Two years after the controversial decision to strip Pluto of its status as a planet, astronomers are still unable to agree on how to define "planet". And the task is growing more difficult as scientists grapple to account for the increasing number of weird worlds found circling other stars.rnLast week, astronomers discussed the current definition at the Great Planet Debate, a meeting at the Johns Hopkins University Applied Physics Laboratory in Laurel,rnMaryland. It states that a planet must be in orbit around the sun, have enough gravity to make itself round and, crucially, must have "cleared the neighbourhood around its orbit" of other objects. Since Pluto is just one of many small icy objects in the outer solar system's Kuiper belt, it does not meet these requirements.rnThis definition was adopted by the International Astronomical Union in 2006 but does not enjoy full support. Scientists are largely divided among those who favour the definition's emphasis on an object clearing its neighbourhood and those who argue anything massive enough to be round should quality, including Pluto.
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