Pluto demoted

On this day in 2006, the International Astronomical Union decided that Pluto would no longer be considered a planet. The controversial decision to reduce the number of planets in the solar system came after the 2005 discovery of a rocky object, since named Eris, that is more massive than Pluto. If astronomers had designated Eris the 10th planet, then the planet list probably would have continued to grow as more worlds were discovered in the outer solar system. As a result, IAU astronomers voted at a meeting in Prague to redefine the term planet. A planet needs to orbit the Sun, have enough mass to become round, and clear other objects from its orbital path. Pluto satisfies the first two criteria but not the third. Instead Pluto was reclassified as a dwarf planet, along with Eris and the asteroid Ceres. To try to placate Pluto lovers, the IAU also added a designation called Pluto-class objects, which are dwarf planets beyond the orbit of Neptune. Of course, many scientists and nonscientists were not happy about Pluto’s demotion (for one thing, they had to learn a new mnemonic device to memorize the eight planets!). Recently we’ve learned that although Pluto is not a planet, it’s still an amazingly dynamic place. In July 2015 the New Horizons spacecraft (which launched when Pluto was still a planet) buzzed by the demoted world, revealing vast plains of nitrogen ice and even a bright heart-shaped region. (Image credit: The International Astronomical Union/Martin Kornmesser)

Date in History: 24 August 2006