Pluto Becomes a Dwarf Planet
Next: August 24, 2027
Small world, big debate.
The story
Where it comes from
This Story marks the anniversary of 24 August 2006, when astronomers at the General Assembly of the International Astronomical Union (IAU) in Prague voted on what a planet is. New discoveries in the outer Solar System had made a clear definition necessary. Under the one they agreed, a planet orbits the Sun, is massive enough to pull itself nearly round, and has cleared the neighbourhood around its orbit. Pluto has not, so it became a 'dwarf planet', a new class of its own, and the Solar System was left with eight planets.
How it's celebrated
There is no formal observance, but the day still sparks friendly debate among stargazers and in classrooms. It is a good moment to learn the eight planets in order, read about the other dwarf planets, or look at the close-up pictures NASA's New Horizons spacecraft sent back from Pluto in 2015, and decide for yourself where you stand.
Did you know
- The resolution naming Pluto a dwarf planet passed with 237 votes in favour, 157 against and 17 abstentions.
- The first three dwarf planets were Ceres, Pluto and an object then called 2003 UB313, which its discoverers named Eris, after the Greek goddess of strife, a nod to the arguments it caused.
- The IAU noted that the word 'planet' first meant 'wanderer', a light seen moving across the sky.
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