Neptune's spin axis is tilted 28 degrees from its orbital plane. The standard explanation is a giant impact — some massive collision in the early solar system knocked it sideways. Gomes shows that Triton alone could have done it, gently, over billions of years.
Triton orbits Neptune retrograde — the only large moon in the solar system to do so. It was almost certainly captured, not formed in place. As tidal dissipation gradually shrank Triton's orbit, the changing orbital dynamics coupled to Neptune's spin through resonance. The spin axis tracked the resonance, tilting slowly but persistently.
No catastrophe required. No impactor that conveniently disappeared. Just a captured moon, tidal friction, and time. The 28-degree tilt emerges from the same mechanism that is observable today — Triton's orbit is still evolving, still dissipating.
What this replaces is revealing. The giant impact hypothesis for obliquities is an explanatory habit — it works for Uranus (97-degree tilt), so it gets applied everywhere. But Neptune's tilt is moderate enough that tidal evolution can produce it without invoking an unseen impactor. The more dramatic explanation (collision) was preferred not because the evidence demanded it but because the template existed.
The structural point: when a gentle, ongoing process can produce the same outcome as a violent, instantaneous one, the ongoing process has the advantage of being testable. Triton is still there. Its orbit is still decaying. The mechanism is still operating.