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Chapter 1 The Solar System（第2页）

&oisnearperihelion，itisiofhereishemg。Theirdifferingorbitalinsprevehsfr，auneisalwaysoesideoftheSuopassesiu。Thisispossiblebecauseforeverythreeorbitspletedbyopletesexactlytwo。Sucharelationshipisreferredtoas3:2orbitalresonance。

&heirorbitsbeingnearlyar，everyplahesamewayrouheytravelanticlockwiseasseenfromanimaginaryvafarabovetheEarth’siotionisalsomahedirelaVenusandUrasaxis。Betiotionissoon，itiscalled‘prograde’。Clootionorrotatiardedasbadisreferredtrade’。

WiththeexofUranus，theaxisaboutlaesislessthan3atrightasorbitalplane。Merearly‘perfect’，withatiltofonly0。1°，whereasEarth’saxisistiltedat23。5°。Thedirela’saxispointsaoftiltbothvarywheensofthousatheyareeffectivelyescaleofasi。Axialtiltiswhyplashaveseasoh，summerothenorthernhemisphereduringthatpartoftheorbitwheheh’saxisistiltedtowardstheSun，aerissixmoheEarthisohersideoftheSun，sothattheheaxisistiltedawayfromtheSuwopladon’t，Venus’saxisistiltedatonly2。7°butitrotatesveryslowlyirivingitadaylengthof116。7Earth-days），whereasUranus’saxisistiltedby82。1°idretrograderotation。Uranusprobablysufferedacatastrophethatkstartedwithprograderotatioippedoverby97。9°（97。9°being180°minus82。1°）。Thiswouldresultiuationwithoutaseparateevesdireofspin。

&esofplas

AllplaMerdVees，or‘moons’。ThesearesmallerbodiesoughtoorbittheplaheSun。Strictlyspeaking，aplassatelliteeachorbittheiroreofmass（or‘barytre’）。However，plasaresomuassivethaellitesthattheirbarytreisi，anditisusuallyperfectlyadequatetardsatellitesasgoingrou。Mostsatellites’orbitslieclosetotheirplaorialplaneandalmostallthelargeoneshaveprogradeorbits，whichisdefinedasiio’sspin。

&h’ssatelliteis，ofcourse，theMoon（italM）。Thisisexaliivelylargeinparisontoitspla，havier27%andamass1。2%oftheEarth’s。Byce，theMooaharesuchthatitappearsalmostthesameastheSun，whichismuchlargerbutdinglyfurtheraway。WhentheMoolybetweehahidestheSun’sdisk，gasolareclipse。IftheMoon’sorbitrouhlanarwiththeEarth’sorbit，thereseeverylunarorbit（thatis，everymoheMoon’sorbitisi5。2°totheecliptic，soeclipsesarerare，oheMooopassbetweehaowopoisorbitcrossestheeravellingtheatureoftheseeventsaingwheneclipseswhwithoutfullyuandingthereasons）wasoatsofBabylonianastro2，6o。Lunareclipses，whentheMooheEarth’sshadow，aretrolledbythesamecycle，butaremoreoh’sshadowissiderablybiggerthantheMoon。

Marshastwoties。Jupiterhasfourexg3，000kilometresier（whicharethosethatGalileodiscovered），plusatthelastt59otherslessthan200kilometres（mostlessthares）across。SaturnhasasimilartotalnumberofkhoughohemrivalsJupiter’slargest。Uranushasfivesatellitesbetween400and1，600kilometresad22knownsmallerones，aunehasesatelliteandadozenknownsmalloofthesmall（fewkilometresacross）outersatellitesofJupiterandSaturnweredisctelescopes（ratherthanbyvisiti），aellitesofthegiassurelyremaintobefouUrauionbytelescopeisespeciallyhardfortworeasons:theyarefurtherfromtheSunandsoarelessbrightlylit，andtheyarefurtherfromtheEarthandsowouldlookfaiheywereequallywelllit。

ThelargersatellitesaregeologicallyveryiingandIsaymoreaboutthemlater，butallsatellitesareusefulfortheplaarystistbecausetheyetobeweighed。Theorbitalperiodofasatellitedependsonlyoaheplareandtheirass（whibecalculatedusion’sgravitatioiohirdLaw）。Becausesatellitesaresomuchsmaller，themassoftheplaesalmostehesamelasroundtheSundependondistanass。

&raus，as

ThisbookisaboutplaherthanthewholeSolarSystem，butitiswthatobjectsofothertypesvastlyoutsaellitesbihoughadmittedlythesearesmallaalmassisrelativelyinsignifit。Althoughplaarystistshaveetorealizethattheboundariesaresomewhatblurred，these‘jusbedividedintothreebroadclasses:asteroids，traus，as。

Asteredownwardsinsizefrom950kilometresacross（thediameterofCeres，thelargestexample），with。AsteroidsoeresacrosshavebeeheypassclosebytheEarth，andtheremainsofsmallerofalltothegroundeteorites。Formerlyassumedtmentsofadestroyedplahinkthatasteredtoapla。ThetotalmassofalltheasteroidsisprobablylessthanathousandthoftheEarth’smass。Somehaveclearlyexperieuals，asattestedbytheirirregularshapes。

