--Contents:--- *Pluto no longer a planet* PLUTO DECLARED A DWARF PLANET *IAU 2006 General Assembly: Result of the IAU Resolution votes* Pluto's New Place in Space Could Be as a 'Pluton' Astronomers in a Quandary Over Pluto's Status Plan Would Add Planets to Solar System Pluto's Brave New Worlds -- Discoveries in Our Solar System Force Astronomers to Ask: What's a Planet? ============================= *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. * 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.* ** * ** http://www.world-science.net/othernews/060824_planet.htm ================================================================= *_What's Up in Space -- 24 Aug 2006 _Subscribe to Space Weather News * *PLUTO DECLARED A DWARF PLANET: *Today at the International Astronomical Union (IAU) General Assembly in Prague, astronomers decided that the Solar System has eight planets, and Pluto is not one of them. Instead, Pluto is a "dwarf planet." To be a planet, the assembly ruled, a world must meet three criteria: / (1) It must have enough mass and gravity to gather itself into a ball./ / (2) It must orbit the sun./ /(3) It must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies. / So, /e.g./, mighty Jupiter, which circles the sun supreme in its own orbit, is a planet--no adjective required. Pluto, on the other hand, shares the outer solar system with thousands of Pluto-like objects. Because it has not "cleared its own neighborhood," it is a dwarf planet. This decision clarifies the vocabulary of planetary astronomy while simultaneously upturning 76 years of "Pluto is a planet" pop-culture. Will non-specialists heed Pluto's demotion? That remains to be seen. Meanwhile, according to the IAU, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; and three dwarf planets: Ceres , Pluto and 2003 UB313 . IAU Press Release http://spaceweather.com/index.cgi ================================================================= *IAU 2006 General Assembly: Result of the IAU Resolution votes* 24 August 2006, Prague *The first half of the Closing Ceremony of the 2006 International Astronomical Union (IAU) General Assembly has just concluded. The results of the Resolution votes are outlined here. * It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more. In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly: 1. Resolution 1 for GA-XXVI: "Precession Theory and Definition of the Ecliptic" 2. Resolution 2 for GA-XXVI: "Supplement to the IAU 2000 Resolutions on reference systems" 3. Resolution 3 for GA-XXVI: "Re-definition of Barycentric Dynamical Time, TDB" 4. Resolution 4 for GA-XXVI: "Endorsement of the Washington Charter for Communicating Astronomy with the Public" 5. Resolution 5A: "Definition of 'planet' " 6. Resolution 6A: "Definition of Pluto-class objects" The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known. The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects. Below are the planet definition Resolutions that were passed. *RESOLUTIONS* Resolution 5A is the principal definition for the IAU usage of "planet" and related terms. Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects. *IAU Resolution: Definition of a Planet in the Solar System* Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. *RESOLUTION 5A* The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet"^1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape^2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects^3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies". --begin footnotes------------------------------------------------------- ^1 The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. ^2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories. ^3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. --end footnotes--------------------------------------------------------- *IAU Resolution: Pluto* *RESOLUTION 6A* The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects. Planet Definition Questions & Answers Sheet: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_Q_A.html IAU - INTERNATIONAL ASTRONOMICAL UNION / UAI - UNION ASTRONOMIQUE INTERNATIONALE 98bis Bd Arago, FR - 75014 Paris, FRANCE - Phone +33 (1) 43 25 83 58 - Fax +33 (1) 43 25 26 16 © International Astronomical Union, 2005 http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html *Notes for editors* A press conference about the Closing Ceremony of the General Assembly, including the results of the planet-definition vote, will be held at 18:00, in Meeting Room 3.3 of the Prague Congress Center. (It will NOT be possible for journalists to ring in to this conference: they must be there in person.) The panel for the press conference will be: * Ron Ekers (outgoing IAU President) * Catherine Cesarsky (incoming IAU President, Member of the Planet Definition Committee) * Jan Palous (Chair of the National Organising Committee) * Richard Binzel (Member of the Planet Definition Committee) * Karel van der Hucht (incoming Secretary General) This press conference will conclude around 18:30 CEST. The IAU is the international astronomical organisation that brings together distinguished astronomers from all nations of the world. