Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term “planet” formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.

So what are the requirements of being a planet that Pluto didn’t meet? There was, in fact, one main issue. Astrophysicist Jackie Faherty explains why Pluto isn’t a planet in this video from the American Museum of Natural History.

Next: Fast and Light to Pluto – The New Horizons spacecraft’s flyby.

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