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Pluto and the Formation of its Moons:

The discovery of two additional moons of Pluto could not have come at a worse time - for Pluto that is. For a planet recently demoted to dwarf-planet, Pluto sports a 3 moon system with bodies that mimic the resonance of the moons of much larger planets. None the less, while Pluto may have lost its status as a planet it may well provide valuable insight to Solar System dynamics.

Last year the Hubble Space Telescope, our greatest tool of the heavens, discovered two additional tiny moons of Pluto: Nix and Hydra. These two moons orbit Pluto in the same plane as Charon - with Charon, Nix and Hydra in a 6:4:1 resonance (that is Charon orbits 4 times for every 1 of Nix and 6 times for every 1 of Hydra). In addition, Nix and Hydra are orbiting Pluto (and Charon) is near circular orbits.

It is generally accepted that a multi-moon system with an outwardly migrating inner moon traps the outer moons in a near circular orbital resonance - this is due to tidal forces that must be conserved, but is usually lost as a result of greater orbits of the outer moons need of this energy to maintain its orbit. This results in an elliptical orbits of one or more inner moons. In simple terms, the inner moon looses its energy because the outer moon needs this energy to maintain its orbit. The inner moon's orbit becomes elliptical.

While astronomers are still trying to understand orbital dynamics, Pluto and its system of moons gives us a real world laboratory to understand not only capture theories of moons, but also how transfer of orbital momentum occurs between the orbits. It is believed that Pluto and Charon were independent bodies that impacted and formed a close orbiting binary, but this capture/impact model does not explain the exciting discovery of the two smaller, circular moons of Pluto. The image below, and quote from a Science Magazine article, explains a possible, and intriguing scenario:

Pluto Orbit Dynamics

Formation and evolution of Pluto's satellite system. (A) A giant impact results in the (almost) intact capture of the large moon Charon. (B) This leads to a highly eccentric orbit for Charon and a disk of small debris orbiting the Pluto-Charon pair. (C) The tiny moons Nix and Hydra accreted from this disk, which produces nearly circular orbits. As tidal forces caused Charon's orbit to recede from Pluto, the moons Nix and Hydra became trapped in the 4:1 and 6:1 orbital resonances of Charon. (D) Charon pushed the moons ahead of itself until tides damped Charon's eccentricity and the resonances were broken, leaving the substantially expanded system of moons on circular orbits that is observed today (Science 25 August 2006: Vol. 313. no. 5790, pp. 1054 - 1055).
Pluto's migration is locked in 3:2 with Neptune, and its elliptical orbit is evidence to this. Using this as a base, the model for Pluto and its moons is that rather forming from a giant impact, Charon as an independent body was already in an elliptical orbit when its inelastic impact occurred. Debris from this collision would have formed Nix and Hydra close to Charon and Pluto, and as Charon migrated out Nix and Hydra were pushed out as well by transfer of momentum - as demonstrated by the illustration.

This is an interesting system, and provides an interesting prediction that most larger TNO's (Tans-Neptunian Objects) and KBO's (Kuiper Belt Objects) will also have one or more moons of their own.


Original Story: Science 25 August 2006: Vol. 313. no. 5790, pp. 1054 - 1055 DOI: 10.1126/science.1128447


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