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.
