I still have a big pile of textbooks from when I studied astronomy up at the RGO. Almost overnight they have become museum pieces / collectors items / landfill (* delete as appropriate) thanks to the International Astronomical Union's (IAU) recent Resolution GA26-5-6.
The good news is that we now have a formal definition of a planet in the solar system. Unfortunately for Pluto, it doesn't cut it.
It seems inconceivable that we never had a formal definition before now, but by the time Clyde Tombaugh spotted another big rock orbiting the sun it was obvious it was a planet, right? Well no actually. One thing I picked up in my studies is that Pluto was always a bit weirder than all the other planets:
- Firstly it's inclined at 17 degrees to the plane of the ecliptic whereas all the other planets sit within a few degrees of it,
- Secondly its orbit ranges from as far away as 49.5 AU (from the sun) to as close as 29.7 AU.
- Thirdly, at this distance it's orbit actually crosses the orbit of Neptune.
As planets went, it was always the odd one out. So on August 24, 2006 Pluto was redefined as a 'dwarf planet'. Bye bye Pluto, hello asteroid number 134340.