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 鈥減lanet鈥�. 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 鈥渃leared the neighbourhood around its orbit鈥� of other objects. Since Pluto is just one of many small icy objects in the outer solar system鈥檚 Kuiper belt, it does not meet these requirements.
鈥淪ince Pluto is just one of many small icy objects in the Kuiper belt, it does not meet the requirements to be a planet鈥�
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This 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鈥檚 emphasis on an object clearing its neighbourhood and those who argue anything massive enough to be round should qualify, including Pluto.
What astronomers do agree on, however, is that the definition needs to account for objects orbiting other stars, something the current one fails to do, given its requirement that a planet must orbit our sun.
Accounting for the growing motley crew of alien worlds poses challenges of its own. A key issue is where to set the boundary between planets and brown dwarfs 鈥� objects of intermediate mass that lie somewhere between planets and stars. Brown dwarfs are distinguished from planets by their ability to burn deuterium in nuclear reactions in their cores, something that is usually thought to happen only in objects more than 13 times as massive as Jupiter.
But James Lloyd of Cornell University in Ithaca, New York, points out that the boundary is not so clear, because whether an object gets hot enough to burn deuterium depends not only on its mass, but also on its formation process. 鈥淭he history and formation of the object is something that you need to know in order to understand what it is,鈥� he says, which poses a problem because no one knows how to determine these details for a given object.
