It's official: Pluto is no longer a planet. It is now only a "dwarf planet", one of three in the solar system.
The fate of Pluto was determined on Thursday by a vote among members of the International Astronomical Union in Prague.
They approved an initial resolution that requires a "planet" to dominate its neighbourhood, clearing most other stuff out of its path. "Dwarf planets", on the other hand, are large enough for gravity to make them round, but not big enough to clear out their orbits.
The decision to demote Pluto was not even close. One amendment would have left its status open to debate by creating two categories of planets - the eight "classical planets" as well as "dwarf planets" - that might have seemed to be on equal footing. But it was voted down overwhelmingly.
Astro-lobbyists
"The scientific community has realised that the classification used for Pluto for 75 years was not correct" says Gonzalo Tancredi of the University of the Republic in Montevideo, Uruguay.
Tancredi was part of the group who lobbied to change the original draft definition, which would have included Pluto as a planet and possibly hundreds more icy objects. "I'm pretty happy," Tancredi told New Scientist.
So where does that leave the solar system? There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Then there are three recognised dwarf planets: Pluto, Ceres (formerly an asteroid), and the largest dwarf, UB313, popularly known as Xena. Many more dwarf planets will follow. Most will be icy middleweight objects in the Kuiper Belt out beyond Neptune, and one or two may be asteroids.
Tricky issues
A crumb of comfort for Pluto fans is that it will now become the prototype of a new class of object. After some confusion, the IAU members passed resolution 6a, which means that all the dwarf planets beyond Neptune will be named after Pluto.
But resolution 6b, naming them "plutonian objects", did not quite gain an overall majority - so nobody knows what they will be called. The decision will probably be left to an internal IAU committee.
A few tricky issues in the first draft definition have been avoided. One contentious point was that Pluto and its companion Charon would have become a double planet. But that idea has been dropped. Charon remains a moon.
And all these definitions are intended only for our solar system, rather than planets everywhere. "I think one of the problems with the original proposal was that they tried to hard to make sure to define every single thing that might ever be found," says Mike Brown of Caltech, who discovered Xena.
Ambiguous systems
"It's OK if we find planetary systems where things are more ambiguous. In fact, it would be extremely exciting," he told New Scientist.
The resolutions were changing right up to the last minute. A proposal to change dwarf planet to "planetino" was rejected, but the wording was changed to recognise the existence of satellites, which had been carelessly ignored in the previous draft.
Astronomers could yet discover another full-fledged planet, maybe a Mars-sized ball of ice orbiting way beyond Pluto, and sharing its orbit with only much smaller snowballs.
"It's not out of the question," Mark Bailey, an astronomer at Armagh Observatory in Northern Ireland, told New Scientist. But simulations of planet formation suggest that it is unlikely, so chances are that the final planet count for our solar system will be eight.
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