|volume=157|issue=2|pages=79|date=28 January 2019|doi=10.3847/1538-3881/aafa72|bibcode=2019AJ....157...79K|arxiv=1810.01277|s2cid=119091388 |doi-access=free }}</ref> In order of distance from Pluto, they are [[Charon (moon)|Charon]], [[Styx (moon)|Styx]], [[Nix (moon)|Nix]], [[Kerberos (moon)|Kerberos]], and [[Hydra (moon)|Hydra]].<ref>{{cite web|url=http://www.tweentribune.com/article/tween56/moons-dance-around-pluto/?page=4|title=Moons dance around Pluto|publisher=Smithsonian Institution|language=en|date=9 June 2015|access-date=9 April 2016}}</ref> Charon, the largest, is mutually [[tidally locked]] with Pluto, and is massive enough that Pluto and Charon are sometimes considered a [[Double planet|binary dwarf planet]].<ref>{{Cite news|url=https://www.space.com/16535-plutos-moons.html|title=Pluto's Moons {{!}} Five Satellites of Pluto|work=Space.com|access-date=2018-10-27}}</ref>

==History==
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 designation in astronomy|provisional designations]] S/2005&nbsp;P&nbsp;1 and S/2005&nbsp;P&nbsp;2. The [[International Astronomical Union]] officially named these moons Nix (Pluto&nbsp;II, the inner of the two moons, formerly P&nbsp;2) and Hydra (Pluto&nbsp;III, the outer moon, formerly P&nbsp;1), on 21 June 2006.<ref name="IAUC 8723">
{{cite journal
| first=Daniel W. E. |last=Green
| title = Satellites of Pluto
| journal = IAU Circular
| volume = 8723
| date = 21 June 2006
| url = http://www.cbat.eps.harvard.edu/iauc/08700/08723.html#Item1
| access-date = 26 November 2011
}}
{{cite web
| url = http://www.nasa.gov/mission_pages/hubble/science/pluto-moon.html
| title = NASA's Hubble Discovers Another Moon Around Pluto
| publisher = NASA
| date = 20 July 2011
| access-date = 20 July 2011
}}
</ref> 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''.<ref>{{Cite web |title = Hubble Discovers a Fifth Moon Orbiting Pluto |url = http://hubblesite.org/newscenter/archive/releases/2012/32/full/ |website = hubblesite.org |access-date = 2015-07-29 |date = 29 July 2012 |archive-url=https://web.archive.org/web/20121117053628/http://hubblesite.org/newscenter/archive/releases/2012/32/full/ |archive-date=17 November 2012 |url-status=dead}}</ref> <!-- <ref name="MoonsName">{{cite web|title=2 Pluto Moons Get New Names}}</ref> -->
[[File:Nh-pluto moons family portrait.png|thumb|450px|center|The small moons to approximate scale, compared to Charon.]]

==Charon==
[[File:PIA19856-PlutoCharon-NewHorizons-Color-20150714.jpg|thumb|upright=2.5|Charon and Pluto, to scale. Photo taken by ''New Horizons'' on approach.]]
{{Main|Charon (moon)}}

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 {{convert|960|km}} above Pluto's surface.<ref>{{cite web |author=Staff |title=Barycenter |url=http://www.education.com/science-fair/article/barycenter-balancing-point/ |date=30 January 2014 |work=Education.com |access-date=4 June 2015 }}</ref><ref group=lower-alpha>{{cite web|url=http://wwwsa.boulder.swri.edu/P1P2_motion.avi |title=P1P2_motion.avi |format=AVI |url-status=dead |archive-url=https://web.archive.org/web/20051104094343/http://wwwsa.boulder.swri.edu/P1P2_motion.avi |archive-date=4 November 2005 }} and barycenter for animations</ref> Charon and Pluto are also tidally locked, so that they always present the same face toward each other. The [[International Astronomical Union#The XXVIth General Assembly and the definition of a planet|IAU General Assembly]] in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned.<ref name="Draft Resolution 5">{{cite web|title=The IAU draft definition of "planet" and "plutons"|url=http://www.iau.org/public_press/news/release/iau0601/|publisher=International Astronomical Union|date=16 August 2006|access-date=4 June 2015 }}</ref>
Like Pluto, Charon is a perfect sphere to within measurement uncertainty.<ref name="Nimmo2017">
{{cite journal |last1=Nimmo |first1=Francis |display-authors=etal |title=Mean radius and shape of Pluto and Charon from New Horizons images|journal=Icarus |date=2017 |volume=287  |pages=12–29 |doi=10.1016/j.icarus.2016.06.027|bibcode=2017Icar..287...12N |arxiv=1603.00821|s2cid=44935431 }}</ref>

