Flipping the orbital planes of primordial trans neptunian binaries by close encounters with Neptune during the planetary instability migration phase of the outer solar system

Abstract

The distribution of inclinations of the mutual orbits of trans-Neptunian binaries contains valuable information about the formation of these binaries. The inclinations measured so far show a clear preference of orbital motion in the same sense as the orbits around the Sun. This fact strongly favours binary formation by the gravitational collapse of nebular clumps of small solid particles. A few binaries, however, are orbiting with a retrograde motion, challenging an origin by this mechanism. Here we show that an implanted population of binaries, formed by gravitational cloud collapse in direct orbits, flipped their orbital planes during close encounters with Neptune. The cloud collapse formation mechanism is therefore the most plausible way to form trans-Neptunian binaries in the innermost part of the outer Solar system, which were then implanted in the Kuiper belt during the planetary instability phase.