WHEN IS a planet not a planet? In August 2006, the International Astronomy Union, meeting in Prague, created consternation when it downgraded Pluto to the status of a dwarf planet.
Until then it enjoyed elite status as one of the nine planets in our solar system.
At a stroke the decision rendered every wall chart and model detailing the planets of the solar system out of date, along with every mnemonic which children used to remember the sequence of the planets such as “my very efficient mother just served us nine pizzas”.
The decision in particular angered Americans, who have a special attachment to Pluto given it was discovered by an American astronomer, Clyde Tombaugh, in 1930.
It also came too late for the Nasa probe New Horizons which was already on its way to Pluto when the decision was announced. It made history last week by becoming the closest spacecraft to Pluto, and is more than halfway to its scheduled rendezvous in July 2015.
The astronomer who facilitated the fateful meeting was Irish-born scientist Dame Jocelyn Bell-Burnell, who last night, in Astronomy Ireland’s Christmas lecture in Trinity College, Dublin, defended the decision taken five years ago.
Dame Jocelyn, who was the head of the UK Institute of Physics until last year, achieved worldwide fame as a young scientist when she helped discover pulsars, a type of rotating neutron star, for which her supervisor received the Nobel Prize for Physics.
She said the Pluto decision was taken because the outer solar system is full of millions of potential objects such as Pluto which will be discovered with bigger and better telescopes. The planet was simply not unique, and was not even a boss in its celestial neighbourhood, one of three main definitions that make a planet.
At the time astronomers were finding many large objects in the outer solar system. “The consensus among astronomers is that something had to be done. It was getting silly. There are a lot of these so-called trans-Neptunian objects. It is absolutely clear that there are millions of them.”
