h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Actually, it's quite bullshitty to ask, "Is pluto a planet?"
If you ask that, the first question that comes is "How do you define a planet?" Isn't anything that revolves around the sun a planet? Or at least aren't all bodies larger than certain size planets? If they are, then pluto is also a planet. And whether it 'is' a planet or not won't make any difference since the word "planet" is man made. Man has to decide what it should mean. He can just change the definition a bit to include "Alpha Centauri" in the list of planets.

Getting my point?

Yes.

I believe there are nine planets... but I haven't yet come up with a precise definition to allow me to legitimately cling to this belief.

On the TV show, QI, recently it was revealed there are only 8 planets and pluto apparantly doesn't fit the reasonable definition (not given on the programme) and that a definition that included pluto would unfortunately include too many insignificant asteroids.

So what is the real consensus, and why?

Neil

The consensus is that the IAU gets to decide. They have final authority in naming anything above Earth's atmosphere. Currently there definition of a planet, in this solar system, is anything on their list of nine planets. The problem with the "anything that revolves around the sun definition" is that that includes all the little meteroids that burn up in the atmosphere whenever there's a meteor shower. Surely they aren't "planets".

In August of 2006 the IAU got around to saying that things going around the sun fall into three groups. They are either planets, dwarf planets, or small solar system objects.
here is their definition of a planet:1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. That last caveat elimitates anything in the Asteroid, or Kuiper belt ( however they can still be dwarf planets (see A13943261)

Thanks