== Circumbinary moons ==
{{multiple image | align = right | direction = horizontal | width = 
| header = Animation of moons of Pluto around the barycenter of Pluto - [[Ecliptic]] plane
| image1 = Animation of moons of Pluto - Front view.gif
| caption1 = Front view
| image2 = Animation of moons of Pluto - Side view.gif 
| caption2 = Side view
| footer ={{legend2|khaki| Pluto}}{{·}}{{legend2|RoyalBlue| Charon}}{{·}}{{legend2|Lime| Styx}}{{·}}{{legend2|Magenta| Nix}}{{·}}{{legend2|Cyan| Kerberos}}{{·}}{{legend2|Orangered| Hydra}}
}}
{{multiple image |align=right |direction=horizontal
|image1 = Pluto system 2006.jpg
|caption1 =The ''Hubble'' discovery image of Nix and Hydra
|image2 = Pluto moon P5 discovery with moons' orbits.jpg 
|caption2 = 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 [[direct motion|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,<ref name=":0">{{Cite web|title = New Horizons 'Captures' Two of Pluto's Smaller Moons|url = http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150721|website = New Horizons|access-date = 2015-07-29}}</ref> and Styx and Kerberos are 7 and 12 kilometers respectively.<ref name="styx">[https://web.archive.org/web/20151010222508/http://www.nasa.gov/nh/new-horizons-picks-up-styx/ New Horizons Picks Up Styx]</ref><ref name="kerberos">[http://www.nasa.gov/feature/last-of-pluto-s-moons-mysterious-kerberos-revealed-by-new-horizons Last of Pluto's Moons – Mysterious Kerberos – Revealed by New Horizons]</ref>  All four are irregularly shaped.

== Characteristics ==
The Pluto system is highly compact and largely empty: prograde moons could stably orbit Pluto out to 53% of the [[Hill sphere|Hill]] radius (the gravitational zone of Pluto's influence) of 6 million km, or out to 69% for retrograde moons.<ref name=Steffl_2006/> However, only the inner 3% of the region where prograde orbits would be stable is occupied by satellites,<ref name="Sternetal 2005">{{cite arXiv | first1=S. Alan |last1=Stern| first2=Harold A. Jr. |last2=Weaver| first3=Andrew J. |last3=Steffl | display-authors=3| first4=Max J. |last4=Mutchler| first5=William J. |last5=Merline| first6=Marc W. |last6=Buie| first7=Eliot F. |last7=Young| first8=Leslie A. |last8=Young| first9=John R. |last9=Spencer| title = Characteristics and Origin of the Quadruple System at Pluto | date = 2005| eprint = astro-ph/0512599}}</ref> 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.<ref name="Kenyon2015">{{cite journal|last1=Kenyon|first1=S. J.|title=Astronomy: Pluto leads the way in planet formation|journal= Nature|volume= 522|issue= 7554|date= 2015-06-03|pages= 40–41|doi= 10.1038/522040a|bibcode = 2015Natur.522...40K|pmid=26040888|s2cid=205085254|doi-access= free}}</ref>
An intense search conducted by ''New Horizons'' confirmed that no moons larger than 4.5&nbsp;km in diameter exist out to distances up to 180,000&nbsp;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).<ref name=Stern_2015/>
{{multiple image |image1=Masses of Plutonian moons.png |caption1=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.
|image2=Pluto-Charon system-new.gif
|caption2=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 orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005.<ref>{{cite web |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/19/image/h |title=Orbits of 4 Bodies in Pluto System about Barycenter as Seen from Earth |publisher=Hubblesite |access-date=21 June 2006}}</ref>

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,<ref name="Pasachoff 2006">
{{cite journal
| first1=Jay M. |last1=Pasachoff
| first2=Bryce A. |last2=Babcock
| first3=Steven P. |last3=Souza
| display-authors=3
| first4=Joseph W. |last4=Gangestad
| first5=Anne E. |last5=Jaskot
| first6=James L. |last6=Elliot
| first7=Amanda A. S. |last7=Gulbis
| first8=Michael J. |last8=Person
| first9=E. A. |last9=Kramer
| first10=Elisabeth R. |last10=Adams
| first11=Carlos A. |last11=Zuluaga
| first12=Rosemary E. |last12=Pike
| first13=Paul J. |last13=Francis
| first14=R. |last14=Lucas
| first15=Amanda S. |last15=Bosh
| first16=David J. |last16=Ramm
| first17=John G. |last17=Greenhill
| first18=A. Barry |last18=Giles
| first19=Stefan W. |last19=Dieters
| date = 2006
| title = A Search for Rings, Moons, or Debris in the Pluto System during the 2006 July 12 Occultation
| journal = Bulletin of the American Astronomical Society
| volume = 38
| issue = 3
| page = 523
| bibcode = 2006DPS....38.2502P
}}
</ref> and later by ''New Horizons'', suggest that no ring system is present.

