Gravitational Instability
Planetesimal Formation by Gravitational Instability:
The growing understanding of planetesimal formation is that it occurs by gravitational instability, explaining their spherical contours; however, excess angular momentum often causes them to fragment as they gravitationally collapse, forming gravitationally-bound binary comets, asteroids and trans-Neptunian objects (TNOs). And perturbed binaries may frequently spiral in to merge and form peanut-shaped 'contact binaries'.
An alternative hypothesis suggests that gravitationally-collapsing stars may form bar-mode instabilities which become isolated pairs of giant planets when the protostar collapses to form a core, abandoning its two 'bar-mode' arms. If the arms are gravitationally bound within their own Roche spheres, they may go on to gravitationally collapse to form proto-planets.
And giant proto-planets may go on to fragment (bifurcate) due to excess angular momentum, forming binary planets. Then the energy and angular momentum of their binary orbits cause them to spiral out from their progenitor stars until their binary components spiral in and merge, forming solitary planets.
Moons may similarly spin off from gravitationally-collapsing proto-planets during their own bar-mode instability phase, forming moons that bifurcate and spiral out from their progenitor planets. Saturn's moon Iapetus may be a contact binary without sufficient gravity to form a completely spherical surface, hence its contact-binary walnut-shape and the raised ridge around its equator.