A NEW APPROACH TO AN OLD (1936) SUGGESTION FOR THE ORIGIN OF PLUTO AS AN ESCAPED SATELLITE OF NEPTUNE

Any successful model for the origin of the Pluto-Charon system has to explain, or be consistent with, the following features: (1) the eccentricity and inclination of the orbit of Pluto, (2) the retrograde direction of the orbit of Charon and the other satellites of Pluto, (3) the masses of both Pluto and Charon and the mass ratio of about 10 to 1, (4) the density and composition of the bodies in the Pluto-Charon system, and (5) the surface features in photos from the New Horizons spacecraft.

Raymond Lyttleton (1936, Mon. Not., Royal Astron. Soc., v. 97, p. 108) suggested that Pluto may be an escaped satellite of Neptune. An additional complication for any such explanation, added in 1978, is the presence of Pluto’s large satellite, Charon. We have developed a model for the origin of Pluto as an escaped satellite of Neptune in which Charon is formed during a very close, near grazing, encounter between a captured retrograde satellite of Neptune (i.e., Triton) and an original satellite of Neptune (i.e, proto-Pluto) . In this model the mass of Triton is about twice that of proto-Pluto. During the very close encounter, ice is “peeled off” the sub-Triton portion of proto-Pluto and goes into a retrograde orbit around Pluto. Although the newly formed satellite (Charon) is in a retrograde orbit about Pluto, Pluto and its newly formed satellite escape from Neptune and are inserted into a prograde heliocentric orbit of notable eccentricity and inclination relative to solar system standards. We have done some four-body numerical simulations (sun, neptune, triton, proto-pluto) in which a pluto-like body is inserted into a pluto-like heliocentric orbit.

The major difference between Lyttleton’s (1936) suggestion and ours is that Triton is captured into an elliptical retrograde orbit rather than being an original satellite of Neptune in a prograde orbit. In Lyttleton’s model the pluto-like body escapes from the Neptune system during the encounter in which the triton-like body is changed from a prograde neptocentric orbit into a retrograde neptocentric orbit that it has today. No one has been able to demonstrate that such a drastic change in the orbit of the triton-like body is physically possible.