On the origin of Triton and Pluto

Lyttleton hypothesized long ago that Triton and Pluto originated as adjacent prograde satellites of Neptune(1). With the presently accepted masses of Triton and Pluto–Charon(2,3), however, the momentum and energy exchange that would be required to set Triton on a retrograde trajectory is impossible. The mass of Triton has probably been seriously overestimated(4,5), but not by enough to relax this restriction. It is implausible that the present angular momentum state of Pluto–Charon has been significantly influenced by Neptune(6). It could not acquire such angular momentum during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer Solar System planetesimals. Triton is simply captured, with potentially spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH(4), CO and N(2) (ref. 7). Condensed remnants of this proto-atmosphere could account for features in Triton's unique spectrum(8–11).