JLP’s Blog

Up until today our Solar system had 9 planets. From this day forward it only has 8. No, one of them didn’t get destroyed by some Death star or something. What happened was that stronomers at International Astronomical Union (IAU) General Assembly 2006 redefined the term planet. So the new definition is this:

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.

The consequence of this is that Pluto doesn’t qualify as a planet any more and now belongs to a new group called dwarf planets, which are defined as (pay attention to points (c) and (d)):

A dwarf 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

And then we have a third group:

All other objects except satellites orbiting the Sun shall be referred to collectively as Small Solar-System Bodies.

So there you have it. Some books will have to be rewritten, people will have to learn some new facts, and celestial bodies still don’t care about any of this

Tags: astronomy, space, planet, planets, solar system, iau, science