Synchronisation of the Pluto-Charon binary by inward tidal migration.

The dwarf planet Pluto and its largest moon Charon represent a fully tidally evolved system: their orbital eccentricity is almost zero and their respective rotational periods are equal to the mutual orbital period. According to a widely accepted hypothesis, Charon as well as other Pluto moons originated in a giant oblique impact (e.g., Canup et al., 2005; Arakawa et al., 2019), forming on a tight orbit above the synchronous radius, and evolved by tidal recession from the primary, which was endowed with a large angular momentum and thus fast rotation. A recent, alternative scenario proposes formation by collisional capture (Denton et al., 2025), resulting in Charon’s emplacement on an initially circular close-in orbit and a primordial synchronisation at high spin rate.

A tidally evolving binary is subjected to surface stresses that are strongly dependent on the mutual distance and, for small orbital separations, may lead to the formation of tidally-oriented fractures in the ice shell similar to those on Enceladus or Europa. The orientation of fractures identified on images from the New Horizons mission is, however, not correlated with expected tidal stresses and has instead been attributed to ocean freezing, which would have postdated the full orbital evolution (Rhoden et al., 2020). Moreover, an initially quickly rotating Pluto (and Charon) consistent with the giant impact scenarios would lead to a considerable rotational bulge that would only be able to relax before present in the case of a thin lithosphere and a weak ice shell above a subsurface ocean (McKinnon et al., 2025).

Here, we present a model of the Pluto-Charon synchronisation that predicts lower tidal stresses and does not require initial fast rotation of the partners, thus potentially alleviating some of the challenges posed by the standard tidal recession scenario. We propose that the binary was formed by a capture of a highly inclined retrograde minor planet (proto-Charon) by a prograde-rotating Pluto and subsequently evolved by tidal approach. Following this line, we perform numerical simulations of the binary’s orbital evolution, studying the effect of various initial spin rates, eccentricities, and interior properties. During the evolution, Pluto acquires its present-day retrograde rotation and, depending on ice viscosity, Charon may experience episodes of higher spin-orbit resonances (such as 3:2 or 2:1). Since the evolution of a planet with a retrograde moon proceeds at distances greater than the present-day semi-major axis, both Pluto and Charon experience tidal heating and stresses two orders of magnitude lower than in the tidal recession scenario.