The dullest debate in science

“I have found your Planet X” said Clyde Tombaugh to Vesto Slipher on 18 February 1930. Tombaugh was a humble observer, while Slipher was a top astronomer, and Tombaugh's boss at the Lowell Observatory in Arizona. But finding Pluto put Tombaugh into the record books and ensured that he is probably better known today than Slipher.

Back then, things were simple. You just needed to give the new planet a name from classical mythology, and announce that the solar system had grown a little. But in the twenty-first century, we have realized that Pluto is only one of the objects that make up the outer solar system. More tellingly for some, it is not even the biggest. The romantically named 2003 UB313 is about 2400km in diameter, compared to 2320km for Pluto, which is dramatically smaller than everyone thought in 1930.

These objects – primitive remnants of the early solar system – are the biggest known members of the Kuiper Belt, a zone beyond Neptune that may contain thousands of objects over 100km in diameter.

Our increasing knowledge of the outer solar system has led to a lengthy debate about whether Pluto deserves its planet status. Finally, in 2006, the International Astronomical Union decided that Pluto should be demoted to the status of “dwarf planet” along with UB313, now called Eris, Ceres, the biggest of the asteroids, and Charon, Pluto's own satellite. Perhaps this long-running discussion can now end.