The decision by astronomers to strip Pluto of its status as a planet has been sharply criticised by the lead scientist on Nasa's robotic mission to the former planet. On Thursday, experts at the International Astronomical Union's (IAU) 10-day General Assembly in Prague approved a new definition of planets.
Dr Alan Stern, who leads the US space agency's New Horizons mission to Pluto and did not vote in Prague, told BBC News the decision was "embarassing": "It's an awful definition; it's sloppy science and it would never pass peer review ... Firstly, it is impossible and contrived to put a dividing line between dwarf planets and planets. It's as if we declared people not people for some arbitrary reason, like 'they tend to live in groups'.
And the chair of the committee set up to oversee agreement on a definition implied that the vote had effectively been "hijacked".
The meeting had unceremoniously removed Pluto's 76-year status as a planet, demoting it to a new category of astronomical body, a "dwarf planet".
"This issue produced one of the most-heated and dynamic debates I have ever seen at an IAU meeting before," stated official Irish delegate to the meeting Dr Paul Callanan.
"It wasn't so much an argument about Pluto per se, it was about the definition of what a planet was and wasn't," said Dr Callanan, a lecturer at University College Cork. "The public was much more interested in whether Pluto remained a planet."
The whole question about unfortunate Pluto only arose after the discovery of several planet-like bodies out beyond the planet Neptune. The first was Charon, about the same size as Pluto and in orbit around it.
The real crunch came, however, with the discovery in 2003 of a new body much further out.
Officially named UB313 and nicknamed Xena (as the princess warrior of TV fame), this new body was twice the size of Pluto, yet astronomers were slow to allow it the full status of a planet. If UB313 wasn't a planet, then how could diminutive Pluto retain its status as one?
The IAU sought to answer the question for the first time by setting up a committee to adjudicate on the fate of Pluto and other small solar system bodies.
Earlier this month, the committee recommended that Pluto retain its planetary tag, but the full IAU meeting in Prague rejected this view after a two-day debate, said Dr Callanan.
The exchanges were particularly fierce on Tuesday, he said. "There was a lot of heated debate and discussion because the majority of delegates could see the committee's resolution was vague."
Two critical meetings chaired by the Irish scientist who discovered pulsing stars or "pulsars" and narrowly missed receiving a Nobel Prize, Jocelyn Bell Burnell, took the heat out of the debate. "She played a blinder," Dr Callanan said. Her words paved the way for yesterday's agreement.
"We had two options. One was to increase the number of planets in the solar system by allowing Pluto to remain and including similar bodies. Or tightening up the classification we use to define a planet and effectively demoting Pluto to a dwarf planet."
