A long-standing fact beloved of school-children across the world is about to change. Ever since American astronomer Clyde Tombaugh discovered Pluto in 1930, it has been commonly understood that the Solar System contains 9 planets. In order of distance from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (although Pluto occasionally goes nearer to the Sun than Neptune due to its eccentric orbit). However, that list is about to grow. On August 16, the International Astronomical Union (IAU) announced that it is planning to add three new members to the exclusive club of large celestial objects orbiting our Sun.
The additions come after the organisation concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the new definition is approved by the astronomers gathered at the triennial IAU General Assembly in Prague (14-25 August 2006), the Solar System will include 12 planets, with more to come. These will be the 8 closest planets to the Sun, the largest asteroid (Ceres) and three remote ‘plutons’. This new category contains Pluto itself, its erstwhile moon and now double planet Charon and the provisionally sexy-named 2003 UB313. Plutons are distinguished from classical planets in that they reside in highly tilted orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). The draft "Planet Definition" Resolution will be discussed and refined during the General Assembly and then it will be presented for voting on 24 August.
The word "planet" comes from the Greek word for "wanderer”. There is no formal definition for the term but it is generally considered to be a relatively large mass of accreted matter in orbit around a star. Astronomers also drew a distinction between planets, and large asteroids (sometimes confusing called minor planets). With a diameter of almost a thousand kilometres, Ceres, discovered by an Italian monk in 1801, is by far the largest of the asteroids. For the next fifty years it was classified as a planet but once its true size was known, it was reclassified as an asteroid. Ceres has remained an asteroid for the last 150 years.
But the meaning of the word planet has come under increasing scrutiny due to recent discoveries. Pluto was initially added to the list of 8 planets because it was believed to be as big as Earth. Later its diameter was measured to be just 18% of Earth’s. Pluto is smaller than the Moon which is 25% of Earth’s diameter. From the 1990s onwards, astronomers became aware of a vast population of small bodies orbiting the sun beyond Neptune. But the trouble really started with the discovery of 2003 UB313. As the name would suggest it was discovered in 2003 by a team at Mt Palomar observatory in San Diego. Dubbed “Xena” by its discoverers, it is currently classified as a “Scattered Disk Object” but because it is slightly larger than Pluto and even has its own moon, many have argued that it too should be given planet status. Xena is three times further away from the Sun as Pluto.
The discovery drastically heated up the debate over how to define a planet. Some astronomers claim Pluto is just an overgrown Kuiper-belt object and there are really only eight planets. However, if the new definition is accepted, then the list will rise quickly from 12. Currently a dozen other "candidate planets" are listed on IAU's "watchlist" which keeps changing as new objects are found and the physics of the existing candidates becomes better known. The number could rapidly accelerate to the thousands as objects in the Kuiper belt are identified by prospective planet-finders. This should make an interesting dilemma for those attempting to stay within the current naming convention: all planets must be named after Roman gods.
The days of an easily memorised and numbered Solar System are itself numbered.