A quadrillion previously unnoticed small bodies beyond Neptune have been spotted as they dimmed X-rays from a distant source. Models of the dynamics of debris in the Solar System's suburbs must now be reworked. Until the time of Galileo, the Solar System was believed to consist of the Sun, the Moon and the six inner planets — including Earth — as far out as Saturn. Since then, we have added other gas giant planets (Neptune and Uranus), moons of other planets, the asteroid belt and, in 1930, Pluto.On page 660 of this issue, Chang and colleagues1 present observational estimates of the make-up of the Solar System's most distant components — the trans-neptunian objects. Their results call for a rethink of models of how these bodies formed.
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