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Pluto no longer a planet

Aug. 24, 2006
Special to World Science  
Updated Aug. 25

As­tronomers from across the globe meet­ing in Prague adopt­ed a new def­i­ni­tion of “plan­et” Thurs­day. But the word­ing ex­clud­ed Plu­to, a mem­ber of the club of So­lar Sys­tem plan­ets since its dis­cov­er­y in 1930. And some re­searchers crit­i­cized the def­i­ni­tion as vague.

Pluto in a computer reconstruction of Hubble Space Telescope data.


The de­ci­sion fol­lows years of grow­ing frus­tra­tion among sci­en­tists over the lack of a de­fi­ni­tion. New find­ings have made it  in­creas­ingly ob­vious that tra­di­tion­al notions of “plan­et”—usu­ally de­scribed as a large, round bod­y or­bit­ing a star—are too fuzzy to be of much use.

Some as­ter­oids are al­most as large and round as plan­ets. And some plan­ets are al­most large and hot enough to be con­sid­ered a type of star called a brown dwarf.

Af­ter tu­mul­tu­ous de­bate in Prague, the pres­tig­ious In­ter­na­tion­al As­tro­nom­i­cal Un­ion voted to de­fine a plan­et as a ce­les­tial body that or­bits the sun; is mass­ive enough for its self-grav­i­ty to pull it into a ball shape; and “has cleared the neigh­bour­hood around its or­bit.” 

Advocates of the wording argued that a planet must be the dominant object in its neighborhood.

This de­fi­ni­tion leaves eight plan­ets in our So­lar Sys­tem, as­tronomers said: Mer­cu­ry, Ve­nus, Earth, Mars, Ju­pi­ter, Sat­urn, ura­nus and Nep­tune. Plu­to is out be­cause its ob­long or­bit over­laps with that of larg­er Nep­tune. 

The de­ci­sion at a con­fer­ence of 2,500 as­tronomers was a stark shift from a week ago, when the or­ga­ni­za­tion lead­ers floated a pro­pos­al that would have reaf­firmed Plu­to’s plan­etary sta­tus and made plan­ets of its larg­est moon and two oth­er ob­jects. That plan proved un­pop­u­lar.

The guide­lines ul­ti­mate­ly adopt­ed al­so did­n’t sit well with re­searchers who still hold that Plu­to is a plan­et. But they of­fered some sol­ace by cre­at­ing a cat­e­go­ry of “d­warf plan­et,” which in­cludes Plu­to. This clas­si­fi­ca­tion is sim­i­lar to the plan­et cat­e­go­ry but com­prises those ob­jects that have not cleared their or­bital neigh­bor­hood.

The dwarf cat­e­go­ry is to in­clude al­so the as­ter­oid Ce­res, con­sid­ered a plan­et in the 1800s be­fore it got de­mot­ed; and 2003 UB313, nick­named Xena, an icy ob­ject slight­ly larg­er than Plu­to.

“The clas­si­fi­ca­tion does­n’t mat­ter. Plu­to—and all So­lar Sys­tem ob­jects—are mys­te­ri­ous and ex­cit­ing new worlds that need to be ex­plored and bet­ter un­der­stood,” Lou­is Fried­man, ex­ec­u­tive di­rec­tor of the Plan­e­tar­y So­ci­e­ty in Pas­a­de­na, Cal­i­for­nia, told the BBC News.

The de­ci­sion set­tles a con­tro­ver­sy over wheth­er Xena would rise to plan­etary sta­tus, said Mike Brown of Cal­i­for­nia In­sti­tute of Tech­nol­o­gy. Brown and col­leagues an­nounced Xe­na’s dis­cov­er­y last year. “I’m of course dis­ap­point­ed that Xena will not be the tenth plan­et, but I def­i­nite­ly sup­port the IAU in this dif­fi­cult and cou­ra­geous de­ci­sion,” said Brown. “It is sci­en­tif­i­cally the right thing to do.”

Not all as­tronomers agreed. Be­sides those who in­sist Plu­to is a plan­et, oth­ers com­plain that the new def­i­ni­tion is still mud­dled. 

“What ex­act­ly is meant by a plan­et ‘clear­ing its neigh­bor­hood?’ said Hal Weav­er of Johns Hop­kins uni­ver­si­ty in Mary­land. De­bate may al­so con­tin­ue be­cause, al­though the new def­i­ni­tion is meant to dis­tin­guish plan­ets from smaller bod­ies, it did­n’t take on the ques­tion of how to sep­a­rate super-large plan­ets and stars. That ques­tion does­n’t af­fect the clas­si­fi­ca­tion of plan­ets in our So­lar Sys­tem, but will be rel­e­vant to some oth­ers.

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