And then there were eight…
As of this morning, the solar system officially has 8 planets.
The IAU have finally ruled on the question of Pluto’s status, and rejected the proposal that would have seen us with 12 or more planets. Instead they have passed a resolution to demote Pluto to a new classification of Dwarf Planet.
The critical distinction comes in the requirement that a planet not only have sufficient mass for gravity to make it spherical, but to also sweep out all the other debris within its orbit. In this respect, Pluto does not qualify, and neither does Ceres, Xena, Sedna or Quaoar.
I’m actually quite satisfied with this, as it speaks more to a process that helps to create a planet – the accretion of other bits of stuff to create a larger whole. It implies some critical size, below which you will never get any bigger than you are, and above which you will continue to grow until you’re the only thing in your orbit. There’s science there, not just taxonomy.
Yesterday we woke up in a world where dark matter had been shown to exist. Today we have one planet fewer. I wonder what will happen tomorrow?
Update: The Bad Astronomer has more.