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 to shepherd the Kuiper Belt objects into their similar paths. Analytical calculations and N-body simulations established that Planet Nine could plausibly account for the similarities, provided it is at least 10 times as massive as Earth and orbits in the plane of the six objects along the trajectory illustrated in yellow. Surprisingly, the theoretical work also implied that some Kuiper Belt denizens have orbits nearly perpendicular to Earth’s. Five such bodies have been spotted. The Caltech model is silent as to where the putative Planet Nine currently lies on its 10-millennium journey around the Sun. If it’s not too close to its aphelion, telescopes should have already spotted it, and it might be in old, overlooked data. At aphelion, the biggest telescopes on Earth could still spot it. (K. Batygin, M. E. Brown, Astronom. J. 151, 22, 2016, doi:10.3847/0004-6256/151/2/22 .)