Where is planet nine?

Michael E. Brown is often called "the guy who killed Pluto." But he takes the moniker in stride. Sitting in his sunny Pasadena office at the California Institute of Technology, Brown jokes that Pluto, which was reclassified as a dwarf planet in 2006, had it coming. The year before, Brown had discovered Eris, a frosty dwarf in the outer solar system more massive than Pluto and named, fittingly, for the Greek goddess of strife.