An asteroid masquerading as a planet

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My Very Educated Mother Just Served Us Nine Pizzas -the mnemonic you learned at school to list the order of the nine planets (outwards from the Sun) may soon be without Pizza for Pluto. Your very angry mother would be serving nine nothings.

Pluto isn't rocky like Mercury, Venus, Earth and Mars, nor is it a gas giant like Jupiter, Saturn, Uranus and Neptune. It is a relatively tiny ball of ice; with a diameter of 2,274 kilometres it is smaller than the Moon. Pluto's orbit is also peculiar. The other outer planets orbit the Sun in roughly circular orbits, but Pluto's orbit is elliptical, which at times brings it closer to the Sun than Neptune. The oddball Pluto is not really a planet - that's what some astronomers have been arguing for years. They say it's the largest object in the Kuiper belt, the outpost of icy asteroids beyond the orbit of Neptune.

The existence of such a belt was first suggested in the early 1950s but the first Kuiper belt object, called 1992 QB1, was found in 1992. This small icy body, similar in size to an asteroid, suggested that there might be more than just Pluto in the distant reaches of the solar system. Since then hundreds of objects like QB1 have been found in the Kuiper belt. Their diameters range from 50 to almost 1,200 kilometres. Though these objects are smaller than Pluto, they are thought to be similar to Pluto in composition.

The Kuiper belt is believed to contain about 100,000 objects larger than 100 kilometres across. This swarm of Pluto-like objects gives weight to some astronomers' argument that Pluto is not a planet but a very large ball of ice. Even those astronomers who defend Pluto's status as a planet agree that were it discovered today it probably wouldn't be called a planet.

The discovery in 2004 of the most distant planet-like object in the solar system made the Pluto debate more interesting. Named Sedna after the Inuit ocean goddess, the new object is three-quarters the size of Pluto and way outside the Kuiper belt. The discovery will now hot up the debate about whether Pluto is the puniest planet or the king of the Kuiper belt. This debate has a historical precedent. More than two centuries ago when Ceres was discovered, it was also proclaimed a planet.

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