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What is a planet?

Oct 03, 2014

Academics are arguing over whether Pluto counts as a ‘proper’ planet or not.

Stuck out on the cold edges of the solar system, Pluto is now at the centre of a heated debate. Although it was long accepted that there were nine planets orbiting the sun, in 2006 the IAU (International Astronomical Union) reduced that to eight after downgrading Pluto to the status of ‘minor planet’, saying it didn’t fulfil their definition of a planet, which is:

  • Must orbit the sun
  • Must be a sphere
  • Must be the biggest thing in its orbit

Pluto lost out because other bodies around it were bigger, but not everyone agreed with the definition, and some said that a ‘minor planet’ was still a planet in the same way a dwarf hamster is still a hamster. Now scientist Dimitar Sasselov has proposed a new definition, saying that a planet is the smallest spherical lump of matter that formed around stars or stellar remnants’, which would make Pluto a ‘proper’ planet again. He seems to be winning the argument for now, after a vote over the two definitions taken during a public debate at Harvard came out in his – and Pluto’s – favour.

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