A kindergartener in 2005 and a kindergartener in 2006 would have learned very different facts about the number of planets in the solar system. 2006, of course, was the year Pluto was reclassified as a dwarf planet — a move that sparked outrage among a public that tends to romanticize our solar system.
But long before the Pluto "controversy," other objects moved on and off the official list of solar system planets. Indeed, a kindergartener in the early 1800s would have learned that Ceres was a planet.
