PHYSICS & ASTRONOMY_Exoplanets

The Search for a Second Earth

To date, astronomers have discovered nearly 800 planets orbiting distant stars. So far, only three of them have been found to potentially offer life-sustaining conditions. However, there are probably many second Earths in the Milky Way. But how can traces of life be detected on exoplanets? At the Max Planck Institute for Astronomy in Heidelberg, Lisa Kaltenegger is trying to answer this question.

TEXT THOMAS BÜHRKE

48 MaxPlanckResearch 3 | 12 Photo: ESO M Technology andtheKarl-FranzensUni- to andfrobetweentheUniversity of meant shewasconstantlycommuting find outwhatfascinatedher most.This ing physicsandastronomy in orderto dia studies,business engineer- plines, studyingJapanese,filmandme- did startwithabroadrangeofdisci- provided byacommittedteacher. wide-ranging interestsandtheimpetus what theyhaveincommonaretheir the pioneerofquantumphysics,but est enoughnottocompareherselfwith tail.” LisaKalteneggeriscertainlymod- study thelawsofnatureingreaterde- his mathteacherandthe“desireto that heoptedforphysicswasthanksto Unlike MaxPlanck,Lisaactually as theclassics.Thefact well havestudiedmusic school, hecouldjustas himself that,onleaving ax Planckoncesaidof the mostcreativeandcompetent spe- three monthsoftheyear. Sheisoneof ter forAstrophysics,whereshe spends Cen- renowned Harvard-Smithsonian taneously aResearch Associateat the Astronomy inHeidelberg,and issimul- Group attheMaxPlanckInstitutefor Kaltenegger leadsanEmmyNoether hold herowninthefield,hesaid. It wouldbedifficultforawomanto against acareerinthenaturalsciences. though acareeradvisorhadurged engineering physicsandastronomy– obviously, andfinallyshesettledon She couldn’t keepthisuplongterm, OF OURPLANET SPECTRAL FINGERPRINT coped withthispentathlonofcourses. bike ride,”shesays,toexplainhow versity inGraz.“Itwasaten-minute Less thantwentyyearslater, Lisa on oneoranotherofthem. In thecomingyears, researchers wanttofindoutwhetherlife exists billion terrestrial planetsorbitingdistantstarsintheMilkyWay alone. The homeofextraterrestrials? Astronomers suspectthere are several who alsoofferedcoursesinastronomy. byagoodphysicsteacher encouraged year atschool.Thisenthusiasm was electrified LisaKalteneggerin herfinal branch ofastronomyin1995, butalso created notonlyanew, fast-expanding a celestialbodyorbitingdistantstar, of theexoplanets. to search forsuchtracesoflifeonone Conversely, itshouldbepossibleforus et anduseittodeduceourexistence. also makesuchaspectrumofourplan- Inhabitants ofdistantplanetscould recorded withthe gerprint oftheEarth,whichhadbeen Carl Saganpublishedthespectralfin- German Ministry ofResearch.German Ministry German Research Foundationandthe Maier-Leibnitz Prizeforphysicsbythe she wasawardedtheprestigiousHeinz cialists inextrasolarplanets.Thisyear The discovery ofthefirstexoplanet, The discovery In 1993,Americanastrophysicist 3 | 12 MaxPlanckResearch MaxPlanckResearch Galileo spaceprobe. 49 >

It must be said that her home town of edge of both engineering and astrono- Her work also focuses on Earth’s evolu- Kuchl, near Salzburg, Austria, is less my was clearly an asset in this position. tion in order to learn about habitable known for astronomy than for its wood Though Darwin was put on ice, as planets. The idea is simple in principle: processing and agriculture. But her par- was a similar NASA mission, this didn’t “When we are able to investigate a dis- ents ensured lively discussion on a wide mean that the search for a second Earth tant rocky planet spectroscopically, scope of topics at the dinner table. was abandoned – especially not by Lisa what could be the indicators of life? Kaltenegger. NASA’s Kepler mission in We can’t assume that any possible life PALEONTOLOGISTS AND particular excited her, because its fasci- there is at the same stage of evolution BIOLOGISTS ALSO ON BOARD nating discoveries greatly increased the as we are now.” So the first thing she number of potential terrestrial planets, did was to investigate how Earth’s at- “At the end of the 1990s, nobody in which made missions such as Darwin mosphere had evolved since the forma- Austria was working on characterizing seem more realistic again. “Discovering tion of our planet, which involved exoplanets,” says Lisa. She would have traces of life on another planet would close collaboration with biologists and to leave the country. Research visits to be one of the truly great steps in the ex- paleontologists. the Instituto de Astrofísica de Canari- ploration of the universe,” says Lisa. In the beginning, carbon dioxide

as on Tenerife, Johns Hopkins Univer- “This great discovery would have (CO2), nitrogen and water dominated sity in Baltimore and the European social, religious and philosophical con- the primeval atmosphere. When the Space Agency in the Netherlands fol- sequences, of course. From the scientif- first organisms appeared about 3.5 bil- lowed. Here, she was part of a three- ic point of view, it would also provide lion years ago, they produced meth- person design team working on the the opportunity to learn something ane, which enriched rapidly in the air,

