In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. Famously, Pluto no longer met the criteria and was demoted to a dwarf planet. Things have been a bit of a mess since then — so is it time to redefine the planet?
To be fair, Pluto had it coming. The word "planet" never had an official definition, and astronomers had always played fast and loose with its use. To the ancient Greeks, a planet was any "wandering star," which included the sun and the moon. With the Copernican revolution, the definition changed: Earth was considered a planet in its own right, the moon was demoted to a satellite, and the sun got promoted.
