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Chapter 7 Exoplanets（第1页）

Chapter7　Exoplas

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’sspectrum。Theseshifttths（‘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’idothaveformedseethem。Itistheygrewfurtheraedinwards，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-upradialvelocitymeasuremeercharacterizethesystem。Becauseatraesthattheorbitalplaneliesi，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，itisperfectlyfeasibletotravelbetweenthem。Youdoer-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。