Whew! So Pluto stays. The International Astronomical Union (IAU) meeting in Prague has drafted a new set of rules that officially recognise Pluto as a planet. Once ratified, the unseemly controversy swirling around Pluto — whether it’s big enough to be a planet — will be quelled. Curiously, the IAU’s new definition of a planet — as any object orbiting a star, pulled by its own gravity into a ball shape, and not a satellite of another planet — means the addition of three more names to the roster. Ceres (so far a lowly asteroid) apparently makes the grade along with Charon, Pluto’s large moon, and the prosaically-named UB313, an iceball slightly bigger than Pluto and farther from the Sun.
Since its discovery in 1930, Pluto has been something of an oddity in the outer solar system. It looked even more out of place following the discovery of swarms of ‘ice dwarfs’ that graze in the Kuiper Belt, billions of miles from the Sun. Hence suggestions that Pluto could be just another Kuiper Belt object, or KBO.
Disappointing generations of schoolchildren apart, this was always a question of semantics. For Pluto is big enough for gravity to give it a round shape like any planet (unlike KBOs that tend to be misshapen), and it revolves around the Sun like other planets. The question should have been what Pluto is like, not what it is. Now that the silly debate is buried, astronomers’ time would be better spent peering out the unique window that Pluto and KBOs provide to ‘see’ 4.5 billion years back in time when planets formed in the infant solar system.