[fw] When is a planet not a planet? http://www.newscientistspace.com/article.ns?id=dn8039 When is a planet not a planet? a.. 17:43 22 September 2005 b.. NewScientist.com news service c.. Maggie McKee 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. "The hope was that we would come to some agreement before anything else dramatic happened," says the working group's chairman, Iwan Williams of Queen Mary, University of London, UK. "But then 2003 UB313 turned up." This large object was discovered in July 2005 in a ring of rocky bodies beyond Neptune called the Kuiper Belt. It was dubbed the "tenth planet" 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 "just been going in circles", 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's size - 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. "There is no scientific value in maintaining that there are nine planets, including Pluto as one out of many similar small bodies," says group member Brian Marsden, head of the IAU's 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 - 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 "planet" - an option Williams calls a "fudge factor". It uses qualifying adjectives to define subclasses of planets based on characteristics like location, composition or culture. In this scheme, Earth might be a "terrestrial planet" and Pluto a "historic planet". Marsden supports this idea, explaining it "allows us to educate a public that thinks Pluto is somehow very special." But Stern is vehemently opposed to it. "Our charter is to define a planet, not subgroups," he told New Scientist. "Either we do our job or we don't." "If 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," he says. "Everyone will ask why egghead PhDs can't tell when an object is a planet if regular people can." The working group may vote on the proposals within the next two weeks. But Williams says: "We may just decide to start from scratch again rather than going on the two proposals." Have your say about how to define a planet by voting in the poll on our home page. Print this page Email to a friend RSS Feed begin 666 icon_print.gif M1TE&.#EA$@`/`+,``&:$I?___[_+V?/U^,S6X=KAZ7^8M%IWF&.!H[/"TJ6W MRG&-JXJANOCY^_GZ^P```"'Y! ``````+ `````2``\```17,,@P@+V7E3D! MYX"Q<56#><"1#)_W2:E2L%(5G YF"*S]W@) 4%0Y88)!`((@%#B?3Z%T::PZ C0X4F="M55;;@R_)6/0TW7_ UN.#5RI<=K58@V._WV20"`#L` ` end begin 666 icon_email.gif M1TE&.#EA$@`-`+,``(.;MK7$U)*GOZJ[S7^8M'2/K=;>YYBLPJ"SQYROQ>GM M\GF3L(F@N?___YNNQ&6#I"'Y! ``````+ `````2``T```1DL(A#JSVB!-4R M`6 8$LL"#,I!$,C@O@-")$#!& <@)_"0L(,:`7>BM5P(`"]8&.:0A18B^A(2 M7TF!#F9]OA+@GM5QZIG')I$:M&@:`K6"?$Z7/VZ*`&/%[_,9&PT*!H2%AH8< #$0`[ ` end begin 666 icon_xml.gif M1TE&.#EA$0`,`+,``,_3U_;X^FZ+JJ>XRH^BMU5UF='9XYVMO[G&UD=KDUQ\ MH",T1MSBZ4QIB62 GU%PE"'Y! ``````+ `````1``P```1P<,A)JRSIX?=2 MV=U7" :A(,[email protected]!($JL&,P0%$A $,%Q, 69(*!K``*(G(_QD"F.C@> QJ@= M#,W8,# H`!"&`6!PS3X1-H<!46A8N0,!2: FD)N('<,`<#@^?TT/+ J"#0T* /"XJ+C(V*`)"1DI.0$0`[ ` end