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Founded in 1919, the IAU is the world's largest professional body for astronomers. The IAU General Assembly is held every three years and is one of the largest and most diverse meetings on the astronomical community's calendar. *Contacts* Following the vote, some of the members of the planet definition committee will be available for interviews (after the final vote): Richard Binzel Member of the Planet Definition Committee Prague Conference Center, Meeting Room 3.1 Tel: via the IAU Press Office +49-89-32-00-63-06 Junichi Watanabe Member of the Planet Definition Committee Prague Conference Center, Meeting Room 3.3 Tel: via the IAU Press Office +49-89-32-00-63-06 Iwan Williams President, IAU Division III Planetary Systems Sciences Prague Conference Center, Meeting Room 244 Tel: via the IAU Press Office +49-89-32-00-63-06 Owen Gingerich Chair of the IAU Planet Definition Committee Tel: via the IAU Press Office +49-89-32-00-63-06 Professor Ron Ekers IAU President Tel: via the IAU Press Office +49-89-32-00-63-06 Catherine Cesarsky IAU President-Elect and member of the Planet Definition Committee Tel: via the IAU Press Office +49-89-32-00-63-06 *PIO contact* Lars Lindberg Christensen IAU Press Officer IAU GA 2006 Press office, Meeting Room 3.2 Prague Congress Center Tel: +49-89-32-00-63-06 Cellular: +49-173-3872-621 E-mail: lars@eso.org *Links* * Programme for the Closing Ceremony: http://www.astronomy2006.com/second-session-and-closing-ceremony.php * Live public webcast of the Closing Ceremony: http://astronomy2006.com/tv/ * The IAU Web page: http://www.iau.org * IAU News during the 2006 General Assembly: http://www.iau2006.org * IAU General Assembly: http://www.astronomy2006.com * Free registration for the media: http://www.astronomy2006.com/media-accreditation.php ================================================================= Pluto's New Place in Space Could Be as a 'Pluton' By Rob Stein Washington Post Staff Writer Wednesday, August 16, 2006; Page A01 Hoping to end the agonizing over whether Pluto is really a planet, an international committee of astronomers has come up with a new definition that would save the tiny body's place in the sun's family. Under the long-awaited proposal, Pluto would remain in the pantheon of planets by becoming the prototype of a new subcategory of small, outer solar system objects dubbed "plutons" -- planets, but distinct from the eight larger "classical" planets closer to the sun. The changes would require astronomy textbooks to be rewritten and every schoolchild to be taught a new vision of the solar system, because three other orbs would get promoted to planet status, as well -- expanding the total from the traditional nine to 12. "Everybody's been wanting to know: 'Is Pluto a planet?' " said Richard P. Binzel of the Massachusetts Institute of Technology, who served on the seven-member committee assembled by the International Astronomical Union (IAU) to settle the explosive issue. "The answer is: 'Yes, Virginia, Pluto is a planet.' " The proposal to resolve the dispute is being officially unveiled today at the IAU's general assembly in Prague. It will be hotly debated until Aug. 24, when about 1,000 astronomers will vote on it. Some astronomers expressed misgivings about the new definition, but it generally drew initial praise, and several predicted it will be ratified. "I think it's a good compromise," said Larry W. Esposito of the University of Colorado, who had opposed maintaining Pluto and similar bodies as planets. "They're really too small and don't amount to much. But it would be too difficult to demote Pluto. This way, we don't have to scratch it off the list." The status of Pluto, the smallest of the nine planets, has been called into question by the discovery in recent years of other objects of similar size and distance from the sun. But suggestions that Pluto be demoted prompted heated debate and angry denunciations. In an attempt to settle the issue, the IAU assembled a 19-member committee, which deadlocked after two years of intensive debate. That led to creation of the smaller committee, which met in Paris June 10 and July 1 to find a way out of the thicket. Under the new definition, a planet would be defined as any body massive enough to be round that is not a star but is orbiting one. "These are the most fundamental physical parameters that apply not only in our solar system, but everywhere in the universe," Binzel said. "That's what's so appealing about the definition -- it can be applied universally." The eight "classical" planets would be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Ceres, an object located between Mars and Jupiter that has long been considered an asteroid, would be considered a planet. "One might call it a 'dwarf planet,' but that's not an official term," Binzel said. Pluto, another object discovered orbiting it in 1978 called Charon, and a body discovered in 2003 that is slightly farther from the sun -- temporarily named UB313-- would be plutons. A pluton would be any planet beyond Neptune. "Currently, we know of three, but there are other objects that are close in size to Pluto that will have to go through an evaluation process to determine whether they will be considered plutons," Binzel said. "We fully expect there are even more discoveries to come that are likely to be in this class of plutons." Binzel and other committee members stressed that categorizing Pluto as a pluton was in no way meant to downgrade its longtime status as the ninth planet. "We might be demoting it from the list of eight classical planets, but we're promoting it by making it the head of its own special class," said Owen Gingerich of Harvard University, who chaired the panel. Other astronomers praised the committee for developing a