HOW many planets are in the solar system? The official answer is eight – unless you happen to live in Illinois. Earlier this year, defiant Illinois state governors declared that Pluto had been unfairly demoted by the International Astronomical Union, the authority that sets the rules on all matters planetary.
Three years ago, the IAU decided to draw up the first scientific definition of the term planet. After days of stormy arguments at its general assembly in Prague, the delegates voted for a definition that excluded Pluto, downgrading it to the new category of dwarf planet.
The decision caused outrage among many members of the public who had grown up with nine planets, and among some astronomers who pointed out that only 4 per cent of the IAU’s 10,000 members took part in the vote. The governors of Illinois saw the decision as a snub to Pluto’s discoverer, Clyde Tombaugh, who was born in the state.
Next week the IAU’s general assembly will convene for the first time since Pluto was axed from the list of planets. Surprisingly, IAU chief Karel van der Hucht does not expect anyone to challenge the ruling made in Prague, but Pluto fans can take heart: resistance remains strong.
If Pluto is reinstated, it will probably be thanks to discovery rather than debate. Mark Sykes of the Planetary Science Institute in Tucson, Arizona, believes that revelations within and beyond our solar system over the coming years will make the IAU’s controversial definition of a planet untenable (see diagram). “We are in the midst of a conceptual revolution,” he says. “We are shaking off the last vestiges of the mythological view of planets as special objects in the sky – and the idea that there has to be a small number of them because they’re special.”