Once upon a time there was a planet called Pluto, and a cartoon character dog too, lest we forget. Then, the powers that be decided that Pluto wasn’t a planet and it was demoted. I thought that this must’ve been very demoralising for said planet, to be a former planet and now just a …?
Apparently, in order for an object to be a planet, you would need a satellite or moon orbiting around you, but both Venus and Mercury don’t have moons and it’s not the size of the planet that counts, so I wasn’t sure what the issue was.
What happened when Pluto was discovered?
In 1930 staff at the Lowell Observatory issued a circular entitled “Discovery of a solar system body apparently trans-neptunian” for distribution to astronomers around the world. The announcement describes a new “object” and makes no claim of a planet discovery. This object later became known as Pluto.
There are many things that make Pluto quite different from the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. But one difference is truly fundamental, and it explains why Pluto is not classified as a planet. Unlike any of the planets, Pluto is embedded in a vast swarm of bodies similar to itself. Pluto is therefore analogous to the asteroid Ceres in the asteroid belt between Mars and Jupiter. Pluto has many friends orbiting nearby, which is not the case for any of the planets. The planets accumulate, eject, or otherwise control all the mass in their immediate proximity. Pluto and Ceres are not able to do that; therefore they belong to a class that is really quite distinct from the eight planets.
In August of 2006, the International Astronomical Union (IAU) voted to update the definition of what makes a planet. According to their decision a planet must satisfy the following three criteria:
- It must be an object which independently orbits the Sun;
- It must have enough mass so that gravity pulls it into a roughly spheroidal shape;
- It must be large enough to ‘dominate’ its orbit (i.e. its mass must be much larger than anything else which crosses its orbit).
And Bob’s your uncle, Pluto was no longer a planet. Pluto the Dog’s still going strong though, so there’s that. What does this inter planetary sojourn have to do with all things water, you ask? Well, seeing as I’m literally lurking at the water cooler harassing, listening for ideas most of the working day, that’s as close as dammit. In my not so humble opinion.