Astronomers Debate Status of Pluto

Astronomers are proposing that there are now twelve planets in our solar system. At issue is the question when is a so-called minor planet or asteroid big enough to justify being called a planet. The proposal to increase the number of official planets to twelve (with more likely to be confirmed in the near future) is being discussed this week at the 26th triennial General Assembly of the International Astronomical Union (IAU) in Prague, and is the culmination of months of effort amongst specialists to reach agreement on the question.

Pluto has been regarded as the solar system's ninth planet since its discovery in 1930. However, hundreds of other such objects have been discovered in the same trans-Neptunian part of the solar system since the early 1990s, albeit most very much smaller than Pluto. Nevertheless, it has become clear that Pluto is just one of a substantial population of small solar system objects, and this has raised the question whether it should be considered a "planet" at all.

The new discoveries are widely thought to be cometary in nature, but have all been classified as minor planets. They occupy a region of the solar system beyond Neptune which has become known as the Kuiper, or Edgeworth-Kuiper belt, named after the two astronomers in the middle of the twentieth century who first seriously predicted a population of comets in this region. Gerard Kuiper, based in the USA, was one of the leading professional planetary scientists of his day, whereas Kenneth Edgeworth, an independent Irish astronomer, was largely an amateur. Nevertheless, Edgeworth's prediction of a substantial population of trans-Neptunian comets, in a paper published in 1943 and in unpublished work in 1938, predated that of Kuiper by more than a decade.

For a while, at least when Pluto was the largest known trans-Neptunian object, astronomers were content to count Pluto as a planet, and the others as minor planets. However, several such objects have now been shown to be comparable in size to Pluto and some even larger. In particular, the fact that the object designated 2003 UB313 is larger than Pluto dictates either that there are more than nine planets or that Pluto should not count as a planet at all.

Drawing the line between what is and is not a planet has proved problematic. The IAU is proposing that a planet should orbit a star, and be sufficiently massive (and not a star!) to have a nearly spherical form shaped by its own gravity. According to the draft proposal, all objects currently counted as planets, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, would remain planets. But in addition, the first minor planet, namely Ceres, and both 2003 UB313 and Pluto's principal satellite, Charon, would be promoted to full planetary status, making currently twelve "planets" in all.

Although 2003 UB313 is so far away, it is large enough to be seen with a relatively small telescope. It was discovered by a team of Californian astronomers on 5th January 2005 from images taken in 2003, and the discovery was announced on 29th July 2005. Soon after, in August that year, images of the new "planet" were obtained with the Faulkes Telescope in Hawaii, controlled remotely from Armagh by Observatory summer student Elizabeth Connolly. These were the first such observations from the island of Ireland.

FOR FURTHER INFORMATION CONTACT: David Asher or John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; djaarm.ac.uk; jmfarm.ac.uk

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