Dwarfing Pluto

Pluto when first discovered by Clyde Tombaugh in 1930 was considered the 9th planet from the Sun. However, in August of 2006 the International Astronomical Union (IAU) redefined the term "planet", and classified Pluto, the asteroid Ceres, and Eris as dwarf planets. Pluto is also classified as the prototype of a family of trans-Neptunian objects. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.

It orbits between 29 and 49 AU from the Sun, and was the first Kuiper Belt Object to be discovered. Approximately one-fifth the mass of the Earth's Moon, Pluto is primarily composed of rock and water ice. It has an eccentric orbit that is highly inclined with respect to the planets and takes it closer to the Sun than Neptune during a portion of its orbit. Pluto and its largest satellite, Charon, have often been considered a binary system because they are more nearly equal in size than any of the planetoid/moon combinations in the solar system, and because the barycentre of their orbits does not lie within either body. Two smaller moons, Nix and Hydra, were discovered in 2005.

According to the IAU resolution passed on 24 August 2006, there are 3 main conditions for an object to be called 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 neighborhood around its orbit.

Pluto fails to meet the third condition.

There has been resistance amongst the astronomical community towards the reclassification including that of Alan Stern, principal investigator with NASA's "New Horizons" mission to Pluto. Many members of the public have even expressed their views and concerns of the reclassification, with some even questioning if this move by the astronomers was deliberately done to justify their existence.