I first learned about the Solar System‘s planets when I was six years old. My music teacher conditioned our class to learn their names in the order of their proximity to the sun through a silly song to which I don’t remember the tune, only the lyrics.
When you’re taught something at a very young age, generally that something is immutable – or so you assume. The alphabet, addition and subtraction tables, making juice into frozen popsicle treats. Yet in 2006, when I was 10 years old, it was announced that the top-notch astronomers had suddenly changed their minds about the solar system: Pluto was no longer a planet.
To be clear, the scientists didn’t arrange for Pluto to dissipate into thin air or anything like that. It is now considered a dwarf planet; it still looks as it always had and does what it always did, i.e. resembles a sphere and orbits the sun. But in August 2006 astronomers reconstructed the definition of ‘planet’ such that it also has to be the dominant gravitational body in its orbit in the Solar System. Because Pluto is so small, it does not hold the ability to either fling away or consume all other objects in its orbit; in fact, it’s only .07 times the mass of the average object! For this reason, astronomers voted against allowing Pluto to co-exist with the likes of Mars and Neptune in the prestigious category of ‘planet,’ scooting the term ‘dwarf’ in front of its label.