The Pluto Files by Neil deGrasse Tyson Norton HK$192 When is a planet not a planet? When it's Pluto, that's when. Pluto was discovered in 1930 but was stripped of its planetary status in 2006 when astronomers decreed it should be classed as one of many objects in the icy Kuiper Belt. The decision was controversial within the science world and outside it. Many scientists opposed the new classification and the debate inspired thousands of children to write letters in support of Pluto's claim to planethood. The Pluto Files: The Rise and Fall of America's Favorite Planet is a light-hearted but factual account of the ensuing hoopla by Neil deGrasse Tyson, one of the figures at the centre of the fuss. It is idiosyncratic in style and self-publicising but it succeeds in compressing the science into a witty and digestible read. Pluto, formerly the ninth and outermost planet in our solar system, was discovered by Clyde Tombaugh in 1930. It was the first planet to be discovered by an American and it caught the public imagination. It influenced the naming of entities as diverse as Walt Disney's famous cartoon dog and plutonium. Pluto's future seemed secure. Until astrophysicist Tyson, the director of New York's Hayden Planetarium, thought otherwise. Drawing on scientific research, Tyson subtly omitted Pluto from an exhibit when the planetarium opened its Rose Centre for Earth and Space in 2000. This quiet act made its way to the front pages of the world's newspapers. Tyson's only crime was to heed current scientific thought. But the public didn't like their faith in the solar system rocked. So they unleashed a stream of letters protesting against the planet's absence from the display. This brought the Pluto issue to a head. Many scientists fought against its demotion, claiming it would be better for astronomy if the public were allowed to keep their favourite planet. But the International Astronomical Union (IAU) finally voted it out of planethood in 2006. So what caused Pluto's demise? Proof of the Kuiper Belt's existence in 1992 was the culprit. The Kuiper Belt is an enormous disc-shaped collection of icy debris that circles the solar system beyond the orbit of Neptune. It mainly consists of frozen remnants of the solar system's formation. Pluto's icy composition, small size and unusual orbit led astronomers to decide it would more accurately be classified as a Kuiper Belt Object (KBO) than a planet. It has more properties in common with the KBOs than with planets in the solar system. And if Pluto was defined as a planet, it would mean big chunks of rock such as Eris would have to be granted planethood too. The decision to declassify was not taken quickly because no one had ever defined what a planet actually was. In 2006 the IAU finally came up with criteria to distinguish a planet from anything else. To quote the book, a planet is an object that 'is in an orbit around a star, but not in an orbit around another planet; is large enough for its own force of gravity to shape it into a sphere, but not so large it becomes a sun; and has cleared its orbit of wayward debris'. The third criterion means that a planet cannot be found floating around in a cloud of rocky or icy debris. Pluto does not pass the last test because it is surrounded by space debris. So astronomers demoted it to 'dwarf planet' status, which means it isn't a planet at all. As Tyson points out, the whole fuss was about classification - how we label things. Labelling is important, he says, because it is a way for us to know what is connected to what. But he also adds that celestial bodies such as Pluto will go on unperturbed by whether we humans call them planets, KBOs or jellybeans.