===Resonances===
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.<ref name="ShowalterHamilton2015"/><ref name="Regius">{{cite book| title=Pluto & Charon| author=Codex Regius| url=https://books.google.com/books?id=dTs2DQAAQBAJ&pg=PT197| page=197| year=2016| publisher=XinXii| isbn= 9781534633520| access-date=13 March 2018}}</ref> The ratios should be exact when [[Apsidal precession|orbital precession]] is taken into account. Nix and Hydra are in a simple 2:3 resonance.<ref group=lower-alpha name=resonance1>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.</ref><ref name="ShowalterHamilton2015">{{cite journal|last1=Showalter|first1=M. R.|author1-link=Mark R. Showalter|last2=Hamilton|first2=D. P.|title=Resonant interactions and chaotic rotation of Pluto's small moons|journal=Nature|volume=522|issue=7554|date=3 June 2015|pages=45–49|doi=10.1038/nature14469|pmid=26040889|bibcode = 2015Natur.522...45S |s2cid=205243819}}</ref><ref name="Witze2015">{{cite journal|last1=Witze|first1=Alexandra|title=Pluto's moons move in synchrony|journal=Nature|year=2015|doi=10.1038/nature.2015.17681|s2cid=134519717}}</ref> Styx and Nix are in an 9:11 resonance, while the resonance between Styx and Hydra has a ratio of 6:11.<ref group=lower-alpha name=resonance2>The ratio of '''18:22''':33 in the 3-body resonance corresponds to a 2-body resonance with ratio ''9:11'' between Styx and Nix. In analogy, the ratio of '''18:'''22:'''33''' in the 3-body resonance corresponds to a 2-body resonance with ratio ''6:11'' between Styx and Hydra.</ref> The Laplace resonance also means that ratios of [[synodic period]]s are then such that there are 5 Styx–Hydra conjunctions and 3 Nix–Hydra conjunctions for every 2 conjunctions of Styx and Nix.<ref group="lower-alpha" name="resonance3">This is calculated as follows: for every <math>1</math> orbit of Hydra there are <math>\frac{33}{22}=\frac 32</math> orbits of Nix and <math>\frac{33}{18}=\frac{11}{6}</math> orbits of Styx. The conjunctions then occur at a relative rate of <math>\frac{11}{6}-1=\frac{5}{6}</math> for Styx-Hydra, <math>\frac{3}{2}-1=\frac 12</math> for Nix-Hydra and <math>\frac{11}{6}-\frac{3}{2}=\frac 13</math> for Styx-Nix. Multiplying all three rates by <math>6</math> (to make them integers) yields that there are <math>5</math> Styx-Hydra conjunctions and <math>3</math> Nix-Hydra conjunctions for every <math>2</math> Styx-Nix conjunctions.</ref><ref name="ShowalterHamilton2015"/> If <math>\lambda</math> denotes the [[mean longitude]] and <math>\Phi</math> the [[libration]] angle, then the [[Resonant trans-Neptunian object#Toward a formal definition|resonance]] can be formulated as <math>\Phi=3\lambda_{\rm Styx} - 5\lambda_{\rm Nix} + 2\lambda_{\rm Hydra}=180^\circ</math>. As with the [[Orbital resonance#Laplace resonance|Laplace resonance]] of the [[Galilean moons|Galilean satellite]]s of Jupiter, triple conjunctions never occur. <math>\Phi</math> librates about 180° with an amplitude of at least 10°.<ref name="ShowalterHamilton2015"/>

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 [[orbital resonance#Coincidental 'near' ratios of mean motion|near resonances]], with Styx approximately 5.4% from its resonance, Nix approximately 2.7%, Kerberos approximately 0.6%, and Hydra approximately 0.3%.<ref name="Matson">{{cite web
| last = Matson | first = J. | date = 11 July 2012 | title = New Moon for Pluto: Hubble Telescope Spots a 5th Plutonian Satellite
| work = [[Scientific American]] web site | url = http://www.scientificamerican.com/article.cfm?id=pluto-moon-p5
| access-date = 12 July 2012}}</ref> 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|date=June 2015}}

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.<ref>{{cite arXiv|eprint=0802.2951|class=astro-ph|last=Lithwick|first=Y.|author2=Y. Wu|title=On the Origin of Pluto's Minor Moons, Nix and Hydra|year=2008}}</ref> The existence of Kerberos and Styx may support this idea.{{clarification needed|date=March 2024}}{{Citation needed|date=April 2024}}