Withoutex，asteroidorbitalmotirade。Mosthaveorbitalinsoflessthan20°，butetricityistypicallygreaterthanforplasofmostasteroidsliebetweenthoseofMarsaheso-called‘asteroidbelt’），butsomeuchclosertotheSun，passinginwardsoftheEarthandeven（inahandfulofexamples）inwardsofMercury。AfewasteroidsareknbeyondSatureoritesderivedfromthem，mostasteroidsarestonyorposition，butsomearemadeofironandnickel。Sofaraswetell，asteroidpositioobelessstonyandmoreadeventuallyicierwithdistaheSun。

&heorbitofweenabout30and55AUfromtheSun，smallicybodiesbeon，andthereareseveralthatexceedthelargestasteroidsihisregionisusuallycalledthe‘Kuiperbelt’，ch-AmeriGerardKuiperwhopredi1951asazonewhereicylumpsshouldbeleftoverfromthebirthoftheSolarSystem。AnIrishmahEdgeworth，saidmuchthesameinamoreobsalin1943，sosomeprefertocallthisthe‘Edgeworth-Kuiperbelt’。ThefirstKuiperbeltobjecttobedisdreizedassuchwasfoundin1992，butnowmahemhavebeencatalogued，andithasbeeclearthatPlutoshouldbehem。Similarobjectswithperihelionubutreagabout100AUataphelioedas‘ScatteredDisk’objects。TogetherwiththeKuiperbelt，thesemakeupafamilycalled‘traus’，orTNOs，allis。ThetotalmassofTNOsisprobablyarouoftheasteroidbelt（ohofah-mass），andintotaltheremaybenearly100，000bodiesmorethan100kilometresinsize。OeredDisk’objectdis2005alynamedErisseemstinallybiggerthanPluto。Webemoretabouttheirmassesbecausetheybothhavesatelliteswithwell-dotedorbits，showingthatthemassofErisis28%greaterthanthatofPluto。

&shavebeenkybeetbrieflylookveryspectacular，thankstothedevelopmentofatailofgasanddustthatstretchacrosstheskywhepassesclosetotheSuhesolidpartofaetisjustakofdustyice（famouslydescribedasa‘dirtysnowball’），oresaostcases。AetspendsmostofthetimefarfromtheSun，aailopassesoughfortheSuntowarmit。Thishappensrarelybeetshaveextremelyetricorbitswithperihelionusuallyih’sorbitbutaphelionnearorwellbeyo。Somesofarbeyondthattheirorbitslooklikeparabolas（infinitelylongellipses），andmakeonlyootheSunthroughoutrecordedhistory。Th-period’ets，aohavebeendislodgedfromanill-definedshellsurrouabout50，000AUknowtrast，‘short-period’etspriteredDiskobjectsthatwereperturbediricorbiteriheliondistancebyaterwithafellowobject。ThosewithorbitalperiodsofhundredsofyearsstillhavetheiraphelioeredDisk，butaphelionudgedclosertotheSuofaterla。Forexample，Halley’sethasapheliou，andanorbitalperiodof75years，whereasEhasaphelioer’sorbitandaperiodofonly3。3years。assbydegassiheSuwarmsthem，soafterfewerthanathousandperihelioisprobablyreduieofice-freeroddust，hardtodistinguishfromanasteroid。Asyoumightexpetheirsource，theorbitsof‘short-period’etsareprogradeaobeclosetotheecliptic。However，ioolong-periodetswhoseorbitsbehighlyinedrade。

la?HowPlutocametobethrownoutoftheclub

&NOtobediscoveredluto，ierPluto’ssmallsizebecameapparent（aly，thankstothediscoveryestsatellitein1978，itssmallmass），peopletehinkofPlutoastheni。However，whentheknownpopulationoftheKuiperbeltblossomedintohundredsofobjects，severaloflutoibeglyanomaloustoclassifyPlutoasaplaherKuiperbeltobjeethi。WhenEriswasedtobemoremassiveandprerthanPluto，thenlogicallyeitherallthelargeTNOshadtobecalledplas，orhem。HolearguedtoretainPlutoasaplaalrounds。

Deakingeredbythefactthattheterm‘pla’hadneverbeenfullydefiually，atameetiionalAstronomiheldinPraguein2whisraesvotedtoaedefinitionsthathavelargelysettledtheissue。TwocriteriaforplahoodwerenoheIAUruledthat，firstly，aplahavesuffitmassforitsself-gravitytidbodyforces’sothatitassumesahydrostaticequilibrium（nearlyround）shape，ahatitmustbeinorbitabouttheSueriolargesatellitessuoon。

Athirdwasthee。Itstatesthattobetedasapla，abodymusthave‘clearedtheneighbourhoodarounditsorbit’exceptmuchsmallerobjects。ThisisthetestthatPlutofails。Plutohassneighbourhood，becauseitsharesitwithmanybodiesofsimilarsizeandindeedalsowiththevastlymoremassiveheotherhauhetest，becauseitismanythousandsoftimesmoremassivethanahesameion（suchasPluto）。

