Did a massive ice giant planet once orbit in the outer solar system? And what could evidence for such a world teach us about the original positions of Jupiter and Saturn? The ancient Solar System was formed from a disk of gas and dust spiraling around the nascent Sun. At first, most astronomers believe, the earliest planets formed in regular, closely-packed, orbits. Soon, however, gravitational tugs from the most massive of these worlds played havoc with the regular orbits of their neighbors. It was once thought that solar systems like our own — with small, rocky planets placed close to their parent…

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