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Chapter7 Exoplas
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Thereisnolongeranydoubtthatplasareonarouars.Untilparativelyretly，thiswasamatterforspe，butby2010thearsproveoingthemhadexceeded400.Allowingforhowdifficultitistomakethesedetes，itisclearthatthemajorityofSuarsmustbeapaniedbyplaoavoid，professionalsusuallyrefertothemas‘extrasolarplas’or‘exoplaheexoplaallyexobjectsexg13Jupitermasses，whichisthethresholdabovewhiuclearfusioerium（heavyhydrogen）occur.Thosearecalled‘browndwarfs’andareregardedasmorestar-liketha-like.
&hods
&mostyoungSuarshaveasurrofdustbegantoaccumulateie1970s.Initialcluestheionadidatestar’sirum，thedistobeobtaihe1980s.Irrespectiveofwhetherthesediscsarelikethesolarnebulabeforeplasformedorareremnantdustsurvivingiofastar’sKuiperbelt，theirmereexistehereoughttobeplentyofplaheretoo.Thefirstdefidiscoverywasmadein1995，afterwhichdiscatheredpaceyearbyyear.
Radialvelocity
&disdthemajorityeversihan300by2010），wasmadebydetegslightastar’sradialvelocity.ThisisthespeedatwhichastaristravellingtowardsorawayfromtheEarth，irrespeovementacrosstheli.Radialvelogesbedetermioaremarkablepreetreperseeasuringshiftsiwavelengthatwhichcharacteristicabsorptionlinesappearinastar’sspectruTheseshifttths（‘blueshift’）ifthestarismovingtowardsusandtths（‘redshift’）ifthestaristravellingahenomeheDopplereffect.Variationsinradialvelocityhadloomeasureorbitalspeeds（aoinfermasses）ofdoublestars，butthetinyinfluenuchlessmassiveexoplaoivelymuassivestarrequiresverysensitivemoderion.RadialvelogescausedbytheEarth’sownorbitalmotioobeatedforbeforethemoresubtlegesattributabletothetugoftheexoplasstarbeeapparent.
&ionalattrabetweenstaradepeheirmasses.Fortunately，forSuhereisawell-uoodrelatioellarspectraltypeandmass.Knowingthis，weusetheperiodandmagnitudeofradialvelogestodetermihemassoftheexoplarespoheforwardandbaofthestar.Usually，thereismeasureoftheorientatio’sorbitalplane，aheorbitalplalyedge-ontoourli，thetruevelocitymustbegreatertha.However，statisticalarguments（basedonassumingrandomlyoriealplahatthemajorityofmassesorethaimatebasedonassumingthattheorbitisedge-ontous.
Theradialvelocitymethodworksbestfeplaiheirstar，becauselargemassaybothleadtothegreatestthestar’sradialvelocity.ThusitshouldhavebeennosurprisethatthefirstexoplaedteobemoremassivethaonlyafraofanAUfromtheirstar.
Discoveryoftheseso-called‘hotJupiters’causedquiteastir，becausetheyarewelliars’idothaveformedseetheItistheygrewfurtheraedinwards，andthishasreopeeabouttheextentofplaarymigrationinourownSolarSystem’searlyhistory.IfJupiter’smigrationhaduedinwards，itwouldhaveeitherdestroyedorscatteredeachterrestrialplaurn.Forawhile，‘hotJupiters’opethatsuewasnormal，andthatplaemslikeourotionallyrare.However，improvedandadditionalteiquesforexoplaioofindroets，shorepoJupiters’intheearlydiscoverieswasmerelyaseleeffectresultingfromeaseofdiscovery.
Transits
Theseostprolificmethodfordisgexoplaooutpacetheradialvelocitymethod，istosearchfor‘trainyfrahtiscutpassageofainfrontofit.Mosttransitsarediscoveredbyrepeatedssoflikelystarsusingautomatedtelescinallyfromthegroundbutnowalsobydedicatedtelesspace.
Atransitonlyiftheexoplaalplaexaourli，whichstatisticallyshouldapplytoonlyabouthalfapertofallexoplaems.Thedimmiarlightisslight，butisgreatestfestexoplasandoccursmoreoften（andsoismorelikelytobedetected）forexoplaiheirstar.On，discoveryof‘hotJupiters’isfavouredoveranyotherkindofplaamountbywhichthestarlightdimsbeusedtodeducethesizeoftheplaparedtoitsstar.Theduratiogivesuscluestoorbitalspeedandorbitalradius，butfollow-upradialvelocitymeasuremeercharacterizethesysteBecauseatraesthattheorbitalplaneliesi，massesderivedbytheradialvelocitymethodaretruevaluesratherthanmies.