HaviheboldbutentirelylogicalsteptoexpelPlutofromtheplaaryclub，theIAUseemsimmediatelytrettedit，aowotobelongto。Atthe2uemeeting，thenewly‘dla’wasdefinedas‘acelestialbodythatisinorbitroundtheSun，hassuffitmassforitsoullitintoanearlyrouclearedtheneighbourhoodofitsorbit，andise’。Determiherornotshapeis‘nearlyround’isdiffiotely，andtroversialtodefiinadoptiiontheIAUgavePluto，Eris，ahelargestasteroid）thesolationprizeofbeingcalled‘dlahetime，itwasaowledgedthateTNOswouldberankedasdlaheyhadbeeelymeasured，andsureenoughin2008aKuiperbeltobjeamedMakemake（pronouncedasfoursyllables），estimatedtobeabouttwo-thirdsthesizeofPluto，wasjudgedtopasstheshapetestandwasadmittedasafourthdla，closelyfollowedbyafifthcalledHaumea。

TheIAUseemilumpedPluto-likeobjethesamecategoryasdin2008ierm，‘Plutoid’，toderauniandlahustheonlydlaisnotaPlutoid，andthereissurelynouerhevertojoinitinthatcategory。However，thereareprobablynumerousundiscoveredorielydotedlargeTNOsthatwilljoinPluto，Eris，Makemake，ahPlutoidsanddlas。Ially，Erisisnamed（appropriately，gthetroversythatitcaused）afteraclassicalGreekgoddessofdiscord，whereasMakemakeandHaumeaareerPacifidfertilitydeities。

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Uossibletuethatplasarerareintheos，butitnowseemsclearthatplasareausualby-productofstarformatioenceofourSolarSystemisthusaceiself。

Astarisbelievedtoformwhenavastiellarostlyen，butmixedwithafewasesandtinysolidparticlesreferredtoasdust，tradertheisowngravity。Asthetracts，mostofthematterbeestratedire，inabodythatbeesinglyhotbecauseofthegravitatiyvertedtoheatbytheprofall。Eventually，thetralpressureaurerisesohighthathydrogeofusetogethertomakehelium，atwhichstagethetralbodybecalledastar。Theplasgrowfromsomeofthematerialleftbehindduringthefinalstages。servationofangularmomentumyslightinitialrotatioospeedupduringatternotihestarbeestratedintoadiskiar’sequatorialplaingiioar。

Thisrotatingdiskiswhereplasform。TheogavebirthtoourSolarSystemisreferredtoasthesolarnebula，‘infor‘dusedbyastroodenoteanylargemassofgasandordustiherearestrongreasohatthesolarnebula’spositionwasabout71%hydrogen，27%helium，1%oxygen，0。3%d0。1%eaitrogen，neon，magnesium，silidiron。AlmostalltheinaldustinthesolarnebulaorizedbyheatfromtheyoungSun，butsoonsinthenebulabecameoughforgrainstose，notasindividualelementsbutaspoundsproducedbychemibination。Heliumdoesnotbiochemipounds，sothemostabundantpoundsthatdenseiherhydren。

Thankstotheavailableelementsaemperatureandpressure，oxygeobondwithsilidvariousmetalsteofpouesioftheheseareineralsocrystallizewhenmoltenrockcools，butinthesolarheygrewdiregas。Hydrogenoratedintosolidpartilywherethetemperaturewaslowenen-bearingpoundstoform，andformostpurposesthisseemstohavebeenbeyondabout5AUfromtheSun。Atahisso-called‘ie’，water（madeenplusoxygen）denseasspecksofice。FurtherfromtheSuileedwherehydrogehethahomakeammonia，andwithoxygentoformeitheronoxideordioxide。Atabout30AU，itwasentoseassolidparticlesehequirksofplaaryscevocabulary，anysolidmadeofwater，methane，ammonia，onoxide，dien（orindeedahese）isreferredtoas‘ice’，regsimilaritiesininahismeansthat，toavoidambiguity，plaarystistshavetospecify‘water-ireferringspecificallytofrozenlithatrarelyarisesoh，wheretemperaturesaretoohighfororevolatilethaofreezenaturally。

&ionhappenedinsuchawaythatthefirstdustgrains–microscopicspecksmadeofsilicatesclosetotheSunandices（plussomeleftoversilicates）furtherfromtheSun–didnotgrowasdense，rigidspestead，theyhadintricate‘fluffy’shapes，ahesebumpediher，theyteicktogetherratherthanboungapart。Withinaslittleastenthousaertheoofsatioiclescouldhavegrownintoglobulesaboutatimetreacrhthebisofgsationaigtogether）whentheycollided。Afterperhaps100，000years，theSolarSystemwouldhavesistedofhordesofbodiesabout10kilometresacross，dubbed‘plahesewereallswirlingrouhesame，prograde，diredenadiffusehazemadegasanddust。