relatively simple but scientifically valid definition to categorize the new objects being discovered, thanks to advances in technology and more powerful telescopes. "I think the group brilliantly came up with a simple scientific sieve that can be used to decide whether something should be considered a planet," said S. Alan Stern of the Southwest Research Institute in Boulder, Colo., who served on the committee that deadlocked. "I'm very pleased." But Mike Brown of the California Institute of Technology in Pasadena said he is disappointed, even though the proposal would characterize the object he discovered, UB313, as a planet. "My first reaction was, 'Wow, this would mean this thing I found is a planet,' which is pretty exciting," Brown said. "Then I started looking at the details, and I don't think they got it quite right." Brown questioned the caveats in the details of the proposal, which, for example, would make Pluto a planet but not Earth's moon. "I find the definition oddly inconsistent. It makes no sense to me," Brown said. "They are sort of trying to have things both ways: They want to have a scientific definition, but they also don't want to offend cultural sensibilities." Brown questioned why a committee would need to vote on what gets planet status. "That's weird," Brown said. "That's not science." But the proposal came as welcome news to Neil deGrasse Tyson, director of the Hayden Planetarium in New York. Tyson was inundated with angry and anguished e-mail and letters from schoolchildren when he opened an exhibit that displayed Pluto apart from other planets. "This is about the only way you could define planethood in a way that would include Pluto. So I find it a little suspicious," Tyson said. "But I'm happy to finally have an unambiguous definition, so I don't have to worry about it after this." © 2006 The Washington Post Company http://www.washingtonpost.com/wp-dyn/content/article/2006/08/15/AR2006081500330.html ================================================================= Astronomers in a Quandary Over Pluto's Status By DENNIS OVERBYE Published: August 23, 2006 Pluto was looking more and more like a goner yesterday as astronomers meeting in Prague continued to debate the definition of a planet. "I think that today can go down as the 'day we lost Pluto,'" said Jay Pasachoff of Williams College in Williamstown, Mass., in an e-mail message from Prague. Under fire from other astronomers and the public, a committee appointed by the International Astronomical Union revised and then revised again a definition proposed last week that would have expanded the number of official planets to 12, locking in Pluto as well as the newly discovered Xena in the outer solar system, as well as the asteroid Ceres and Pluto's moon Charon. The new definition offered yesterday would set up a three-tiered classification scheme with eight "planets"; a group of "dwarf planets" that would include Pluto, Ceres, Xena and many other icy balls in the outer solar system; and thousands of "smaller solar system bodies," like comets and asteroids. The bottom line, said the Harvard astronomer Owen Gingerich, chairman of the Planet Definition Committee of the union, is that in the new definition, "Pluto is not a planet." "There's not happiness all around, believe me," he added. The new proposal was hashed out in a couple of open meetings, the first of which was described by participants as tumultuous, and the second as more congenial. Astronomers are supposed to vote Thursday on this or some other definition, but whether or not a consensus is emerging depends on whom you ask. Some astronomers expressed anger that the original definition of a planet had been developed in isolation and then dropped on them only a week before the big vote. Others continued to question whether it was so important to decide the question now. Among its defects, some astronomers say, the newer definition abandons any pretense of being applicable to planetary systems beyond our own solar system. To many astronomers, Pluto's tiny size and unusually tilted orbit make it a better match to the icy balls floating in the outskirts of the solar system in what is known as the Kuiper Belt than to the traditional planets like Jupiter and Mars. The issue has been forced on astronomers by the discovery of such a ball even larger than Pluto, nicknamed Xena by its discoverer, Mike Brown of the California Institute of Technology . If Pluto is a planet, Xena should be, too, Dr. Brown has argued. The committee's original prime criterion was roundness, meaning that a planet had to be big enough so that gravity would overcome internal forces and squash it into a roughly spherical shape. But a large contingent of astronomers, led by Julio Fernández of the University of the Republic in Montevideo, Uruguay, has argued that a planet must also be massive enough to clear other objects out of its orbital zone. Dr. Gingerich admitted, "They are in control of things." So the newest resolution includes the requirement for orbital dominance as a condition for full-fledged planethood, Dr. Gingerich said. That knocks out Pluto, which crosses the orbit of Neptune; Xena, which orbits among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt. "Vociferous objectors have said they could accept this," Dr. Gingerich said. Reached in his office at Caltech, Dr. Brown, who as the discoverer of Xena has the most to lose by its and Pluto's demotion, said he thought he could live with the new proposal. "It