Moons of Pluto
|Moons of Pluto|
The dwarf planet Pluto has five known moons. 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 June 22, 1978, nearly half a century after Pluto itself. 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. 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. Styx, announced on 7 July 2012, was discovered while looking for potential hazards for New Horizons.
Theoretical simulations suggest that there may be as many as 10 moons and one or more ring systems. Further information is expected as the New Horizons probe approaches the system, arriving on 14 July 2015.
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.[a] Charon and Pluto are also tidally locked, so that they always present the same face toward each other. The IAU General Assembly in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned.
Nix and Hydra orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Their sizes are estimated from albedos. If their albedo is similar to that of Charon (0.35), then their diameters are 46 kilometres for Nix and 61 kilometres for Hydra. Upper limits on their diameters can be estimated by using the albedo of the darkest Kuiper-belt objects (0.04); these bounds are 137 ± 11 km and 167 ± 10 km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% that of Pluto.
Kerberos has an estimated diameter of 13 to 34 km and is located between the orbits of Nix and Hydra. Kerberos has a much lower albedo, approximately 0.04, than the other moons of Pluto, which is difficult to explain with a giant collision. Styx is believed to have a diameter of between 10 and 25 km and orbits Pluto at a distance between Charon and Nix.
The Plutonian system is highly compact and largely empty. 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. The possibility of additional objects exists, including a small ring system. In contrast, the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits.
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.
When discovered, Hydra was somewhat brighter than Nix, and therefore thought to be larger by 20%. Follow-up observations found them to be nearly identical. It is likely that the change in brightness is due to the light curve of Hydra, but whether this is due to an irregular shape or to a variation in surface brightness is unknown.[dubious ] The diameters of objects can be estimated from their assumed albedos; the estimates above correspond to a 35% albedo like Charon, but the moons could be as large as 130 km if they have the 4% albedo of the darkest Kuiper belt objects. However, given their color and suspected chemical similarities to Charon, it is likely that their albedos are also similar and that the diameters are closer to the lower estimates.
The presence of Nix and Hydra suggests that Pluto may have a variable ring system. Small-body impacts can create debris that can form into a ring system. Data from a deep-optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1,000 km in width. Similar conclusions have been made from occultation studies.
Styx, Nix, and Hydra are in a 3-body orbital resonance with orbital periods in a ratio of 18:22:33. 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. 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. 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%. 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. The existence of Kerberos and Styx may support this idea.
Nix, Hydra, and possibly Styx and Kerberos rotate chaotically. According to Mark R. Showalter, author of a recent study, "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." 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. Only one other moon, Saturn's moon Hyperion, is known to tumble.
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. 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, which is consistent with a common origin. Their difference in color from Pluto, one of the reddest bodies in the Solar System due to the effects of sunlight on the nitrogen and methane ices of its surface, may be due to a loss of such volatiles during the impact or subsequent coalescence, leaving the surfaces of Pluto's moons dominated by water ice. Such an impact would be expected to create additional debris (more moons), but these must be relatively small to have avoided detection by Hubble. It is possible that there are also undiscovered irregular satellites, which are captured Kuiper belt objects.
The Plutonian moons are listed here by orbital period, from shortest to longest. Charon, which is massive enough to have collapsed into a spheroid, is highlighted in light purple. Pluto has been added for comparison.
|Mass (×1019 kg)||Semi-major
(relative to Charon)
(to Pluto's equator)
|Pluto||//||±202368||1305 ± 7||2,035||6.387230||1 : 1||0.0022[b]||0.001||15.1||1930|
|±31207||158.7 ± 1.5||17,536 ± 3*||6.387230||1 : 1||0.0022[b]||0.001||16.8||1978|
|Pluto V||Styx||//||16–56||0.000–0.002||42,656 ± 78||20.16155 ± 0.00027||1 : 3.16||0.0058 ± 0.0011||0.81 ± 0.16||27||2012|
|Pluto II||Nix||//||56 km × 26 km ± ?||0.005 ± 0.004||48,694 ± 3||24.85463 ± 0.00003||1 : 3.89||0.002036 ± 0.000050||0.133 ± 0.008||23.7||2005|
|Pluto IV||Kerberos||//||31 km ± ?||0.002 ± 0.001||57,783 ± 19||32.16756 ± 0.00014||1 : 5.04||0.00328 ± 0.00020||0.389 ± 0.037||26||2011|
|Pluto III||Hydra||//||58 km × 34 km ± ?||0.005 ± 0.004||64,738 ± 3||38.20177 ± 0.00003||1 : 5.98||0.005862 ± 0.000025||0.242 ± 0.005||23.3||2005|
The maximum distance between the centers of Pluto and Charon is 19,571 ± 4 km.
The Plutonian system will be visited by the New Horizons spacecraft in July 2015. Besides Pluto and Charon, good images should be returned of Nix. The other small moons will be imaged at lower resolution.
Nix and Hydra can be seen here: http://www.nasa.gov/nh_new-horizons-spots-small-moons-orbiting-pluto/
- See "P1P2_motion.avi" (AVI).[dead link] and barycenter for animations
- 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).
- Many astronomers use this idiosyncratic pronunciation, rather than the classical //, but both are acceptable.
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- Orbital elements of small satellites from Showalter and Hamilton, 2015; mass and magnitude from Buie & Grundy, 2006
- Buie: Mapping the surface of Pluto and Charon
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- NASA's Hubble finds Pluto's moons tumbling in absolute chaos
- "Welcome to the Year of Pluto", Mountain Living, Jan/Feb 2015, p. 49.
- 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
|Wikimedia Commons has media related to Moons of Pluto.|
- Animation of the Plutonian system[dead link]
- Hubble Spots Possible New Moons Around Pluto (NASA)
- Two More Moons Discovered Orbiting Pluto (SPACE.com)