The origin of Pluto's peculiar orbit

THE origin of Pluto's unusual orbit - the most eccentric and inclined of all the planets - remains a mystery. The orbits of Pluto and Neptune overlap, but close approaches of these two planets are prevented by the existence of a resonance condition¹: Pluto's orbital period is exactly 3/2 that of Neptune, which ensures that the conjunctions always occur near Pluto's aphelion. Long-term orbit integrations^2-5 have uncovered other subtle resonances and near-resonances, and indicate that Pluto's orbit is chaotic yet remains macroscopically stable over billion-year timescales. A suggestion⁴ that the orbit may have evolved purely by chaotic dynamics appears unlikely in light of recent orbital stability studies⁶, unless one appeals to a well-timed collision to place Pluto in its stable orbit¹⁹. Here I show that Pluto could have acquired its current orbit during the late stages of planetary accretion, when the jovian planets underwent significant orbital migration as a result of encounters with residual planetesimals⁷. As Neptune moved outwards, a small body like Pluto in an initially circular orbit could have been captured into the 3:2 resonance, following which its orbital eccentricity would rise rapidly to its current Neptune-crossing value.