Astronomers are arguing bitterly over how to define a planet, with some proposing that the term be abandoned completely in favour of more specific labels based on where objects are located. Two competing proposals are expected to be put forward to a formal task group on Friday, but astronomers say the debate could drag on indefinitely.
The International Astronomical Union (IAU), which is responsible for settling such issues, assembled a special working group to come up with a new definition about 18 months ago, when a large new body called Sedna was found in the outer solar system.
鈥淭he hope was that we would come to some agreement before anything else dramatic happened,鈥� says the working group鈥檚 chairman, Iwan Williams of Queen Mary, University of London, UK. 鈥淏ut then 2003 UB313 turned up.鈥�
Advertisement
This large object was discovered in July 2005 in a ring of rocky bodies beyond Neptune called the Kuiper Belt. It was dubbed the 鈥渢enth planet鈥� by its discoverers because it is larger than Pluto, reigniting the debate over what constitutes a planet.
Going in circles
But the 19-member working group has been unable to arrive at a consensus. The discussion has 鈥渏ust been going in circles鈥�, says group member Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado, US.
One of the two proposals to be submitted on Friday is based simply on an object鈥檚 size 鈥� a definition that Stern favours. That would increase the number of planets in the solar system, with Pluto being just one of several known Kuiper Belt objects of about the same size that would qualify as a planet.
But some of the team say choosing a size cutoff for the definition is arbitrary. 鈥淭here is no scientific value in maintaining that there are nine planets, including Pluto as one out of many similar small bodies,鈥� says group member Brian Marsden, head of the IAU鈥檚 Minor Planet Center in Cambridge, Massachusetts, US.
The second proposal would decrease the number of planets in the solar system by demoting Pluto. This definition says a planet is the dominant body in its immediate neighbourhood 鈥� a title based on its size relative to its neighbours and the dynamics of its orbit. Objects in the asteroid belt beyond Mars, for example, would not be planets because there are so many of them in the same region, while Pluto would not count because it crosses the orbit of its more massive neighbour Neptune.
Fudge factor
But a third proposal has been discussed that does away entirely with the term 鈥減lanet鈥� 鈥� an option Williams calls a 鈥渇udge factor鈥�. It uses qualifying adjectives to define subclasses of planets based on characteristics like location, composition or culture. In this scheme, Earth might be a 鈥渢errestrial planet鈥� and Pluto a 鈥渉istoric planet鈥�.
Marsden supports this idea, explaining it 鈥渁llows us to educate a public that thinks Pluto is somehow very special.鈥� But Stern is vehemently opposed to it. 鈥淥ur charter is to define a planet, not subgroups,鈥� he told New 杏吧原创. 鈥淓ither we do our job or we don鈥檛.鈥�
鈥淚f the working group actually ratifies a statement that says there is no such thing as a planet, the IAU will be the laughing stock of the world,鈥� he says. 鈥淓veryone will ask why egghead PhDs can鈥檛 tell when an object is a planet if regular people can.鈥�
The working group may vote on the proposals within the next two weeks. But Williams says: 鈥淲e may just decide to start from scratch again rather than going on the two proposals.鈥�
Have your say about how to define a planet by voting in the poll on our .
