Pluto is a planet. Ninth from the sun, smallest of the planets, the cute one. It was discovered in 1930 by Clyde Tombaugh, named by an 11-year-old British girl, and lived on every solar system poster in every classroom in Australia for seventy years. Saturn had rings, Jupiter had storms, Pluto had distance and a slightly tilted orbit. Saying 'Pluto is not a planet' is what we said when we wanted to feel smart. The IAU said it. Neil deGrasse Tyson said it. We were assured.
Pluto was demoted in 2006 because we kept finding things like it. Eris, discovered in 2005, was originally thought to be larger than Pluto (turns out it's slightly smaller, but more massive). Then Haumea, then Makemake, then Sedna, then dozens of trans-Neptunian objects in the same orbital neighbourhood. The choice was either to add ten new planets to the lineup or change the rules. The IAU defined a planet as a body that (a) orbits the sun, (b) has enough mass to be roughly spherical, and (c) has cleared its orbital neighbourhood of other comparable objects. Pluto fails on the third one. It shares its orbit with the Kuiper Belt, including objects of comparable size. It is now formally a 'dwarf planet,' alongside Eris, Haumea, Makemake, Ceres, and Sedna-class candidates. The downgrade is reasonable. The taxonomy is more honest. The kid in you is still wrong about the poster.
Reception
Sources
- International Astronomical Union: Pluto Definition PRIMARY
- NASA: Pluto REFERENCE