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1、Unit46IceWorlds,PlutoandBeyondCopyright(c)TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.BeyondNeptuneAccordingtotheInternationalAstronomicalUnionthisisthisendstheMajorplanets.SOwhatisbeyondNeptune?BeyondNeptuneSomethingcalled:Tr
2、ans–NeptunianObjectsTheseobjects(knownasTNO)arenearlythesamesizeasPlutoandarebeyondNeptune'sorbitPlutoPlutoItwaspredictedbyPercivalLowell,basedonperturbationtheory,butlaterdiscoveredbyClydeTombaugh(1906-1/17/1997)(thefirstplanetfoundbyanAmerican)in1930b
3、yafortunateaccident.PlutoFromitsdiscoveryin1930until2006,Plutowasclassifiedasaplanet.Inthelate1970s,followingthediscoveryofminorplanet2060ChironintheouterSolarSystemandtherecognitionofPluto'srelativelylowmass,itsstatusasamajorplanetbegantobequestioned.P
4、lutoInthelate20thandearly21stcenturies,whilesearchingformoreTNO,MikeBrowndiscoveredintheouterSolarSystem,anobjectwhichhefinallycalled:Erisin2005,whichis27%moremassivethanPluto.NeveragainwillPlutobetheninthplanet,orthelittlestplanet,orthemostdistantplane
5、t.Dr.MikeBrown,CaliforniaInstituteofTechnology(Caltech),Pasadena,Calif.,associateprofessorofplanetaryastronomyandleaderoftheresearchteamOntheInternationalAstronomicalUnion(IAU)onAugust24,2006,limitingthenumbertoeightanddefines"planet“PLANET:acelestialbo
6、dythat,withintheSolarSystem,isinorbit(pathofoneobjectaroundapointoranotherpoint)aroundtheSun;hassufficientmassforitsself-gravitytoovercomerigidbodyforces(isanidealizationofasolidbodyoffinitesizeinwhichdeformationisneglected.Inotherwords,thedistancebetwe
7、enanytwogivenpointsofarigidbodyremainsconstantintimeregardlessofexternalforcesexertedonit.)sothatitassumesahydrostaticequilibrium(nearlyround)shape;andhasclearedtheneighbourhood(ofitsownorbitalzone,meaningithasbecomegravitationallydominant,andthereareno
8、otherbodiesofcomparablesizeotherthanitsownsatellitesorthoseotherwiseunderitsgravitationalinfluence)arounditsorbit;orPLANET:orwithinanothersystem,isinorbitaroundastarorstellarremnants;hasamassbelowthelimitingmassforthermonuclearfu
