Pluto's a dwarf planet, but not a planet... confused yet?NASA

Pluto has been kicked out of our Sun's planetary family by astronomers who voted today to define a planet by three criteria. It failed on one of them.

Astronomers have been battling over the concept of what defines a planet all week at the general assembly of the International Astronomical Union (IAU) in Prague (see our reports/internationalastronomical_union/">conference newsblog for a blow-by-blow account).

In the end it was decided that to qualify as a planet in orbit around our Sun, a chunk of rock must have been made round by its own gravity; have cleared its neighbourhood of other debris; and not be a satellite of another planetary body.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune all fulfil these criteria. But Pluto is just one of many bits of icy debris in orbit at the edge of our Solar System, known as trans-neptunian objects. Pluto's membership of the trans-neptunians disqualifies it from being a fully fledged planet because it has not 'cleared its orbit'.

Demotion

Instead, Pluto is one of a new category of object to be known as 'dwarf planets' (which, not to be confusing, don't fall under an umbrella term of 'planets', and must, by definition, be written with single quote marks). These objects satisfy the other criteria, in being round and not a satellite. Ceres, which lies in the asteroid belt between Mars and Jupiter, is also now a 'dwarf planet'.

'Dwarf planets' in Pluto's neighbourhood, including the object nicknamed Xena (UB₃₁₃), will be given a category of their own. But the IAU's most recent suggestion, that these be named 'plutonian objects', was narrowly voted down, by 186 votes to 183.

This move had been intended to soften the blow of Pluto's demotion. "There is a large Pluto fan club out there that is going to be incensed by our actions," Owen Gingerich, chair of the planetary definition committee, had warned earlier in the week. The rejection of 'plutonian objects' not only disappoints Pluto fans, it also means the category remains nameless.

Commotion

That astronomers would reach any consensus at all on the concept of a planet looked unlikely earlier this week.

A draft definition issued on 16 August (see 'Planets are round. Will that do?') had received a hostile reaction: it was debated in lively sessions in which astronomers accused IAU officials of keeping them in the dark, and proposing something "silly".

The resolution had to be changed many times before astronomers were even happy voting on it.

But despite the successful vote and the IAU's best efforts at clarity, it seems there is still scope for some confusion. Already at the conference there are hints of problems.

"Dwarf stars are stars. Dwarves are people," reasons Donald Lubowich, coordinator of astronomy outreach for Hofstra College of Liberal Arts and Sciences in Hempstead, New York. So as far as he is concerned, Pluto — the 'dwarf planet' — is still a planet.

Follow the debate on our reports/internationalastronomical_union/">newsblog .