{{Annotated image 
|image        = Hydra, Nix, Styx orbital resonance cycle.png
|image-width  = 450  
|width=450
| height      = 112 <!-- to crop the lower part of the image -->
| float       = center 
| annotations = <!-- this parameter must be there, empty or not! -->
| caption = {{center|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).}}}}

{{Clear}}

===Rotation===
[[File:PIA20052-SmallMoonsOfPluto-Animation-20151110.webm|thumb|right|300px|{{center|Rotations of the small moons of Pluto<br />(animation; 01:00; released 10 November 2015)}}]]
Prior to the ''New Horizons'' mission, 
[[Nix (moon)|Nix]], [[Hydra (moon)|Hydra]], [[Styx (moon)|Styx]], and [[Kerberos (moon)|Kerberos]]  
were predicted to rotate [[Chaos theory|chaotically]] or [[tumbling (rigid body)|tumble]].<ref name="ShowalterHamilton2015"/><ref name="Correia2015">{{cite journal
| last1 = Correia | first1 = A. C. M.| last2 = Leleu | first2 = A. | last3 = Rambaux | first3 = N.| last4 = Robutel | first4 = P.| title = Spin-orbit coupling and chaotic rotation for circumbinary bodies. Application to the small satellites of the Pluto-Charon system | journal=Astronomy and Astrophysics | volume = 580 | pages = L7| doi = 10.1051/0004-6361/201526800 |bibcode = 2015A&A...580L..14C| year = 2015| arxiv = 1506.06733| s2cid = 119098216}}</ref>

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.<ref name="Weaver2016">{{ cite journal | last1 = Weaver | first1 = H. A. | title = The Small Satellites of Pluto as Observed by New Horizons | journal=Science | volume = 351 | issue = 6279 | pages = 1281 | doi= 10.1126/science.aae0030| year = 2016 | bibcode = 2016Sci...351.0030W | arxiv = 1604.05366 | pmid = 26989256 | s2cid = 206646188 }}</ref><ref name="Quillen2017">{{ cite journal | last1 = Quillen |first1 = A. C. | last2 =  Nichols-Fleming | first2 = F. | last3= Chen | first3=Y.-Y.| last4  = Noyelles| first4 = B.| title=Obliquity evolution of the minor satellites of Pluto and Charon | journal=Icarus | volume=293 | pages= 94–113 |doi = 10.1016/j.icarus.2017.04.012 |bibcode = 2017Icar..293...94Q |year = 2017 |arxiv = 1701.05594 |s2cid = 119408999 }}</ref> New Horizons imaging found that all 4 moons were at high obliquity.<ref name="Weaver2016"/> Either they were born that way, or they were tipped by a spin precession resonance. 
<ref name="Quillen2017"/> [[Styx (moon)|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."<ref name="NYT-20150603">{{cite news |last=Chang |first=Kenneth |title=Astronomers Describe the Chaotic Dance of Pluto's Moons |url=https://www.nytimes.com/2015/06/04/science/space/pluto-moons-orbits-described-in-nature-article.html |date=3 June 2015 |work=[[New York Times]] |access-date=4 June 2015 }}</ref> 
Only one other moon, [[Saturn]]'s moon [[Hyperion (moon)|Hyperion]], is known to tumble,<ref name="Wisdom1984">{{cite journal| last1 = Wisdom | first1 = J.| last2 = Peale | first2 = S. J. | last3 = Mignard | first3 = F. | title = The chaotic rotation of Hyperion | journal= Icarus | volume = 58 | issue = 2| pages = 137–152| doi = 10.1016/0019-1035(84)90032-0 |bibcode = 1984Icar...58..137W| year = 1984| citeseerx = 10.1.1.394.2728}}</ref> though it is likely that [[Moons of Haumea|Haumea's moons]] do so as well.<ref name="RapidlyRotating">{{Cite web|title = Rapidly Rotating Regular Satellites and Tides |url = http://www.planetary.org/blogs/guest-blogs/2016/1017-rapidly-rotating-regular-satellites-and-tides.html |website = Planetary Society |date=17 October 2016|access-date = 12 September 2017 |first = Darin|last = Ragozzine}}</ref>

