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Planet Nine

Astronomers have theorized that a Neptune-sized planet orbits the Sun in the outer solar system, far beyond the realm of the eight known major planets. The planet was posited based on the alignments of several members of the Kuiper Belt, which are big chunks of ice and rock left over from the birth of the planets. The possible Planet Nine would orbit the Sun once every 10,000 to 20,000 years, at a distance equal to 400 to 800 times the distance from Earth to the Sun. It could have been ejected into its current orbit by a close encounter with Jupiter, the solar system's largest planet.

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Hunting by Gravity Using gravity to track down planets July 24, 2020