“The last full planet to be discovered – in 1930 by US astronomer Clyde Tombaugh – was Pluto. But recently some astronomers have campaigned to have Pluto downgraded to ‘minor planet’ status. It is so small – its diameter is a mere 2,200 kilometres – that it is unworthy of the status of full planet, it was argued. This bid was finally rejected after heated scientific debate.
But now the discovery of Xena, which is only slightly bigger than Pluto, will re-ignite that row. Both Pluto and Xena are components of the Kuiper Belt, which is made up of thousands of small asteroid-like objects, many mere lumps of rock, that sweep the outermost depths of the solar system. As members of the Kuiper Belt, neither Pluto nor Xena should be rated full-fledged planets, it is argued.
The trouble for astronomers is that they do not have an exact definition of a planet. Many say that, if Pluto had been discovered today, it would not have been called a proper planet. “