Neptune's capture of its moon Triton in a binary–planet gravitational encounter

Abstract

Triton is Neptune's principal satellite and is by far the largest retrograde satellite in the Solar System (its mass is approximately 40 per cent greater than that of Pluto). Its inclined and circular orbit lies between a group of small inner prograde satellites and a number of exterior irregular satellites with both prograde and retrograde orbits. This unusual configuration has led to the belief that Triton originally orbited the Sun before being captured in orbit around Neptune. Existing models for its capture, however, all have significant bottlenecks that make their effectiveness doubtful. Here we report that a three-body gravitational encounter between a binary system (of approximately 10(3)-kilometre-sized bodies) and Neptune is a far more likely explanation for Triton's capture. Our model predicts that Triton was once a member of a binary with a range of plausible characteristics, including ones similar to the Pluto-Charon pair.

Binary stripping as a plausible origin of correlated pairs of extreme trans-Neptunian objects

“…Scheeres et al 2000;Sharma et al 2006;Ortiz et al 2012), but the minimum approach distance associated with such events (about 20 planetary radii, see e.g. Keane and Matsuyama 2015) is far shorter than the one required for binary dissociation in the case of wide binaries whose binding energies are rather small (Agnor and Hamilton 2006;Vokrouhlický et al 2008;Parker and Kavelaars 2010). The simplest formation mechanism for dynamically-related asteroid pairs is binary disruption during planetary flybys, but it requires the presence of binary asteroids moving on planet crossing orbits (e.g.…”

Formation of exomoons: a solar system perspective

“…Another form of intact capture, dynamical capture, has been shown to be a viable means of placing satellites in to orbit around a planet with a pre-existing satellite system, however [54,135]. Typically, dynamical capture is invoked as a means of capturing the numerous small satellites of the gas giants.…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…Others are thought to be planetesimals that were captured by the planet (e.g. Agnor and Hamilton 2006). Finally, some moons are thought to have formed due to a major collision, including the Earth's moon (Canup and Asphaug 2001) and Pluto's moon Charon (Canup 2005).…”

“…Neptune's moon Triton has an orbit that is retrograde and highly inclined, suggesting that it is a captured moon presumed to have originated in the Kuiper Belt (Goldreich et al 1989;Agnor and Hamilton 2006). Triton's surface is made up primarily (∼ 55 %) of N 2 ice with water and CO 2 ices making up the remaining ∼ 45 %.…”