Mike Brown is an unlikely destroyer of worlds, but then again it was never his intention to kill off an entire planet. It was an accident. His original plan was to discover a new one, and he nearly did. However, scientific logic intervened and the astronomy professor who bears more of a resemblance to Bill Gates than he does to Darth Vader will go down in history as the man who killed Pluto.

Brown, who was once described by Time magazine as potentially "the most successful planet hunter in the history of the Solar System," did not need a Death Star for his act of cosmic annihilation; a relatively simple telescope sufficed. With it he redrew our understanding of our star system, demoting Pluto from its exalted planetary status to that of a lowly "dwarf planet". It happened four years ago, but he still receives hate mail from "Pluto-huggers".

Pluto's undoing began when Brown embarked on a survey of the outer reaches of the Solar System, in and beyond an area called the Kuiper Belt which stretches past the orbit of Neptune and consists of ice and rock bodies preserved from the time of the Sun's creation. His obsessive quest uncovered several large objects of which one, Eris, appeared larger than Pluto and presented the international astronomical community with a dilemma; was it a planet? If not, it would have to follow that neither was Pluto.

The resulting debate about what constitutes a planet and the final decision to downgrade Pluto caused a firestorm of controversy, most of which was directed at Brown who argued strongly against the case for Pluto and consequently his own status as a space-age Christopher Columbus.

"I knew classing Eris as a planet was a very bad idea," explains Brown. "The word planet is not trivial; it should be reserved for the most important objects in our Solar System. I discovered a lot of small things out past Neptune, they are scientifically very interesting but if I found 10 objects similar to Eris, their combined mass would not add up to that of the biggest moon of Neptune. It would have been wrong to class Eris as a planet.

"There was a cartoon view of the Solar System where Pluto is only a little bit smaller than the earth and Jupiter is only a little bit larger than the Earth. In the view that a lot of people grew up with, Pluto is a prominent part of the solar system, but that is not the actual Solar System."

In truth, Pluto's planetary credentials were in doubt long before Brown's discoveries. Discovered in 1930 by American astronomer Clyde Tom- baugh, it was initially believed to be as big as the Earth, but is now known to be smaller than the moon. In 2000 its battle for survival began in earnest when Neil deGrasse Tyson, an influential astrophysicist at the American Museum of Natural History in New York, designed an exhibit which omitted it from the Solar System.

He explains: "In the mid-90s researchers were discovering new objects in the outer solar system, objects that were small like Pluto, icy like Pluto, with elongated orbits like Pluto, and we thought to ourselves, maybe it's not that Pluto was the ninth planet. Maybe Pluto was the first object of a new class of objects that populates this outer zone in the Solar System."

Brown started his search in 1998 and was convinced by the data coming from observations of the Kuiper Belt that there must be many more objects in the area and that some of them were potentially larger than Pluto. Although drawn to the possibility of discovering what would have been the tenth planet (he defines planet-size as an object with the dimensions of Mars), there was a deeper scientific reason for the survey.

"We were looking at the very outer edge of the solar system and that region has the best preserved parts of the solar system left. The objects out there haven't been squashed together, they haven't become hot, they can tell us so much about the earliest conditions in the Solar System" he says.

Using cutting-edge technology such as robotic telescopes and automatic data reduction, Brown and his team at the California Institute of Technology began examining the entire sky for tiny specks of light millions of miles away, a task he likens to "meticulous book-keeping". Once objects were found and positions pinpointed Brown used some of the largest telescopes on the planet, as well as the space-based Hubble telescope, for further inspection. His efforts quickly paid off and, during stretches of clear visibility, the team discovered around one object each week. In June 2002, they made the first of several landmark discoveries. Quaroar, as it would later be named, was half the size of Pluto, had its own moon and was the first nail in Pluto's coffin.

The second groundbreaking discovery was made in November 2004 and became known as Sedna, it remains one of the most important astronomical discoveries of recent times, being the first object discovered beyond the Kuiper Belt. It is currently located 90 times further from the sun than the Earth and is three times more remote than Pluto. It takes 12,000 years to orbit the sun and in 200 years will be too far away to observe.

Explains Brown: "If we can find enough objects like Sedna we can reconstruct the earliest history of the birth of the sun; how and when it was formed, if there were other stars nearby and what their effect on the Sun was. Sedna is basically a fossil record of the birth of the Sun."

Between December 2004 and March 2005 Brown found the other three largest objects in the Kuiper belt, and one of them, Eris (originally nicknamed Xena after the television female warrior), appeared bigger than Pluto. It gave the International Astronomical Union (IAU), mankind's arbiter of planetary order, a dilemma and the body convened in Prague in August 2006 to debate whether Eris and Brown's other discoveries were planets.

"Either they were going to declare me to be the discoverer of more planets than anyone else in human history or not. I felt like I had so much personally at stake that the best thing for me to do was stay as far away from it as possible," recalls Brown, who fled to a remote island off the north-western coast of the US and followed the debate online. However, when the IAU made an initial proposal to accept Eris as the tenth planet, Brown realised he had to act for the sake of accuracy, despite the consequences.

"I knew a decision to demote Pluto would cause a public outcry and the IAU knew demoting Pluto would incur public ire; everybody loves Pluto. They knew the most convenient thing to do would have been to keep things the way they were, but as someone deeply concerned with education and public understanding I couldn't sit by and watch that happen," says Brown.

"I had always believed that it didn't make sense that Pluto was a planet and knew I would be getting hate mail from schoolkids if Pluto did get kicked out the Solar System."

Eventually, on August 25, a rebel faction of astronomers made a counter-proposal to place Eris and Pluto under a new classification; Dwarf Planet. Brown lobbied in support of this alternative view and the IAU reluctantly accepted it. The Solar System was redrawn and text books were revised (Brown maintains publishers were pleased for the extra business). For Brown, the pro-Pluto hate mail continues almost daily. He sighs: "These days the 13-years-olds who were mad back in 2006 have grown up and gone to college and started drinking so I get 3am obscene messages on my office line, which are really quite funny."

Meanwhile his search continues. He maintains there is a possibility of finding a Mars-sized body beyond the Kuiper Belt where Sedna is located.

"If there is we will have to re-define things again, because although it will be part of the debris field, most people will not think twice about calling it a planet. We are looking for just that kind of thing. The goal is not to make scientists argue all over again, but it would sure be fun if we did."

As for his feelings towards Pluto? "It is like the child in the room who will not shut up," he laughs. "There are so many other fascinating things going on out there, maybe if we stopped talking about the planet debate and started talking about science instead, we would discover how interesting the edge of the Solar System really is."

How I Killed Pluto and Why It Had It Coming is published by Spiegel and Grau (£16.99). To order a copy for the special price of £15.29 (free P&P) call Independent Books Direct on 08430 600 030, or visit www.independentbooksdirect.co.uk

The star-makers: astronomers who shaped the Solar System

Copernicus, the Polish astronomer who lived between 1473 and 1543, introduced the heliocentric model that proved all the planets moved in orbits around the sun.

William Herschel was born in Germany in 1738 and lived in England. He built high magnification telescopes and discovered the planet Uranus in 1791.

Johann Gallle discovered Neptune in 1846 working from mathematical calculations by French astronomer Joseph Leverrier and English astronomer John Couch Adams.