2006: a space oddity – the great Pluto debate

Long known as the ninth planet, Pluto was downgraded in 2006, sparking a scientific spat that raises basic questions about how we understand the universe

Imagine that you have nurtured an ambition for 25 years to head up an expedition to the last unexplored planet in the solar system. You've worked your way up and through countless other suborbital, orbital and planetary missions. You've written scores of scientific papers. Finally, you are the principal investigator on Nasa's New Horizons mission to Pluto - that mysterious little entity, a third of the size of our moon, that is located, depending on orbital position, between 2.6bn and 4.7bn miles from Earth.

In January 2006, your probe leaves Earth on its nine-year journey to its historic destination. It is the crowning moment of your career, a landmark project; you've reached the peak of your profession. Then, seven months later, with your spacecraft still in the early stages of its odyssey, it is announced, following a vote at a meeting of the International Astronomy Union (IAU) in Prague, that Pluto is no longer a planet.