Sir

The discovery of 2003 UB313, an object larger and farther away than Pluto, has once again stimulated the debate on how we define a planet (Nature 436, 616; 2005). The official status of 2003 UB313 will be decided by the International Astronomical Union (IAU). Meanwhile, in the manuscript for a forthcoming book, I have just changed a section heading from “Eight planets?” to “Ten planets!”. But how should we decide how many planets there are?

I believe that Pluto should remain the ninth planet, as it has been for the past 75 years, and we should enjoy teaching about how this planet differs from the others. Being different is not reason enough to exclude Pluto from the list of planets: in recent years Uranus and Neptune have been found to differ greatly from Jupiter and Saturn, which themselves differ greatly from the terrestrial planets.

I believe that 2003 UB313 should be called the tenth planet, because it is both larger than Pluto and at an appreciably different distance, although a practical problem the IAU will then have to face is where to draw the line at the lower end of sizes. For example, should Sedna or 2003 EL61 — each roughly three-quarters the size of Pluto — also be named planets? I propose that the size of Pluto should be considered the lower limit, for historical reasons.

It is important to remember that Pluto and 2003 UB313 are also ‘Trans-Neptunian objects’ — bodies orbiting the Sun at a greater distance than Neptune — and that these differ from the eight large planets, especially in their origin as small asteroidal aggregates. In order to preserve this distinction, Pluto and 2003 UB313 should also be given asteroid identifications.