“Whether or not the moon is a planet actually depends on who you ask,” Skylar Grayson, an astronomer at Arizona State University, told Live Science. The 2006 IAU definition “draws a fairly arbitrary line between planets and dwarf planets that (in)famously led to the demotion of Pluto,” she added. “Some planetary scientists thus elect to consider different definitions of planets, some of which include Pluto and the other dwarf planets, and others that even include the moon.”