==Origin==
[[File:Creation of the moons of Pluto.jpg|thumb|200px|right|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 (moon)|Charon]] relax into spherical bodies.]]
It is suspected that Pluto's satellite system was [[Collisional family|created by a massive collision]], similar to the [[Giant impact hypothesis|Theia impact]] thought to have created the [[Moon]].<ref name="Canup">{{cite journal
| last = Canup | first = R. M. | author-link = Robin Canup | title = A Giant Impact Origin of Pluto-Charon
| journal = [[Science (journal)|Science]] | volume = 307 | issue = 5709 | pages = 546–550
| date = 8 January 2005 
| doi = 10.1126/science.1106818 |bibcode = 2005Sci...307..546C | pmid=15681378| s2cid = 19558835 | url = https://authors.library.caltech.edu/51983/7/Canup.SOM.pdf }}</ref><ref name="Stern">{{cite journal |last=Stern |first=S. A. |author-link=Alan Stern |author2=Weaver, H. A. |author3=Steff, A. J. |author4=Mutchler, M. J. |author5=Merline, W. J. |author6=Buie, M. W. |author7=Young, E. F. |author8=Young, L. A. |author9=Spencer, J. R. |title=A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt |journal=[[Nature (journal)|Nature]] |volume=439 |issue=7079 |pages=946–948 |date=23 February 2006 |url=http://ns1.plutoportal.net/~layoung/eprint/Stern2006plutosat.pdf |doi=10.1038/nature04548 |access-date=2011-07-20 |bibcode=2006Natur.439..946S |pmid=16495992 |s2cid=4400037 |url-status=dead |archive-url=https://web.archive.org/web/20120119001455/http://ns1.plutoportal.net/~layoung/eprint/Stern2006plutosat.pdf |archive-date=19 January 2012  }}</ref> In both cases, the high [[Angular momentum|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 resonance]]s 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.<ref name="Kenyon2019"/> An alternative hypothesis is that the collision happened at about 2,000 miles per hour, not powerful enough to destroy Charon and Pluto. Instead they remained attached to each other for up to ten hours before separating again. The faster rotation of Pluto back then, with one rotation every third hour, would have created a centrifugal force stronger than the gravitational attraction between the two bodies, which made Charon separate from Pluto, but remained gravitationally bound with each other. The same process could have created the four other known moons, from material that escaped Pluto and Charon.<ref>[https://www.nytimes.com/2025/01/08/science/pluto-moon-kiss-charon.html Pluto May Have Captured Its Biggest Moon After an Ancient Dance and Kiss]</ref>

