Moons of Pluto
The dwarf planet Pluto has five known moons. In order of discovery they are Charon, Nix, Hydra, S/2011 P 1 ("P4"), and S/2012 P 1 ("P5"). 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 object.
The innermost and largest moon, Charon, was discovered by James Christy on June 22, 1978, nearly half a century after Pluto itself. In May 2005, the Hubble Space Telescope Pluto Companion Search Team imaged two more moons, named Hydra and Nix in reference to the New Horizons mission.[dead link] Two more small moons, so far unnamed, were discovered in 2011 and 2012. Theoretical simulations suggest that there may be as many as 10 moons and a ring system.
The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where prograde orbits would be stable.
Charon is so massive (being nearly half the diameter of Pluto and an eighth its mass) that the system's barycenter is outside of 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.
Following Buie and Grundy's recent recalculations taking into account older images, 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. 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 KBOs. However, given their color and suspected chemical similarities to Charon, it is likely that their albedos are similar as well and that the diameters are closer to the lower estimates.
Formation and resonances
It is suspected that the Plutonian satellite system was created by a massive collision, similar to the "big whack" believed to have created the Earth's 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 achieved its current orbit. If Hydra and Nix turn out to be tidally locked, as Charon is, that will settle the issue, as tidal forces are insufficient to damp their rotations in their present orbits. The color of each is a lunar 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 the 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.
S/2012 P 1, Nix, S/2011 P 1 and Hydra form a 1:3:4:5:6 sequence of near resonances with the Charon–Pluto orbital period: S/2012 P 1 is about 5.4% from resonance, Nix is within 2.7% of resonance, S/2011 P 1 is apparently within 0.6%, while Hydra is within 0.3%, though none appear to be in an exact resonance. It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current geosynchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. Today the Pluto–Charon pair continue to produce strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. At the lower estimated size range, Nix should have no significant precession, while Hydra should have a precession period of 15 years. However, at their maximum projected masses (assuming an albedo of 4%), the two moons may be in a 3:2 orbital resonance with each other, with libration periods of 400 to 450 days, though this may already be ruled out by the low eccentricity of Charon. Thus accurate orbital data can help resolve the sizes of these moons.
However, it was recently 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, while 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 newly found P4 and P5 may support this idea.
The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light purple. Pluto has been added for comparison, for it orbits a point outside itself.
|Mass (×1021 kg)||Semi-major
(to Pluto's equator)
|Pluto||2,306||13.05 ± 0.07||2,035||6.387230||1:1||0.0022[b]||0.001°||15.1||1930|
|1,207 ± 3||1.52 ± 0.06||17,536 ± 3*||6.387230||1:1||0.0022[b]||0.001°||16.8||1978|
|S/2012 (134340) 1||10–25||?||42,000 ± 2,000||20.2 ± 0.1||≈ 1:3||≈ 0||?||27||2012|
|Pluto II||Nix||//||46–137||< 0.002||48,708||24.856 ± 0.001||≈ 1:4||0.0030||0.195°||23.7||2005|
|S/2011 (134340) 1||13–34||?||≈ 59,000||32.1||≈ 1:5||≈ 0||?||26||2011|
|Pluto III||Hydra||//||61–167||< 0.002||64,749||38.206 ± 0.001||≈ 1:6||0.0051||0.212°||23.3||2005|
The maximum distance between the centers of Pluto and Charon is 19,571 ± 4 km.
- See "P1P2_motion.avi" (AVI).[dead link] and barycenter for animations
- Orbital eccentricity and inclination of Pluto and Charon are equal since they refer to the same two-body problem (the gravitational influence of the minor satellites Nix and Hydra is neglected here).
- Many astronomers use this idiosyncratic pronunciation, rather than the classical //, but both are acceptable.
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- Satellite data from Buie & Grundy; a, i, e updated with JPL (site updated 2008 Aug 25).
- Buie: Mapping the surface of Pluto and Charon
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- 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)
- 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
- Animation of the Plutonian system[dead link]
- Hubble Spots Possible New Moons Around Pluto (NASA)
- Two More Moons Discovered Orbiting Pluto (SPACE.com)