IF SOME astronomers have their way, Earth will soon be called a 鈥渢errestrial planet鈥� and Pluto a 鈥渉istoric planet鈥�.
Eighteen months ago, a working group convened by the International Astronomical Union (IAU) started looking for a new way of defining a planet, after a large body called Sedna was found in the outer solar system. The matter became urgent in July when a so-called 鈥渢enth planet鈥�, bigger than Pluto, was discovered in the Kuiper belt of bodies orbiting beyond Neptune.
鈥淭hey鈥檒l ask why egghead PhDs can鈥檛 tell when an object is a planet if regular people can鈥�
Advertisement
The working group will soon vote on two competing proposals submitted on 23 September. One, based on size, recognises Pluto and other bodies of a similar size as planets. The second argues that a planet should be the dominant body in its neighbourhood. This definition would exclude Pluto, because its orbit crosses that of the more massive Neptune.
In the wings is a third proposal which defines subclasses of planets based on characteristics like location, composition or their place in tradition. Hence terms like 鈥渢errestrial鈥� for Earth and 鈥渉istoric鈥� for Pluto. Brian Marsden, head of the IAU鈥檚 Minor Planet Center in Cambridge, Massachusetts, likes that idea. But Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado, objects: 鈥淚f the working group actually ratifies a statement that says there is no such thing as a planet, the IAU will be the laughing stock of the world,鈥� he says. 鈥淓veryone will ask why egghead PhDs can鈥檛 tell when an object is a planet if regular people can.鈥�
