Was Triton originally a regular satellite of Neptune?

The origin of Triton's retrograde orbit remains challenging. Today it is generally accepted that Triton was somehow captured by Neptune when the latter experienced a phase of planetesimal-driven migration. The most likely capture scenario invokes the trapping of one of the members of a binary planetesimal during a close encounter with Neptune (Agnor and Hamilton, 2006). Although viable, this scenario cannot explain the survival of Nereid on an orbit exterior to Triton's, given the extremely eccentric post-capture orbit of the major satellite (Nogueira et al. 2011). Here we propose an alternative scenario in which Triton was originally a regular satellite of Neptune on an equatorial orbit. In this scenario, Neptune experienced at least two collisions with planetary embryos during its formation, leading eventually to its current obliquity; because of these impulsive resets of Neptune's obliquity, Triton may have acquired an orbit with a large inclination with respect to Neptune's equator, a retrograde orbit being a possibility. If we impose the constraints that Neptune final rotation rate and obliquity end near its current values we estimate the final probability of Triton getting a retrograde orbit at roughly 5% to 10%. After the phase of collisions of planetary embryos with Neptune, we apply a simple tidal model to follow the evolution of Triton's retrograde orbit over 4.5 Gy and check its final compatibility with the current Triton-Neptune distance. Our simulations consider Neptune, two collisions with Earth-mass embryos, an equatorial debris disk generated by each collision and temporarily enhancing Neptune's J2, a Triton-sized satellite and the Sun. We tested different Sun–Neptune distances and different timescales for the decay of the equatorial debris disks and show that these two parameters have little influence on the outcome. We also checked that satellites that ended their tidal evolution near Triton's current orbit seldom had their apoapsis greater than Nereid's periapsis, making this scenario compatible with the preservation of Nereid and of other Neptune's irregular satellites.