Pluto ceased to be a planet 10 years ago when members of the International Astronomical Union (IAU) meeting in Prague voted to demote it. The public didn't take kindly to the decision, and it outraged some astronomers.
Nine years later, NASA's New Horizons spacecraft sailed by the controversial body, revealing features striking enough to reopen the demotion wounds.
The decision to redefine Pluto as a "dwarf planet" remains unpopular. Critics argue that demoting it did little to clarify how we understand the solar system. Some set about trying to reverse the decision, but it has stuck. Many were upset at the way the decision was taken - a vote by just 434 astronomers among some 10,000 around the globe.
Textbooks had to be rewritten, and the mnemonics that aided placing the planets in the right order from the Sun were trimmed to eight.
The textbooks originally stated that the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto orbit the Sun, with Mercury the closest and Pluto the most distant. The original hundreds of mnemonics included: Most Volcanoes Erupt Mulberry Jam Sandwiches Under Normal Pressures. Following the demotion, pupils have had to settle with a selection that includes: Many Violent Evil Maniacs Join Swiftly Under Nationalism; and Mercury Viewed Earth's Many Aspects Joyfully Sitting Under Neptune.
In a wholesale redefinition of planethood, the IAU members came up with: "Planets: The eight worlds from Mercury to Neptune. Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite. Small Solar System Bodies: All other objects orbiting the Sun."
Date written/update: 2015-10-05