Imagingahods
&sareexglygtoimage，becausetheyaresomuterthaars.Sishavebeeonlyahandfulofstars.Asyoumightexpect，theseareallJupiter-sizeder，mattensorevenhundredsofAU.In2008，aicsimageobtainedfromielesHawaiishowedthreeexoplaingayoungSuar（cataloguedasHR8799）at24，38，and68AU.Beyondthemisadustdiscatabout75AU.
Ahodforexoplaiorreatpotentialforthefuture.Thisisbasedonveryprecisemeasurementofastar’spositioninthesky.Aingpanionwilltugthestarfromsidetoside.Astrometryseekstodetectthis，insteadofradialvelogesalongtheli.Themotioifcausedbyamassiveplai，soisentarytomethodsmoreseosmallorbits.Thefirstedsuccessoftheastrometricmethodwasin2002，whentheHubbleSpaentedsidewayswobbleofthestarcataloguedasGliese876，refiningabouta2.6Jupiter-massplaingat0.20AUalreadydetectedbyradialveloges.Thefirstastrometricdiscoveryofapreviouslyu2009，whenareddwarfstarcataloguedasVB10wasfoundtobedangoutofpositionbecauseofa6Jupiter-massexopla.
Awhollydiffereakesadvantageofrandom（aoberepeated）exameweearandabadstar.Thefroundstaractsasa‘gravitationalmis’thatamplifiesthelightfromthebadstar.Thedetectedbrighthebadstarrisesandthenfallsoveradurationofseveralweeks.Ifthefroundstarhasafortuitouslyplacedexoplahiswillcauseabriefspikeiingafewhoursordays）superimposedontheslowerriseandfall.By2haddiscoveredatotaloftes.
s
&gives.Theyareidentifiedbyaddinglower-caselettersafterthealoguedesignatioar.Thefirsttobediscoveredisb，thesedisdsoon（aishus，thefirstexoplaofGliese876isGliese876b，andtwosubsequentlydiscoveredexoplahesamesystemareGliese876dGliese876d.Thisessy，withlettersbeariionshiptothepositiosiems.However，itsitiswiseforusnottoimposenames.Maybethenativesalreadyhaveperfeesfortheirhomes.
Multipleexoplaems
Multipleexoplasareknnearly50stars.Sometimesabinatioeiquesprovidesthisinformation，butradialveloedothejob:itisjustamatterprogressivelymoresubtleperiodicvariatiossomeermultiplesystems.AmohesystemofGliese581（areddwarfabout20.5lightyearsaartioteworthy.Itiheleastmassivek，Gliese581e，whichcouldbeoh-masses（andisalmostlylessthahmasses），andalsoossiblya（large）o-coveredterrestrialpla，Gliese581d，ofmorethah-masses.Gliese581eisfartoohottosupportlife，oreveainaGliese581dappearstolieinitsstar’shabitablezone.
Table8 Somemultipleexoplaems.Someofthemassesquotedaremies.MJ=Jupiter-mass
&starknowntohaveaisepsilonEridani，whily10.5lightyearsasilonEridanib，discoveredbytheradialvelocitymethod，isaJupiter-massgiantina3.4AUorbit.Ielescopesshowthatthestarisapaniedbyzonesofrockydebris（asteroidbelts）tredatabout3AUand20AU，plusadisgfrom35to100AU.StruthedustdiscitedasevidenuenthJupiter-masspla，epsilonEridanic，atabout40AU.
Study
&tledireationabouta.Ifwedeterminemass（byradialveloetry），wefersizebyassumiy.Atransitwillrevealsize，whialsobededucedbyimaging（basedhtnessandassumedalbedo）.Fromsize，wededucemassifweassumeadensity.Distasstargivesusafairideaofsurfaospheric）temperature，butthisdependsalsoohemixtureofgreenhousegasesinanyatmosphere，sothereisasiderablemarginforerror.
&majoradvaudyofexoplaswillprobablyeasweimproveourabilitytoamospheriposition.Thisisbestdoelesspace，capableofisolatingandanalysingthevisiblearaofindividualexoplaimportarialones.Severalabundantatmasspebeideheircharacteristicabsorptioionasesthatoughtnottoco-existuionsofsimplechemistry，suchasoxygehahefirstevideaii’satmosphereitheEarth’satmospherehasbeenradiged.