essentially demotes Pluto to something other than a real planet, which is reasonable," he wrote in an e-mail message. Dr. Gingerich cautioned that there were still many things to be sorted out. For example, the I.A.U. may or may not create a special name for Pluto and other dwarf planets, like Xena and others yet to be discovered, that dwell beyond Neptune. If so, he said that "plutonians" seemed a likelier choice than the previous suggestion "plutons." That term was protested by geologists, who pointed out that it was already used in earth science for nuggets of molten rock that have solidified and reached the surface. With two more days before the scheduled vote, there was no guarantee that Pluto would not make a comeback and that the definition of planethood might be rewritten again. "Some people think that the astronomers will look stupid if we can't agree on a definition or if we don't even know what a planet is," said Dr. Pasachoff of Williams College. "But someone pointed out that this definition will hold for all time and that it is more important to get it right." Copyright 2006 The New York Times Company http://www.nytimes.com/2006/08/23/science/space/23pluto.html ================================================================= Plan Would Add Planets to Solar System By WILLIAM J. KOLE The Associated Press Wednesday, August 16, 2006; 7:46 AM PRAGUE, Czech Republic -- The universe really is expanding _ astronomers are proposing to rewrite the textbooks to say that our solar system has 12 planets rather than the nine memorized by generations of schoolchildren. Much-maligned Pluto would remain a planet _ and its largest moon plus two other heavenly bodies would join Earth's neighborhood _ under a draft resolution to be formally presented Wednesday to the International Astronomical Union, the arbiter of what is and isn't a planet. "Yes, Virginia, Pluto is a planet," quipped Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology. The proposal could change, however: Binzel and the other nearly 2,500 astronomers from 75 nations meeting in Prague to hammer out a universal definition of a planet will hold two brainstorming sessions before they vote on the resolution next week. But the draft comes from the IAU's executive committee, which only submits recommendations likely to get two-thirds approval from the group. Besides reaffirming the status of puny Pluto _ whose detractors insist it shouldn't be a planet at all _ the new lineup would include 2003 UB313, the farthest-known object in the solar system and nicknamed Xena; Pluto's largest moon, Charon; and the asteroid Ceres, which was a planet in the 1800s before it got demoted. The panel also proposed a new category of planets called "plutons," referring to Pluto-like objects that reside in the Kuiper Belt, a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. Pluto itself and two of the potential newcomers _ Charon and 2003 UB313 _ would be plutons. Astronomers also were being asked to get rid of the term "minor planets," which long has been used to collectively describe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as "small solar system bodies." If the resolution is approved, the 12 planets in our solar system listed in order of their proximity to the sun would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and the provisionally named 2003 UB313. Its discoverer, Michael Brown of the California Institute of Technology, nicknamed it Xena after the warrior princess of TV fame, but it likely would be rechristened something else later, the panel said. The galactic shift would force publishers to update encyclopedias and school textbooks, and elementary school teachers to rejigger the planet mobiles hanging from classroom ceilings. Far outside the realm of science, astrologers accustomed to making predictions based on the classic nine might have to tweak their formulas. Even if the list of planets is officially lengthened when astronomers vote on Aug. 24, it's not likely to stay that way for long: The IAU has a "watchlist" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits. "The solar system is a middle-aged star, and like all middle-aged things, its waistline is expanding," said Jack Horkheimer, director of the Miami Space Transit Planetarium in the United States and host of Public Broadcasting's Stargazer television show. Opponents of Pluto, which was named a planet in 1930, still might spoil for a fight. Earth's moon is larger; so is 2003 UB313 (Xena), about 70 miles wider. But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth. Moons and asteroids will make the grade if they meet those basic tests. Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet Earth's moon wouldn't qualify because the two bodies' common center of gravity lies below the surface of the Earth. "People were probably wondering: If they take away Pluto, is Rhode Island next?" Binzel quipped. "There are as many opinions about Pluto as there are astronomers. But Pluto has gravity on its side. By the physics of our proposed definition, Pluto makes it by a long shot." IAU President Ronald D. Ekers said the draft definition, two years in the making, was an attempt to reach a cosmic consensus and end decades of quarreling. "We don't want an American version, a European version and a Japanese version" of what constitutes a planet, he said. Neil deGrasse Tyson, director of the Hayden Planetarium at New York's American Museum of Natural History _ miscast as a "Pluto-hater," he