Pluto booted out of league of planets

Then there were eight

The International Astronomical Union's (IAU) general assembly shindig in Prague has voted to boot Pluto out of the league of planets, declaring in a resolution: "The eight planets are Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune."

In the process, it rejected a proposal to actually increase the number of planets to 12, incorporating Ceres, Plutonian moon Charon and distant object 2003 UB313.

Pluto's demotion to "dwarf planet" is bound to cause a bit of a kerfuffle, since astronomers have been arguing over its classification for yonks.

Robin Catchpole, of the Institute of Astronomy in Cambridge, told the BBC: "My own personal opinion was to leave things as they were; I met Clyde Tombaugh [Pluto's discoverer] and thought how nice it was to shake hands with someone who had discovered a planet.

"But since the IAU brought out the proposal for new planets I had been against it - it was going to be very confusing. The best of the alternatives was to leave the major planets as they are and then demote Pluto. So I think this is a far superior situation."

Louis Friedman, executive director of the Planetary Society in California, attempted to downplay the significance of the decision with: "The classification doesn't matter. Pluto - and all Solar System objects - are mysterious and exciting new worlds that need to be explored and better understood."

The full IAU resolution of the definition of a planet reads:

The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A "planet"¹ 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.

(2) 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) shape² , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects³ except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

¹The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.