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.
Last week, astronomers discussed the current definition at the Great Planet Debate, a meeting at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. 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…
