pluto

[Derek]

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

 

1. The object must be in orbit around the Sun.

2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should

pull it into a shape of hydrostatic equilibrium.

3. It must have cleared the neighbourhood around its orbit.

 

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.