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The Demotion of Pluto

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An Artist's impression of the planet Pluto.

The 9th planet of our Solar System was discovered in 1930 by American astronomer Clyde Tombaugh (1906 - 97). The new planet was eventually named after Pluto, the Roman god of the underworld, although children the world over came to know Pluto as Mickey Mouse's canine companion. Pluto measures just 2,360km (1,467 miles) in diameter, making it smaller than some of the known moons1 in the Solar System. Orbiting the Sun at an average distance of 5.9 billion km (3.7 billion miles), Pluto takes 247.9 (Earth) years to complete one Plutonian year. Pluto's path around the Sun is highly elliptical, taking it within the orbit of Neptune. For 76 years, Pluto enjoyed its status as a bona fide planet on a par with Earth, Jupiter and the rest. However, during this time there were some astronomers who felt that Pluto did not deserve the title 'planet' and soon telescope technology improved to such a degree that other comparably sized objects were being discovered.

For 72 years, Pluto was the most distant object known to be a member of the Solar System2. Then in 2002, Quaoar was discovered and took this position until, a year later, the Hubble Space Telescope (HST) detected a Pluto-sized body named UB 3133 at a distance of 98 astronomical units, moving in an orbit even more eccentric4 than Pluto's. This created a dilemma; the astronomy community couldn't agree to give UB 313 planetary status because this action would open the door for later discoveries to attain the title, and the Solar System as we knew it would be in a constant state of flux. But what was to be done about the planet Pluto?

Definition of a Planet

At the time there was no authoritative definition for a planet. This meant that the ruling body in astronomy, the International Astronomical Union5 (the IAU), was given the task of coming up with the criteria required for planetary status.

They eventually decided that the celestial body must:

  • be in orbit around the Sun
  • have enough mass to be round in shape
  • have cleared the neighbourhood around its orbit6.

According to this definition, Pluto did not qualify for planetary status due to the fact that Neptune is in its path (but then the same can be said of Neptune having Pluto in its path).

However, Pluto's transition from being a planet was nowhere near clear-cut. A new category called 'dwarf planet' was proposed by the IAU to include Pluto and UB 313. At a meeting in the Czech Republic capital of Prague, over 400 astronomers, presided over by IAU member Dame Jocelyn Bell-Burnell7, debated Pluto's fate. Dame Bell-Burnell was photographed clutching a toy Pluto (Mickey Mouse's doggie friend) whilst at the event. The final vote on 24 August, 2006, where a multitude of yellow voting papers were held aloft, passed the motion to demote Pluto to dwarf planet status.

I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be.
– IAU member Professor Iwan Williams

Before the ink was dry on the paperwork though, the news travelled the world and people reacted accordingly. Adults with only a meagre interest in astronomy had been brought up in a Solar System consisting of nine planets, and some didn't take kindly to losing the most remote one. Children the world over mourned the loss of a beloved old familiar friend. Protests began to pour in and one of the co-discoverers of UB 313 even complained of receiving hate mail! In 2008 the IAU ruled that dwarf planets which orbit further than Neptune should be called 'plutoids' in Pluto's honour. This hasn't appeased Pluto's supporters, though. At the time of writing there are still protests in favour of reclassifying Pluto, including online petitions. Members of Clyde Tombaugh's family would also like to see Pluto reinstated. Although new astronomy textbooks do not include it as a planet, Pluto is still an object of interest. Further moons, P4 and P5, were discovered by the HST in 2011 and 2012. These were subsequently named Kerberos and Styx. The Pluto system is expecting a visit from NASA's New Horizons spacecraft sometime in July 2015.


Pluto was not the first planet to get demoted. Ceres (974 km in diameter) was the first asteroid to be detected, and it was deemed a planet for 50 years. By the end of the 19th Century, several hundred other such bodies had been discovered8 and the new class 'asteroids' was invented for them. When the new class of dwarf planet was coined in 2006, Ceres was upgraded to that status.

1Ganymede, one of the Galilean moons of Jupiter, is actually larger than the planet Mercury!2If we exclude comets, and consider Pluto's average distance from the Sun.3UB 313 was eventually given the name Eris after the Roman goddess of discord, an appropriate moniker for the object which had caused the debate about Pluto to come to a head.4Elliptical.5They decide on the names of asteroids, comets and other newly-discovered celestial bodies. It was this group which redefined the constellation borders in 1922.6This is being contested by Planet Pluto supporters who have correctly flagged up that the Earth, Mars, Jupiter and Neptune have not cleared their own orbital paths, and they are planets, right?7Herself no stranger to controversy, but that's another story.8Read more about the discovery of the Asteroid Belt.

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