A giant planet in the Kuiper Belt

Before they were discovered, Neptune and Pluto were conjectured to explain discrepancies between planetary orbits and Newtonian expectations. Now a pair of astronomers from Caltech, theorist Konstantin Batygin and observer Michael Brown, have proposed that our solar system includes a new planet. But Planet Nine, as they call it, doesn’t explain small orbital perturbations; instead it accounts for unlikely similarities in the orbits of six objects, among them the minor planet Sedna, located far away in the Kuiper Belt. The figure shows those orbits in purple; the length of Sedna’s semimajor axis is about 500 AU (Earth–Sun radii); Pluto’s is 40 AU. For all six, the semimajor axis points in about the same direction, and all six orbits are inclined by 30° or so with respect to Earth’s orbit. Having calculated that the likelihood of such a coincidence is 0.007%, Batygin and Brown explored the possibility of a gravitational mechanism...