Darwin project, an ambitious plan to about the evolution of our own planet while the CO2 content decreased fur- use several telescopes in space to find and to take a purely statistical look into ther. About 2.4 billion years ago, the Earth-like planets around other stars the future of terrestrial planets,” ex- first organisms began to produce oxy- and to characterize them. Her knowl- plains the Max Planck scientist. gen in abundance; its concentration in

O2 CO2 H2O H2O H2O H2O CH4 CH4 CH4 O2 H2OH2O H2O O3 H2O O3 H2O H2O H2O H2OH2O CH4/H2O H2O H2OH2O H2O

3.9 billion years ago 2.4 billion years ago 800 million years ago 300 million years ago 0 10 20 30 40 30 20 0 10

Proportion light (%) of reflected 0.6 0.8 10.6 0.8 1 0.6 0.8 1 0.6 0.8 1 Wavelength (μm)

Those who want to roam far from home must first be familiar with their local surroundings. This explains why Lisa Kaltenegger is studying the evolution of the terrestrial atmosphere before she transfers these findings to other, as yet unknown planets. She is particularly interested in which fingerprints would be observable in the spectra from different eras. For example, 3.9 billion years ago, in the early history of our planet, water (H2O) and carbon dioxide (CO2) predominated. Then the first organisms produced methane (CH4), and later, oxygen. Its quantity grew and the increase in molecular oxygen (O2) was accompanied by the proliferation of ozone (O3) in the atmosphere. The concentration of oxygen, 21 at percent, has remained almost unchanged for around 300 million years. Photo: ESA, NASA and Frédéric Pont (Geneva University Observatory); graphics: MPI for Astronomy – Lisa Kaltenegger

