Pluto is no longer a planet- official!
The decision was made at the General Assembly of the International Astronomical Union (IAU) which on 24 August 2006 decided that there are just 8 planets in our solar system: Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Period.
Pluto's status had been contested for many years on account of its size.The call for an official definition of the word "planet" gained ground after the discovery of a distant object beyond Pluto's orbit called 2003 UB313, nicknamed Xena. It is slightly bigger than Pluto and thus could lay claim to being a planet.
The scientists agreed that for a celestial body to qualify as a planet:
Pluto was automatically disqualified because its highly elliptical orbit overlaps with that of Neptune. It will now join a new category of "dwarf" planets ( to be distinguished from "planets" as such and " small solar-system bodies" for everything else, except satellites).In addition Pluto will be the prototype for a new class of TNO ( Trans Neptunian Object), though there was no agreement about their name.
The meeting was chaired by former RAS President, Jocelyn Bell Burnell ( pictured holding Walt Disney's eponymous dog). The current President, Michael Rowan Robinson said that astronomers would look stupid if the meeting concluded without reaching agreement about the definition of a planet. Mike Brown, who discovered UB313, said he was happy with the outcome. Some others were not and vowed to renew efforts to have Pluto 'reinstated'. This will please the 7 year old girl who wrote to the RAS, sad at what had been done to her favourite planet! See Anoushka.jpg
The decision at the conference was a dramatic shift from just a week ago, when the Union’s leaders floated a proposal that would have reaffirmed Pluto’s planetary status and made planets of its largest moon and two other objects. But that plan proved unpopular, splitting astronomers into factions that spent days locked in sometimes combative debate.
Coincidentally, the decision to eliminate the 9th planet came a day before the anniverary of the death of Sir William Herschel (in 1822), the first President of the RAS and the discover of Uranus, the 7th planet.
Footnote: Holst wrote his 'Planet Suite' during the First World War, before the discovery of Pluto. He declined to add an extra movement to accommodate the new 'planet'. How prescient!