Answer:
Ever since the discovery of Pluto in 1930, kids grew up learning that the solar system has nine planets. That all changed in the late 1990s, when astronomers started arguing about whether Pluto was indeed a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to designate Pluto as a "dwarf planet," reducing the list of the solar system's true planets to just eight.
