What I want to know is now that Pluto isn’t a planet what does My Very Engergetic Mother Just Sell Us Nine of?
By Nigel Hawkes
Pitythe late Clyde Tombaugh. Until yesterday he was the last man to have discovered a new planet.
It is 76 years ago since he found Pluto. But at a meeting of the International Astronomical Union in Prague, the assembled experts decided that it was not a planet after all. The ruling leaves the solar system with eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Pluto is relegated to a new class, called dwarf planets, that have little to distinguish them from minor planets, also known as asteroids. So instead of being the smallest, oddest and remotest of the Sun’s planets, it becomes the prototype for a class that is expected to grow rapidly as many dwarf planets are considered for admission. A dozen more are listed on the union’s watchlist.
The first three in this new class are Pluto, Ceres, and 2003 UB313, an icy object that is slightly larger than Pluto and which has been nicknamed Xena by its discoverer, Michael Brown of the California Institute of Technology.
The decision at the conference, attended by 2,500 astronomers from 75 countries, was a dramatic shift from just a week ago, when the union’s leaders floated a proposal that would have reaffirmed Pluto’s planetary status and made planets of its largest moon and two other objects.
But that plan proved unpopular, splitting astronomers into factions that spent days locked in sometimes combative debate that led to Pluto’s undoing.
The die was cast by a vote that for the first time defined what a planet is: “A celestial body in orbit around the Sun that has sufficient gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium [nearly round] shape and has cleared the neighbourhood around its orbit.”
Pluto fails to meet this new definition. Its orbit, far from cleared — which means that the object carries the gravitational clout to draw in rocks and other debris that otherwise clutter its path — overlaps that of Neptune.
That makes it a dwarf planet, according to another definition carried by a vote in Prague.
Pluto’s status had been contested for many years by astronomers, who said that its tiny size and eccentric orbit precluded it from being counted among the other acknowledged planets.
The anti-Pluto movement gained ground after the discovery of 2003 UB313, which lies beyond Pluto’s orbit and is as big. If Pluto was a planet, UB313 could also lay claim to be one.
Pluto, named after the god of the underworld in classical mythology, was discovered on February 18, 1930, by Clyde Tombaugh, who was then 24. He had been given a job at Lowell Observatory in Flagstaff, Arizona, and charged with finding the ninth planet.
It orbits the Sun at an average distance of 3,670,050,000 miles, taking 247.9 years to complete a single circuit. Its orbital plane is 17 degrees off the Ecliptic — the horizontal plane taken by the eight true planets. In addition, its path is so elliptical that, for 20 years of its orbit, it ventures inside the orbit of Neptune.
Some astronomers were disappointed by yesterday’s vote. Professor Richard Binzel, of the Massachusetts Institute of Technology, who is a member of the union’s planet definition committee, had contended that Pluto met key tests of planetary physics “by a long shot”.
He consoled himself by saying that many more Plutos remained to be discovered. Only don’t call them planets.