The formation of Uranus and Neptune has puzzled astronomers for years. The assumed disc of gas and dust that formed the two gas giants would have been too thin at their current locations to form the icy planets. It's more likely that the two, and Jupiter and Saturn, were closer together in the earlier days of our solar system, and spread out once the disc was depleted. The "
five great gravitational bullies of the solar system, Jupiter, Saturn, Uranus and Neptune" jockeyed for position after being formed. According to past simulations, which only included the current planets, either Uranus or Neptune at least should have been jettisoned into deep space. "People didn't know how to resolve that," says David Nesvorny of the Southwest Research Institute. He offers a new explanation: "A sacrificial ice giant between Saturn and Uranus."