Far from being unique, Pluto is one of many large Kuiper Belt objects (KBOs).

Credit: NASA

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For thousands of years, the field of astronomy focused on making measurements and catalogues, often painstakingly built up over years or decades. But the modern astronomer's emphasis is now on understanding, rather than just measuring. How are stars born, why do they shine, and how do they die? How old is the universe, what is it made of, and what is making it expand? It's the need to answer these questions, not just to make pretty pictures of the sky, that motivates my colleagues and me.

But recently the focus of world astronomy has been back on measuring and labelling. This past week in Prague, thousands of astronomers gathered together for the general assembly of the International Astronomical Union. And amidst the debates about new scientific theories and the announcements of exciting discoveries, astronomers decided to change the number of planets in the Solar System.

This came about when a resolution passed that a planet should hereon be defined as a spherical object that orbits a star, and which has cleared the neighbourhood around it. The number of planets in the Solar System consequently drops from nine to eight; Pluto has now been demoted, and joins a variety of other small objects, such as Ceres, Sedna and Orcus, in a new yet-to-be-named category of 'dwarf planets'.

Pluto as seen by the Hubble Space Telescope. Image: NASA/Space Telescope Science Institute

The new definition of a planet is simple and sensible. A celestial body can only be round if it is large enough for its gravity to pull it into a spherical shape. This excludes the vast majority of Solar System bodies, most of which are small misshapen asteroids. The requirement that a planet orbit a star also makes sense, since some of the moons of Jupiter and Saturn are otherwise big enough to qualify as planets. Finally, the key criterion that has demoted Pluto, that a planet must be able to clear out its neighbourhood, is based on our understanding that a newly forming planet around a young star clears out a large area around it as it sweeps up surrounding debris. While the first eight planets all rule their orbits, Pluto follows an unusual elongated path that crosses the orbit of Neptune. Clearly it does not make the cut.

Many people feel very passionate about Pluto's status as a planet, and will be disappointed or disbelieving that astronomers have voted to change this. But Pluto's status as a planet was only ever a historical accident. For decades, astronomers had been convinced that the orbits of Uranus and Neptune hinted at the gravitational influence of a massive, more distant body, "Planet X". When Clyde Tombaugh discovered Pluto in 1930, this exciting new object was quickly anointed as the ninth planet, before much was even known about it. But subsequent study showed that Pluto is puny, barely a quarter the size of our own Moon. And we now know that Planet X never existed, the product of some subtle errors in some century-old mathematical calculations.

So Pluto was only a planet because it was in the right place at the right time. Demoting this tiny lump of rock was a good decision. I encourage all Pluto-lovers to have a look at Pluto for themselves. Even through the largest telescope, it is barely indistinguishable from the stars around it. Compare this to the spectacular sight of Jupiter's moons and cloud bands, or the majesty of Saturn's rings. When one factors in the presence of over 200 planets now known around other stars, all with their own weather systems, moons, rings and other features waiting to be studied and understood, it's clear that it was time for little Pluto to step aside.

Bryan Gaensler is an astronomer and Federation Fellow at the University of Sydney, and a member of the Editorial Advisory Board of Cosmos.