contends, merely because Pluto was excluded from a solar system exhibit _ said the new guidelines would clear up the fuzzier aspects of the Milky Way. "For the first time since ancient Greece, we have an unambiguous definition," he said. "Now, when an object is debated as a possible planet, the answer can be swift and clear." ___ AP Science Writers Alicia Chang in Los Angeles and Seth Borenstein in Washington contributed to this story. ___ On the Net: International Astronomical Union, http://www.iau.org © 2006 The Associated Press http://www.washingtonpost.com/wp-dyn/content/article/2006/08/16/AR2006081600321.html ==================================================================== Pluto's Brave New Worlds Discoveries in Our Solar System Force Astronomers to Ask: What's a Planet? By Dava Sobel Wednesday, August 16, 2006; Page A13 Pluto has become the butt of jokes lately, replacing Uranus as the solar system's laughingstock -- and all because scientists find themselves forced, at last, to come to terms with the meaning of the word "planet." Tacit definitions have existed since ancient times, when planetai, meaning wanderers, applied to seven moving lights in the sky: the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. But telescopes have revealed more objects in the solar system than were dreamt of in ancient philosophy, and new discoveries demand strict, useful terminology that will help astronomers categorize a host of newfound worlds. Pluto, discovered in 1930, was hailed as a planet before its true nature came to light. In time, Pluto proved to be far smaller than any of the other planets, and very unlike them in the way it orbits the Sun at an exaggerated tilt. Even so, there seemed no need to coin a new designation for Pluto, and it held on to its planet classification. But in 1992 astronomers made the first of what now amount to several hundred sightings of other solar system bodies at the distance of Pluto and beyond. Suddenly there was reason to reclassify Pluto as a member of this new society, which quickly became known as "trans-Neptunian objects" or "Kuiper Belt objects," in honor of Gerard Kuiper (1905-1973), who had predicted a vast zone of small bodies in the environs of Pluto. As astronomers began to debate the issue, it spilled over into popular awareness and ignited considerable heat, for the planets are held in common, in awe, by all humankind. What might have constituted a purely scientific discussion, akin to deciding whether a particular tree was coniferous or deciduous, instead became public discourse. On one side were major-planet purists who felt that bodies smaller than 1,500 miles in diameter (a size calculated to eliminate Pluto) should be dropped from the planet list. On the other side were Plutophiles who objected to arbitrary size discrimination. One Pluto specialist asked pointedly, "Is a dachshund not a dog?" The lack of consensus on the "planet" definition struck people both within and outside the planetary science community as ludicrous, though several everyday terms we all think we understand are similarly vague. "Life," for example, poses semantic problems for biologists -- as well as for exobiologists, who hope to identify it if and when they find it on Mars or Europa. In 2005, after a Kuiper Belt object tentatively named "Xena" turned out to be larger than Pluto, the question changed from "Should Pluto continue to be called the ninth planet?" to "Are there 10 planets in all?" Also in 2005, Ceres, a small body discovered more than 200 years ago between Mars and Jupiter and long dismissed as a "minor planet" or "asteroid," was observed by the Hubble telescope to be more or less round. Scientifically, a roundish object carries more weight than a potato-shaped one, because roundness signifies the greater mass required to pull itself into a ball, or "hydrostatic equilibrium." Round Ceres raised the question, "Are there perhaps 11 planets?" Not for the first time, but with new urgency in 2006, the International Astronomical Union (IAU) impaneled a committee to define both the word "planet" and the status of Pluto. Our committee -- seven in number, like the planets of old -- met at the Paris Observatory in late June and reached a unanimous agreement. In short: A planet is a body in orbit around a star (as opposed to orbiting another planet) and big enough for gravity to make it round. The full text of our proposed definition is being released today, to be discussed by astronomers from around the world, now in Prague at the IAU General Assembly, and voted upon next week. If approved, our resolution will not only leave Pluto in place but will also add "Xena" (2003 UB313) and Ceres to the current census of planets -- with room for additions as future discoveries warrant. What's more, Pluto will lend its name to a newly defined category of planets -- the "plutons" -- which differ from the other planets by virtue of their highly inclined, elongated orbits, which take more than two centuries to complete and which suggest a different origin. As the prototype of this class, Pluto may still attract funny remarks, but it will have gained new significance. /Dava Sobel, author of "Longitude," "Galileo's Daughter," and "The Planets," served as the sole non-scientist on the Planet Definition Committee./ © 2006 The Washington Post Company http://www.washingtonpost.com/wp-dyn/content/article/2006/08/15/AR2006081501124.html ================================================================