== List ==
Pluto's moons are listed here by orbital period, from shortest to longest. Charon, which is massive enough to have [[hydrostatic equilibrium|collapsed]] into a [[spheroid]] under its own gravitation, is highlighted in light purple. As the system barycenter lies far above Pluto's surface, Pluto's barycentric orbital elements have been included as well.<ref name="ShowalterHamilton2015"/><ref>Orbital elements of small satellites from Showalter and Hamilton, 2015; mass and magnitude from [https://arxiv.org/abs/astro-ph?papernum=0511837 Buie & Grundy, 2006]</ref> All elements are with respect to the Pluto-Charon barycenter.<ref name="ShowalterHamilton2015"/> The mean separation distance between the centers of Pluto and Charon is 19,596&nbsp;km.<ref name="factsheet"/>
{{sticky table start}}
{| class="wikitable sortable sticky-table-row1 sticky-table-col1"
|-
! Label <br /> {{efn|Label refers to the [[Roman numerals]] attributed to each moon in order of their discovery.<ref name="Gazetteer">{{Cite web|title=Planet and Satellite Names and Discoverers|work=Gazetteer of Planetary Nomenclature|publisher=USGS Astrogeology|url=https://planetarynames.wr.usgs.gov/Page/Planets|access-date=2022-06-23}}</ref>}}
! class="unsortable" colspan="2" | Name <br /> ([[Help:IPA for English|pronunciation]])
! class="unsortable"| Named after<ref>{{cite web |title=Planetary Names |url=https://planetarynames.wr.usgs.gov/Page/Planets |website=planetarynames.wr.usgs.gov |access-date=6 January 2023}}</ref>
! class="unsortable"| Image 
! Diameter<br />(km)
! Mass (×10<sup>19</sup>&nbsp;kg)<ref name=":1">{{Cite web|url=http://www.johnstonsarchive.net/astro/astmoons/am-pluto.html|title=(134340) Pluto, Charon, Nix, Hydra, Kerberos, and Styx|website=www.johnstonsarchive.net|access-date=2018-06-22}}</ref>
! Semi-major<br />axis (km)
! Orbital period<br />(days)
! Orbital resonance<br />{{small|(relative to Charon)}}
! Eccentricity
! Inclination (°)<br />{{small|(to Pluto's equator)}}
! Visual<br />magnitude (mean)
! Discovery<br />year
|- style="text-align: center; background: #ccf;"
|
| '''[[Pluto]]''' || {{IPAc-en|ˈ|p|l|uː|t|oʊ}} || [[Pluto (mythology)|Pluto]], Roman god of the underworld || style="background:black" |[[File:Pluto_in_True_Color_-_High-Res_(cropped).jpg|centre|50x50px]]|| {{val|2376.6|3.2|fmt=}}|| {{val|1305|7}} || {{val|2,035|fmt=}}<ref name="factsheet">Pluto data from {{cite web| author = D. R. Williams| title = Pluto Fact Sheet| publisher = NASA| date = 7 September 2006| url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html| access-date =24 March 2007 }}.</ref> || 6.38723 ||1&nbsp;:&nbsp;1|| 0.0022<ref group=lower-alpha name=eccentricity>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).</ref> || 0.001 || 15.1 || 1930
|- style="text-align: center; background: #eef;"
| I || '''[[Charon (moon)|Charon]]''' || {{IPAc-en|ˈ|ʃ|ær|ən}},<ref group=lower-alpha>Many astronomers use this, Christy's pronunciation, rather than the classical {{IPAc-en|ˈ|k|ɛər|ɒ|n}}, but both are considered to be acceptable.</ref><br />{{IPAc-en|ˈ|k|ɛər|ən}} || [[Charon]], ferryman of the underworld in Greek mythology ||style="background: black;" | [[File:Charon_in_True_Color_-_High-Res.jpg|50px|center]] || {{val|1212|1|fmt=}}|| {{val|158.7|1.5}} || {{val|17536|3|fmt=}}|| 6.38723 ||1&nbsp;:&nbsp;1|| 0.0022<ref  group=lower-alpha name=eccentricity /> || 0.080 || 16.8 || 1978
|- style="text-align: center; background: #fff;"
| V || [[Styx (moon)|Styx]] || {{IPAc-en|'|s|t|ɪ|k|s}} || The mythical river [[Styx]] and its eponymous goddess ||style="background: black;" |  [[File:Styx (moon).jpg|50px]]  || {{val|16|x|9|x|8}}<ref name=LPSC>{{cite web|url=https://www.youtube.com/watch?v=ZtoLw0KWzsU?t=7470 |archive-url=https://ghostarchive.org/varchive/youtube/20211221/ZtoLw0KWzsU |archive-date=2021-12-21 |url-status=live|title=Special Session: Planet 9 from Outer Space - Pluto Geology and Geochemistry|publisher=[[Lunar and Planetary Institute]]|website=[[YouTube]]|date=25 March 2016|access-date=27 May 2019}}{{cbignore}}</ref> || 0.00075||{{val|42656|78|fmt=}}|| 20.16155 || 1&nbsp;:&nbsp;3.16 || 0.00579 || {{val|0.81|0.16}} || 27 || 2012
|- style="text-align: center; background: #fff;"
| II || [[Nix (moon)|Nix]] || {{IPAc-en|'|n|ɪ|k|s}} || Egyptian spelling of [[Nyx]], goddess of the night in Greek mythology ||style="background: black;" | [[File:Nix large view.jpg|50px]]  || {{val|49.8|x|33.2|x|31.1}} <ref name="phasecurves">{{cite journal |last1=Verbiscer |first1=A. J. |last2=Porter |first2=S. B. |last3=Buratti |first3=B. J. |last4=Weaver |first4=H. A. |last5=Spencer |first5=J. R. |last6=Showalter |first6=M. R. |last7=Buie |first7=M. W. |last8=Hofgartner |first8=J. D. |last9=Hicks |first9=M. D. |last10=Ennico-Smith |first10=K. |last11=Olkin |first11=C. B. |last12=Stern |first12=S. A. |last13=Young |first13=L. A. |last14=Cheng |first14=A. |title=Phase Curves of Nix and Hydra from the ''New Horizons'' Imaging Cameras |journal=The Astrophysical Journal |date=2018 |volume=852 |issue=2 |pages=L35 |doi=10.3847/2041-8213/aaa486|bibcode=2018ApJ...852L..35V |doi-access=free }}</ref> || {{val|0.005|0.004}} || {{val|48694|3|fmt=}}|| 24.85463 || 1&nbsp;:&nbsp;3.89|| 0.00204|| {{val|0.133|0.008}} || 23.7 || 2005
|- style="text-align: center; background: #fff;"
| IV || [[Kerberos (moon)|Kerberos]] || {{IPAc-en|ˈ|k|ɜːr|b|ər|ə|s|,_|-|ɒ|s}} || Greek spelling of [[Cerberus]], the many-headed dog who guards the Greek underworld || style="background: black;" | [[File:Kerberos (moon).jpg|50px]] || {{val|19|x|10|x|9}}<ref name=LPSC/> || {{val|0.0016|0.0009}}||{{val|57783|19|fmt=}}|| 32.16756 || 1&nbsp;:&nbsp;5.04|| 0.00328 || {{val|0.389|0.037}} || 26 || 2011
|- style="text-align: center; background: #fff;"
| III || [[Hydra (moon)|Hydra]] || {{IPAc-en|ˈ|h|aɪ|d|r|ə}} || The [[Lernaean Hydra|Hydra]], the many-headed serpent who guards the Greek underworld || style="background: black;" | [[File:Hydra_reprocessed.png|50px|center]] || {{val|50.9|x|36.1|x|30.9}} <ref name="phasecurves"/> || {{val|0.005|0.004}} || {{val|64738|3|fmt=}}|| 38.20177 ||1&nbsp;:&nbsp;5.98|| 0.00586 || {{val|0.242|0.005}} || 23.3 || 2005
|}
{{sticky table end}}

