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Science - Poor old Pluto

Turn the planet's demotion into a lesson that's out of this world

Turn the planet's demotion into a lesson that's out of this world

In 1633, Galileo was forced to recant his view that the sun was at the centre of the universe. Pope Urban VIII had threatened him with being burned alive for heresy. Once he recanted, his punishment was reduced to life imprisonment under house arrest and his book, supporting a heliocentric solar system, was banned. In 1992, the Catholic Church apologised through Pope John Paul II, saying that a charge of heresy was baseless.

Controversy in science today may not result in being burned alive at the stake, but a feud broke out in astronomy in 2006 when Pluto was demoted from being a planet. Countless textbooks describing the "nine planets in our solar system" are now incorrect and astronomers still argue about Pluto's demotion.

The planet was discovered in 1930 and a young girl from Oxford, Venetia Burney, suggested it should be called Pluto after the Roman god of the underworld. With a highly elliptical orbit, Pluto's distance from Earth ranges from 4.4 billion to 7.4 billion kilometres and it is just one-fifth the size of Earth. It muddled along for more than 60 years as a small planet until other bodies of a similar size were discovered. The International Astronomical Union (IAU) then had a problem: do we keep adding to the number of planets, or should we redefine what a planet is?

In 2005, the discovery of Eris crystallised the debate. At the 2006 IAU conference in Prague, Pluto was relegated to the status of a dwarf planet - or "plutoid". Apart from the sun, three categories of astronomical bodies were decided upon: planets, the eight worlds from Mercury to Neptune; dwarf planets, Pluto and any other round object that "has not cleared the neighbourhood around its orbit, and is not a satellite"; and small solar system bodies, all other objects orbiting the sun.

It could have been worse for poor old Pluto: two of the names suggested for it honoured the founder of the Lowell observatory or his wife, so it could have been called Percival or Constance.

James Williams is a lecturer in science education at the University of Sussex's School of Education and Social Work

What else?

Try hannahthomson2002's colourful factsheets about the eight planets and Pluto.

Have an out-of-this-world time with pez555's solar system top trumps.

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