Poor ol’Pluto

Isn’t it strange that kids today are being taught that there are eight planets in our solar system? The concept seems a bit foreign to me, because back in my day there were nine. In fact Pluto was one of my favourite planets. I remember doing a poster about it for a school project when I was 8.

That was until it was demoted in 2006. This makes me (and Pluto) sad. Here’s why it happened.

Back before advanced telescopes were a thing, there were no clear definition of what constituted a planet. Planets were simply objects large enough to be observable, which orbited the sun.

In 1930 a new object was discovered by Clyde Tombaugh of Arizona, far beyond the orbit of Neptune. Originally called Planet X, it became known as Pluto; the ninth planet in our solar system.

mildly interesting side note: Pluto’s name was suggested by an 11-year old girl in England, and not because of the Disney character. The fact that an 11-year old knew about the Roman god of the underworld is pretty cool in itself.

Now, astronomers were never certain about the size of Pluto. They thought it must be at least larger than Mercury. This belief was shattered in 1978 with the advent of advanced telescopes and the subsequent discovery of Pluto’s moon, Charon (another cool Roman underworld reference there). Once Charon was discovered, astronomers determined that Pluto’s mass was smaller than that of even our own little moon. In fact, Charon was actually discovered to be bigger than Pluto. Seems a bit strange, right? This was the first threat to Pluto’s planetary status.

Another thing they discovered was that Pluto’s orbit was a bit strange. It didn’t sit with those of the other planets on the orbital plane. This picture’ll explain it better:

NOT TO SCALE. © Vibhuti Patel

This also shook the foundations of Pluto’s planethood.

To add to Pluto’s distress even further, astronomers discovered a bunch of other objects just beyond its orbit in an area called the Kuiper Belt. One of these – Eris – was also larger than Pluto. Furthermore, it was found to be made of the same icy rock-type composition. This was the major catalyst in Pluto’s fall from grace.  If Pluto was a planet, then surely Eris and all the other objects in this area should be considered planets too right? After a few months of heavy debate, it was decided that these discoveries called for a review of what actually constitutes a planet.

In 2006 the International Astronomical Union (IAU for short) gathered to devise an official definition. A bunch of different proposals were put forward which members of the association were to vote on:

  • One proposal defined Pluto and Eris as planets.
  • Another defined planets as those that we are familiar with (9 at the time – including Pluto) without any scientific demarcations.
  • The final definition would drop the number of planets down to 8, and Pluto would be kicked out of the planet club.

In the end they voted to go with the latter, and a planet was defined as an object which:

  1. Was in orbit around the Sun,
  2. Had enough gravity to pull itself into a spherical (or roughly spherical) shape, and
  3. Had to have, or be able to clear its neighbourhood of its orbit.

A bit of clarification for (3) seems to be in order. A planet’s mass makes them the dominant gravitational body within the areas directly surrounding their orbits. This means that any smaller objects nearby would either be pulled and consumed into their mass, or repelled out of the way (kind of like a magnet).

Now, Pluto is definitely able to adhere to the first two parameters. It’s round, and it orbits the sun (in a weird way maybe, but it does all the same). But because it is so small (0.07 times the mass of other objects in its orbit, to be exact), it doesn’t have the gravitational force to consume or repel other bodies in its orbit. Disqualified!

It seemed a bit unfair to discount Pluto entirely. After all, it only failed one of three conditions. In order to classify things further, Pluto became the first dwarf planet – an object that fulfills the first two conditions but not the third. Although the demotion of anything is inherently a bit sad, Pluto is in good company. Its friends include Eris and a bunch of other objects in the Kuiper Belt.




Featured image is sourced from http://i.imgur.com/rB9FS36.jpg. 


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