=== Scale model of the Pluto system ===
<gallery class="center" mode="packed" heights="150px">
File:Pluto and Charon system new.png|Pluto and its five moons, including the location of the system's [[barycenter]]. Sizes and distances of the bodies are to scale.
</gallery>
{{Clear}}

==Mutual events==
{{more citations needed|section|date = July 2021}}
[[File:Charon Eclipses Pluto on 25 February 1989.jpg|200px|thumb|Simulated view of Charon transiting Pluto on 25 February 1989]]

[[Occultations, transits, and eclipses|Transits]] occur when one of Pluto's moons passes between Pluto and the Sun. This occurs when one of the satellites' [[orbital node]]s (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, transits, and eclipses|Occultations]] occur when Pluto passes in front of and blocks one of Pluto's satellites.

Charon has an angular diameter of 4 [[degree of arc|degrees of arc]] as seen from the surface of Pluto; the Sun appears much smaller, only 39 to 65 [[arcsecond]]s. By comparison, the [[Moon]] as viewed from [[Earth]] has an angular diameter of only 31 [[arcminute|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.<ref>{{cite web
 |date = 12 December 1987 <!-- 11:28 UTC -->
 |title = Start of Eclipse
 |publisher = JPL Solar System Simulator
 |url = http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=901&vbody=999&month=12&day=12&year=1987&hour=11&minute=28&fovmul=1&rfov=5&bfov=30&porbs=1&brite=1
 |archive-url = https://web.archive.org/web/20170324031702/http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=901&vbody=999&month=12&day=12&year=1987&hour=11&minute=28&fovmul=1&rfov=5&bfov=30&porbs=1&brite=1
 |url-status = dead
 |archive-date = 24 March 2017
 |access-date = 2014-07-29}} [https://web.archive.org/web/20170324034439/http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=999&vbody=10&month=12&day=12&year=1987&hour=12&minute=12&rfov=20&fovmul=-1&bfov=50&porbs=1&brite=1 (Pluto as seen from the Sun during mid-eclipse)]</ref><ref>{{cite web
  |date = 12 December 1987 <!-- 12:57 UTC -->
  |title = End of Eclipse
  |publisher = JPL Solar System Simulator
  |url = http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=901&vbody=999&month=12&day=12&year=1987&hour=12&minute=57&fovmul=1&rfov=5&bfov=30&porbs=1&brite=1
  |archive-url = https://web.archive.org/web/20170324062349/http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=901&vbody=999&month=12&day=12&year=1987&hour=12&minute=57&fovmul=1&rfov=5&bfov=30&porbs=1&brite=1
  |url-status = dead
  |archive-date = 24 March 2017
  |access-date = 2014-07-29}}</ref>

==Exploration==
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.<ref>{{Cite web |url=http://pluto.jhuapl.edu/Mission/The-Flyby.php |title=New Horizons flyby timeline |access-date=25 July 2015 |archive-url=https://web.archive.org/web/20150715172512/http://pluto.jhuapl.edu/Mission/The-Flyby.php |archive-date=15 July 2015 |url-status=dead }}</ref>

