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1.MapoftheSolarSystem，showiaryorbitsatthecorrectrelativesizes.Orbitsareonlyslightlyetric，solookvirtuallyindistinguishablefromcircles.TheunlabelledsideMars’sorbitistheEarth’sorbit，heorbitsofVenusandMeralltoioisnotaplasorbitisshowisrepresentativeenumberofsmallbodiesbeyo
&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-AmeriGerardKuiperwhopredi1951asazonewhereicylumpsshouldbeleftoverfromthebirthoftheSolarSysteAnIrishmahEdgeworth，saidmuchthesameinamoreobsalin1943，sosomeprefertocallthisthe‘Edgeworth-Kuiperbelt’.ThefirstKuiperbeltobjecttobedisdreizedassuchwasfoundin1992，butnowmahemhavebeencatalogued，andithasbeeclearthatPlutoshouldbeheSimilarobjectswithperihelionubutreagabout100AUataphelioedas‘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，orheHolearguedtoretainPlutoasaplaalrounds.
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.
ThisrotatingdiskiswhereplasforTheogavebirthtoourSolarSystemisreferredtoasthesolarnebula，‘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.
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