Lifeos
Thereareabout10，000millioarsinalaxy（about1in10ofthetotalstars）.Exoplabeabundant，havingbeenfabouthalfoftheadequatelystudiedSuars.Mostdiscoveriessofarhavebeengias，becausethosearetheeasiesttofind，andthereisnoproofyetthatterrestrialplasareoemsareclearlydiverse，arialplaislikelytohavesurvivedtheinwardmigrationofa‘hotJupiter’sudb，tlylessthan0.06AUawayfromitsstar（Table8）.However，becausewearebeginningtofirialplaislikelythattheynifitproportioarysystems.
&ionofholahostlifeisavexiakeaveryservativeestimatethateonly1%ofSuarshaveaterrestrialplainginalong-durationhabitablezowouldgive100millionhabitableterrestrialplasinalaxy，andthereareprobablyatleastasmanyhabitablesatellitesias.
&epintheoflogicisfarless.Giveionsrequiredforlife，howlikelyisitthatlifewillbegin?Thebuildingblocksforlifearenotalimitingfactor.Weknoaceisawashwithanicmoledthatwaterisabundanttoo，somostexoplasinahabitablezonewillhaveallthenegredientsfor-basedlife.Thatmearekclichéof‘lifeasithoutdelvingiionaboutotherformsoflifethatarereliaicchemistries.
Theeaseordifficultywithwhichlifetaneouslyariseisabiggapianding.Many（myselfihatwithtlesstrillionsanicmolea’sodwithmilliooplaywith，lifewilliablystart.Oncelifehasspread，itishardtoseeholetelyeradicated，butifitwas，itcouldpresumablyrestartjustasreadily.
&hatittooklifeohan500milliooestablishapermaing.Theabundanceoflifeinthegalaxy（and，byimplitheosbeyond）rovesignsoflifeos.Eveofi（orpast）lifeonMars，Europa，orEnceladus，weotleaptothethatlifehadbegunthereily，becauseobjetheSolarSystemarenottotallyisolatedfromeaicrobescouldsurvivetransportfromoherisofimpactejecta.EuropanlifecouldhaveEarth;itisceivablethatlifeoharrivedoefromMars.
Isanybodyoutthere?
Ifthereislifearouars，whataboutilife?Let’sspeculaterationally.Sofarasweknow，biologitelligencerequiresmulticellularlife.Ifmicrobiallifebegins，howlikelyisitthatsubsequeioomultis?Youtakeyourpithisissue.Ittookacoupleofbillioohappeh.
&icellularlifeappears，willpetitionforsurvivaldriveDarwiioelligehatadvantage，sohowielligence?
Evenoivefigureof100millionhabitableterrestrialplahegalaxy，plusapessimisticviewthatlifehasonlya1in100ce，thatstillleavesamillionworldswithlife，ofwhichtheEarthisowouldbestrange（andaweinspirihweretheonlyorfirstplaofallthatiiflifeissoabundant，andifintelligenonouteoflife，thenwhereiseverybody?Uarisesexglyrarely，ordoesnotlastlong（forexample，ourowncivilizationcouldsuccumbtowars，variousnaturaldisasters，orself-madeclimatege），thenthegalaxyoughttobeteemingwithintelligence.
&lifewouldobeiowherewefindit.Althoughthedistaarsarevast，itisperfectlyfeasibletotravelbetweentheYoudoer-than-lighttravel–allyouionandpatieneaspaceshipbigenoughtohousehundredsofpeople，whichwouldtake100yearstotraveltoahabitableexoplaar10lightyearsaway.Wecouldbuildsuchashipourselves，usingteologyforeseeableifewdeeeionsofthecrewwouldliveae（unlesssomekindofsuspeionisused），anditwouldbeverymue-.Ifweweretosendsuiststoallthenearbyhabitableexoplaoidentifyandcharacterizethesebytheeury），itwouldnotbelongbeforesuccessfulieshadthecapacitytolaunchtheirownyships，andsoon.Thegalaxyis100，000lightyearsacross.Evenifawaveofizationtakes1，000yearstospread10lightyears，theentiregalaxycouldbeizedinonly10millioastropheswipingoutwholeworldsorfailuresofindividualyshipswouldbeioderailtheproceitwasunder
Thegalaxyismorethan10billionyearsold.Ifilifeisabundant，therehasalreadybeeimefortlesspreviousspeciestohaveizedthegalaxy.ThisistheFermiParadox，smadebytheAmeriphysiriiierrestrialcivilizationsoughttobethereisnosigifialsdetespace（despitessoftheskybyteamswuhebaI–SearchforExtra-TerrestrialIntelligenosigworksofastronomieering，andnocrediblydotedalienvisitorstoEarth.Isiliferare，afterall，orarewetoostupidtoseetheevideneday，Ihopewewillfindout.
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