Pluto and Charon: A case of precession‐orbit resonance?

Pluto may be the only known case of precession‐orbit resonance in the solar system. The Pluto‐Charon system orbits the Sun with a period of 1 Plutonian year, which is 250.8 Earth years. The observed parameters of the system are such that Charon may cause Pluto to precess with a period near 250.8 Earth years. This gives rise to two possible resonances, heretofore unrecognized. The first is due to Pluto's orbit being highly eccentric, giving solar torques on Charon with a period of 1 Plutonian year. Charon, in turn, drives Pluto near its precession period. Volatiles, which are expected to shuttle across Pluto's surface between equator and pole as the insolation varies, might change the planet's dynamical flattening enough so that Pluto crosses the nearby resonance, forcing the planet's equatorial plane to depart from Charon's orbital plane. The mutual tilt can reach as much as 3° after integrating over 8.4×10 6 years, depending upon how close Pluto is to the resonance and the supply of volatiles. The second resonance is due to the Sun's traveling above and below Charon's orbital plane; it has a period half that of the eccentricity resonance. Reaching this half‐Plutonian‐year resonance requires a much larger but still theoretically possible amount of volatiles. In this case the departure of Charon from an equatorial orbit is ∼1° after integrating for 5.6×10 6 years. The calculations ignore libration and tidal friction. It is not presently known how large the mutual tilt can grow over the age of the solar system, but it is probably less than ∼4°. If so, then observing such small angles from a Pluto flyby mission would be difficult.