A surprising number of the icy objects in the Kuiper belt exist in pairs, or binaries. A new model proposes that these two-body systems were created through three-body interactions. n the frigid outskirts of the Solar System, beyond the orbit of Neptune, lies the Kuiper belt. This doughnut-shaped region contains 100,000 frozen bodies, each more than 200 kilometres across. It is scarcely a decade since the first Kuiper-belt object (KBO) was sighted, but now the positions and orbits of, nearly 800 are known. Sizes have been measured for a handful and, for scores more, crude colours have been detected. A few KBOs have been found to be binary―that is, two objects looping around each other while, as a pair, they circle the Sun. On page 518 of this issue, Funato et al. present a promising explanation of how such systems might have come to exist in the outer Solar System.
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