the rare earth hypothesis：稀土假说

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1、AstrobiologyintheClassroomNASA–CERESProject–http://btc.montana.edu/ceresMontanaStateUniversityPreliminaryEdition“TheRareEarth—JusthowrareisEarth-likecomplexlife?”HowspecialarethecircumstancesthathaveallowedcomplexlifelikeanimalsandmammalstodeveloponEa

2、rth?InthisactivitystudentssystematicallyinvestigatethetimeframeforcomplexlifetodeveloponEarth.Theyapplythisinformationtographsrepresentingstellarmass,luminosity,main-sequencelifetime,andstellarabundanceinordertoapproximatehowmanyotherplanetstheremaybe

3、harboringEarth-likecomplexlifeinourgalaxy.ByexaminingtheconditionsonEarththathavemadecomplexlifepossiblestudentssetlimitsonhowold,brightormassiveaneighboringstarcanbeandstillsupportcomplexEarth-likelife.12MontanaStateUniversityNASACERESProject-http://

4、btc.montana.edu/ceresPreliminaryEditionTheRareEarth—JusthowrareisEarth-likecomplexlife?TheRareEarthhypothesissuggeststhatEarth-likeplanetscontainingcomplex(animal)lifeasweknowitarelikelyquiterareintheUniverse.Thislessonwillexploreseveralparametersthat

5、haveledthescientistsPeterWardandDonaldBrownleetoputforththishypothesisintheirbook,RareEarth:WhyComplexLifeisUncommonintheUniverse.YoumaywishtorefertothisbookforamoredetaileddescriptionoftheRareEarthhypothesis.ScientistsgenerallyagreethattheEarthformed

6、about4.5billionyearsagoyetcomplexlifehasexistedontheEarthforaboutthelast500millionyears.Itisstillunclearexactlywhatchainofeventsleaduptotheemergenceofcomplexlifeonthisplanet.Oneofthefactorsthatscientistsbelievetobenecessaryisalongperiodofrelativelysta

7、bleclimateresultingfromastableplanetaryorbitatjusttherightdistancefromanappropriatetypeofstar.Let'sbeginoursearchforanappropriatestarbylookingatthecharacteristicsthatmaketheSunsoappropriateforcomplexlifetoflourishonEarth.TheSunisaG-typestarinthemain-s

8、equencephaseofitslife,whichmeansthatitisengagedinthestableburninghydrogenbynuclearfusiontoproduceheliuminitscore,andradiatingenergymostlyintheformofvisiblelight.Themeasureofastar’sradiationenergyiscalledluminosity.Althoughithasbeenbrighterinth