==Notes==
{{Notelist}}

==References==
{{Reflist|2|refs=
<ref name=Steffl_2006>{{cite journal
 |title  =New Constraints on Additional Satellites of the Pluto System
 |author1=Steffl, A. J. |author2=Mutchler, M. J. |author3=Weaver, H. A. |author4=Stern, S. A. |author5=Durda, D. D. |author6=Terrell, D. |author7=Merline, W. J. |author8=Young, L. A. |author9=Young, E. F. |author10=Buie, M. W. |author11=Spencer, J. R. |date   =2006
 |journal=The Astronomical Journal
 |volume =132
 |issue  =2
 |pages  =614–619
 |doi    =10.1086/505424
 |bibcode=2006AJ....132..614S
 |arxiv  =astro-ph/0511837
 |s2cid=10547358 }}</ref>
<ref name=Stern_2015>{{cite journal
 |title  =The Pluto system: Initial results from its exploration by New Horizons
 |author1=Stern, S. A. |author2=Bagenal, F. |author3=Ennico, K. |date   =2015
 |journal=Science
 |volume =350
 |issue  =6258
 |doi    =10.1126/science.aad1815
 |bibcode=2015Sci...350.1815S
 |pmid   =26472913
 |arxiv  =1510.07704
 |display-authors=etal
 |pages=aad1815|s2cid=1220226 }} ([http://www.sciencemag.org/content/suppl/2015/10/14/350.6258.aad1815.DC1/Stern.SM.pdf Supplements])</ref>
}}

==Sources==
* {{cite arXiv |eprint=astro-ph/0512599|last1=Stern|first1=S. A.|last2=Weaver|first2=H. A.|last3=Steffl|first3=A. J.|last4=Mutchler|first4=M. J.|last5=Merline|first5=W. J.|last6=Buie|first6=M. W.|last7=Young|first7=E. F.|last8=Young|first8=L. A.|last9=Spencer|first9=J. R.|title=Characteristics and Origin of the Quadruple System at Pluto|year=2005}}
* {{cite journal
 |title  =New Constraints on Additional Satellites of the Pluto System
 |author1=Steffl, A. J. |author2=Mutchler, M. J. |author3=Weaver, H. A. |author4=Stern, S. A. |author5=Durda, D. D. |author6=Terrell, D. |author7=Merline, W. J. |author8=Young, L. A. |author9=Young, E. F. |author10=Buie, M. W. |author11=Spencer, J. R. |date   =2006
 |journal=The Astronomical Journal
 |volume =132
 |issue  =2
 |pages  =614–619
 |doi    =10.1086/505424
 |bibcode=2006AJ....132..614S
 |arxiv  =astro-ph/0511837
 |s2cid=10547358 }}
* {{cite journal |arxiv=astro-ph/0512491|doi=10.1086/504422|bibcode=2006AJ....132..290B|title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2|year=2006|last1=Buie|first1=Marc W.|last2=Grundy|first2=William M.|last3=Young|first3=Eliot F.|last4=Young|first4=Leslie A.|last5=Stern|first5=S. Alan|journal=The Astronomical Journal|volume=132|issue=1|pages=290–298|s2cid=119386667}}
* {{cite journal |doi=10.1016/j.icarus.2014.03.015|bibcode=2015Icar..246..317B|title=The orbits and masses of satellites of Pluto|year=2015|last1=Brozović|first1=Marina|last2=Showalter|first2=Mark R.|last3=Jacobson|first3=Robert A.|last4=Buie|first4=Marc W.|journal=Icarus|volume=246|pages=317–329}}
* Codex Regius (2016), ''Pluto & Charon'', CreateSpace Independent Publishing Platform {{ISBN|978-1534960749}}
* [http://www.cbat.eps.harvard.edu/iauc/08600/08625.html IAU Circular No. 8625], describing the discovery of 2005 P1 and P2
* [http://www.cbat.eps.harvard.edu/iauc/08600/08686.html IAU Circular No. 8686], reporting a more neutral color for 2005 P2
* [http://www.cbat.eps.harvard.edu/iauc/08700/08723.html IAU Circular No. 8723] announcing the names of Nix and Hydra
* [https://web.archive.org/web/20060626220516/http://www.boulder.swri.edu/plutonews/ Background Information Regarding Our Two Newly Discovered Satellites of Pluto] – The website of the discoverers of Nix and Hydra

==External links==
{{Commons category}}
*[[Scott S. Sheppard]]: [https://sites.google.com/carnegiescience.edu/sheppard/moons/plutomoons Pluto Moons]
*[https://thehappykoala.github.io/Harmony-of-the-Spheres/#/scenario/The%20Sun%20and%20the%20Plutonian%20System Interactive 3D visualisation of the Plutonian system]
*[https://web.archive.org/web/20051104094343/http://wwwsa.boulder.swri.edu/P1P2_motion.avi Animation of the Plutonian system]
*[http://www.nasa.gov/vision/universe/solarsystem/hubble_pluto.html Hubble Spots Possible New Moons Around Pluto] (NASA)
*[http://www.space.com/scienceastronomy/051031_pluto_moons.html Two More Moons Discovered Orbiting Pluto] (SPACE.com)
*[http://www.pluto.jhuapl.edu New Horizons Mission Site]

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{{Charon (moon)}}
{{Moons of dwarf planets}}
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