In 2006, Pluto was voted out of the planetary club by members of the International Astronomical Union
Nasa\’s New Horizons mission made a close pass of Pluto this week. For more than 70 years, Pluto was one of nine planets recognised in our Solar System. But in 2006, it was relegated to the status of dwarf planet by the International Astronomical Union (IAU). So why was Pluto demoted? Where did the controversy start? Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, who was using the Lowell Observatory in Arizona. Textbooks were swiftly updated to list this ninth member in the club. But over subsequent decades, astronomers began to wonder whether Pluto might simply be the first of a population of small, icy bodies beyond the orbit of Neptune. This region would become known as the Kuiper Belt, but it took until 1992 for the first \”resident\” to be discovered. The candidate Kuiper Belt Object (KBO) 1992 QBI was detected by David Jewitt and colleagues using the University of Hawaii\’s 2. 24m telescope at Mauna Kea. How did this change things? Confirmation of the first KBO invigorated the existing debate. And in 2000, the Hayden Planetarium in New York became a focus for controversy when it unveiled an exhibit featuring only eight planets. The planetarium\’s director Neil deGrasse Tyson would later become a vocal figure in public discussions of Pluto\’s status. But it was discoveries of Kuiper Belt Objects with masses roughly comparable to Pluto, such as Quaoar (announced in 2002), Sedna (2003) and Eris (2005), that pushed the issue to a tipping point. Eris, in particular, appeared to be larger than Pluto – giving rise to its informal designation as the Solar System\’s \”tenth planet\”. Prof Mike Brown of the California Institute of Technology (Caltech), who led the team that found Eris, would later style himself as the, while deGrasse Tyson would later jokingly quip that he had.
The finds spurred the International Astronomical Union to set up a committee tasked with defining just what constituted a planet, with the aim of putting a final draft proposal before members at the IAU\’s 2006 General Assembly in Prague. Under a radical early plan, the number of planets, seeing Pluto and its moon Charon recognised as a twin planet, and Ceres and Eris granted entry to the exclusive club. But the idea met with opposition. What happened next? The discussions in Prague during August 2006 were intense, but a new version of a planetary definition gradually took shape. On 24 August, the last day of the assembly, members voted to adopt a new resolution outlining criteria for naming a planet: Pluto met the first two of the these criteria, but the last one proved pivotal. \”Clearing the neighbourhood\” means that the planet has either \”vacuumed up\” or ejected other large objects in its vicinity of space. In other words, it has achieved gravitational dominance. Because Pluto shares its orbital neighbourhood with other icy Kuiper Belt Objects, the resolution effectively stripped the distant world of a planetary designation it had held for some 76 years. It was immediately relegated it to the distinct category of \”dwarf planet\”, alongside the biggest body in the asteroid belt, Ceres, and other large Kuiper Belt Objects such as Eris, Quaoar and Sedna. Commenting at the time, the IAU\’s president of planetary systems science Prof Iwan Williams said: \”By the end of the decade, we would have had 100 planets, and I think people would have said \’. \” Was that the end of the matter?
In a word, no. Some experts immediately questioned the part of the definition about a planet clearing its orbital neighbourhood. This is because Earth shares its cosmic turf with more than 12,000 near-Earth asteroids. Thus, some have argued that Earth, Jupiter and other planets also fail to meet the IAU\’s 2006 definition. Speaking just after the vote, Prof Alan Stern, chief scientist for the New Horizons mission, called the outcome and described the new definition as \”internally inconsistent\”. Prof Owen Gingerich of Harvard, who chaired the planet definition committee, revealed that only 10% of the 2,700 scientists who had attended the 10-day meeting were present at the Pluto vote. The low turn-out has been blamed on timing; the vote was held on the last day of the General Assembly when many participants had left or were preparing to fly out from Prague. The debate has rumbled on ever since, on television, in the pages of books and in public talks. Most recently, Alan Stern challenged Neil deGrasse Tyson to a debate on the matter in 2014. But the latter expert turned down the offer, stating: \”I don\’t have opinions that I require other people to have. \” The flyby of Pluto is unlikely to provide any information relevant to a change in Pluto\’s status. But it will bring into clear focus once more what is, and what isn\’t, meant by the term \”planet\”. Follow Paul. When is a planet not a planet? And what is it about Pluto that has got astronomers fighting? examines the sorry fate of the solar system s outcast. Discovered in 1930 and once known as the ninth planet of the solar system Pluto is the smallest, coldest, and most distant planet from the Sun.
In 1978, American astronomers James Christy and Robert Harrington discovered that Pluto has a satellite, which they named Charon. Charon is almost half the size of Pluto and shares the same orbit. Pluto and Charon are thus essentially a double planet. Two additional moons, Hydra and Nix, were discovered in 2005. Pluto s origin and identity have long puzzled astronomers. In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. However this theory has been heavily criticised because Pluto never actually comes near Neptune. The solar system loses a planet In 1992, astronomers began to discover a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt, is believed to be the source of many comets. Astronomers now believe Pluto to be the largest of the known Kuiper belt objects (KBOs). Like other KBOs, Pluto shares features with comets, like the fact that the solar wind is gradually blowing Pluto s surface into space, in the manner of a comet. The discovery of the Kuiper belt and Pluto s relation to it led many to question whether Pluto could be considered separately from others in its population. In short was Pluto really a planet? Moving the goalposts One of the criteria for being classed as a planet is that an object must have cleared the neighbourhood around its orbit. The Earth s mass is 1. 7 million times the mass of the other objects in its orbit. Unfortunately, Pluto s mass is only 0. 07 times the mass of other orbiting objects. Because of this, on September 13th 2006 Pluto was officially downgraded to a dwarf planet.