Astronomers seem to invoke collisions, however improbable, when they cannot explain odd features of the solar system. Triton circles Neptune in reverse of its sister moons, for example, and the prior best guess was that Triton flew in from afar and knocked another moon out, like a well-struck cue ball. But if a pair of objects orbiting each other waltzed past Neptune, the one moving slower relative to the planet could have been snared by its gravity and begun orbiting the planet, and the other would have continued on its way, argue researchers in the May 11 Nature. Many such binary pairs have been spotted on the solar system's periphery over the past few years. Similarly, gravity, rather than a glancing blow from a protoplanet, might also account for Uranus and its moons' 98-degree axial tilt relative to the planet's orbital plane. Based on accepted trajectories for the young gas giants, a simulation reported in the April 27 issue of Nature indicates that the wobbling spin of Saturn could have nudged Uranus on its side during close encounters between the pair.
This article was originally published with the title "Traces of Gravity" in Scientific American 295, 1, 33 (July 2006)