Which celestial bodies deserve to be classified as “planets” has been a source of controversy in astronomy for two thousand years. At the low-mass end, the discovery of numerous additional small bodies in the solar system lead to the International Astronomical Union’s 2006 demotion of Pluto to “dwarf planet” status. At the high-mass end, the discovery of both giant planets and more-massive but still-not-stellar brown dwarfs orbiting stars like the Sun has focused attention on the upper limit for admission to the planet club. I’ll describe the history of the controversy to this point and show that core accretion does not form objects more massive than about 10 times that of Jupiter. That inference can be used to infer the properties of protoplanetary disks and reveals that the Type I and Type II disk migration problems are not problems at all.