50 MaxPlanckResearch 3 | 12 Taken by Dimitris Mislis with the Oskar Lühning telescope at the Hamburg observatory

spheres and,usingfossilfinds, explored computer modeltoexoplanet atmo- negger adaptedanEarthatmospheric low.sphere, islikelytobevery sequently thatofozonein the atmo- inorganically, itsproportion,andcon- million years.Soifoxygenisproduced be asgoodgonewithinarounda producing oxygen,theoxygenwould on ourEarthweretosuddenlystop gen onatemperateplanet.Ifallbiota plained byabiologicalsource ofoxy- both chemicals,whichcanonlybeex- these gasesindicatestrongsources of oxide. Therefore,incombination, duce waterandcarbondioxideormon- act rapidlywitheachotherandpro- example. Butoxygenandmethanere- sun splitscarbondioxideorwater, for through photolysis,whenlightfroma be producedinorganically–oxygen nature. Eithersubstancealonecanalso es incombinationarethebetterbiosig- present inlargerquantities,thesegas- gas suchasmethane,forexample.”If lar oxygenorozonewithareducing things arecombinations–ofmolecu- Kaltenegger explains:“Thecrucial certain indicatorsoflife.ButasLisa oxygen andozonewere,inthemselves, assumed thathighconcentrationsof carbon dioxideandmethanechanged. At thesametime,proportionsof panied bytheincreaseofozone(O in molecularoxygen,O almost unchangedsince.Theincrease about 21percent, whichhasremained 300 millionyearsago,whenitreached first, withfluctuations,untilaround the atmosphereincreasedslowlyat Brightness As a postdoc at Harvard, LisaKalte- As apostdocatHarvard, For alongtime,astronomershad 2 , wasaccom- Time [hours] 3 ). sphere,” saysLisa. known touswouldhaveontheatmo- fects thatdifferentformsoflifenot chemistry. “We can’t simulatetheef- liquid waterandisbasedoncarbon cal principlesasitdoeshere:needs according toroughlythesamechemi- ations requirethatlifetherefunction ets. Ofcourse,alloftheseconsider- hold trueforotherterrestrialexoplan- Something similarcouldthenalso TECTONICS NO GASRECYCLING WITHOUT methane andwater.” combination ofoxygenorozonewith traces oflifeinouratmosphereasa extraterrestrials couldhavedetected about halfofEarth’s todate, history prisingly positiveresultwasthat,“For logical timecanindicatelife.Thesur- fingerprint ofourplanetthroughgeo- logical evolution.The Earth’s atmospherevariedduringgeo- how thechemicalcompositionof a significantroleintheevolution ofa not havetectonics.Yet tectonicsplays on theotherhand,likeMars, itmay small, miniature Neptune.Ifitisvery more likelytobeagasplanet, akindof heavy asEarthatthesamedensity, itis Venus andMarsinoursolarsystem. planet, justliketheEarthandMercury, What iscertainthatitmustbearocky or cooler, drierorhavemorewater. for example,besmallerorlarger, hotter teristics asourplanet,ofcourse.Itmay, Earth tohavethesamephysicalcharac- large andthusaroundtentimesas If aplanetismorethantwiceas Nor canthescientistexpectasecond resulting spectral of theSunonJune transits infront ofitsparent sun–as Venus transited infront theplanet One ofthemusesthetransit: Seenfrom Earth, Stellar eclipse: There are several ways todetectexoplanets. the planet’s atmosphere.the planet’s of conditions, themethodalsoallowsspectral observations duringthepassage;undermostfavorableand thelightcurve characteristics oftheexoplanet from thedecreasing brightness PHYSICS & ASTRONOMY terior canbindcarbondioxideandre- cycles gaseslikeCO allows afeedbackmechanismthatre- planet anditsatmosphere.Itiswhat the nextgenerationoftelescopes foral- their detectablespectralfingerprints for simulates possibleatmospheres and that is,asfarweknow, uninhabited. of acelestialbodywithinthis region of itshabitablezone,isagoodexample Mars, whichorbitsourSunontheedge vironment thatispotentiallyhabitable. but bynomeansproofthatithasanen- the habitablezoneofitssunisexciting, conditions ontheplanet. this isreallythecasedependson stress hereison detectable atmosphericfeatures.The requisite forlife–andcanproduce water canexistinliquidform–apre- on thesurfaceofaplanetaresuchthat star wherethetemperaturesprevailing able zone.Thisisthezonearounda only ifitiswithintheso-calledhabit- an exoplanetmanylight-yearsaway can bedetectedintheatmosphereof was youngandtheSunlessluminous. have beencompletelyfrozenwhenit other hand,Earth,foritspart,would luminosity ofitssunovertime.Onthe influences, suchasanincreaseinthe even withsmallincreasesinexternal mechanism, itcanbecometoohot planet doesn’t havethiscompensation acts likeacarbondioxidebuffer. Ifa the atmosphereagain.Tectonics thus noes, incontrast,introduceCO move itfromtheatmosphere.Volca- 5/6, 2012 The lavadescendingintoEarth’s in- On thecomputer, LisaKaltenegger Conditions forlifeasweknowit So the discovery of an exoplanet in ofanexoplanetin So thediscovery . Astronomers deducethe 3 | 12 can MaxPlanckResearch MaxPlanckResearch 2 , becausewhether . _Exoplanets 2 into 51 52 plains Kaltenegger. would havetocooltheplanet,” ex- sun toalargeextent:“The clouds clouds thatblockoutthelight ofits be surroundedbyadense layerof of thehabitablezone,would haveto HD 85512b,whichisontheinneredge fect toheatitsufficiently. Incontrast, en barinorderforthegreenhouseef- would havetobeatapressureofsev- zone, theCO on theouteredgeofhabitable greatly: OnGliese581d,whichorbits water onthesurface. conditions thatwouldenableliquid model tocalculatetheatmospheric Lisa Kalteneggerusedhercomputer sive asEarth.Forthesehotfavorites, light-years awayand3.6timesasmas- as Earth,andHD85512b,whichis36 years awayandseventimesasmassive zones: Gliese581d,whichis20light- the edgesoftheirrespectivehabitable Two suchsuper-Earthscanbefoundat TWO HOTFAVORITES ATMOSPHERIC MODELSFOR twice asbigEarth,androcky. to tentimesmoremassiveandup Earth. Theseareplanetsthatcanbeup them arewhatisknownasasuper- their largemassandsize.Onlyafewof can bedetectedmoreeasilydueto most ofthemaregasplanets,which day.ones beingaddednearlyevery But 800 exoplanetssince1995,withnew before thishappens. still haveafewhundredmillionyears won’t escapethisfate,either. Butwe comes toohotonitssurface.Earth after acertainperiodoftimeasitbe- et tomigrateoutofthehabitablezone many cases,thiswillcauseanexoplan- adapt tothisgradualchange.Andin during thepasttwobillionyears. Sun hasincreasedbyaround20percent age ofthestar. Theluminosityofour parameter becauseitincreaseswiththe the star. critical Thelatterisalsoavery et, andluminositytemperatureof ters, suchasmassandradiusoftheplan- where shevariesthedifferentparame- well asawidegridofrockyplanets, ready detectedextrasolarplanets,as MaxPlanckResearch MaxPlanckResearch The resultssheobtaineddiffered Astronomers havediscoveredaround The planetanditsatmospherewill 2 intheatmospherealone 3 | 12

of lifeintheuniverse:throughobser- only onewaytoanswerthequestion would theyhave? and thestars.Whatkindofworldview cloud coverandthusneverseethesky that livepermanentlyunderdense ly, itisalsoattractivetoimaginebeings trial exoplanets.Purelyphilosophical- citing varietyofthepotentiallyterres- give usafirstsmallglimpseintotheex- spectral fingerprintsignificantlyand Both conditionschangetheobservable exoplanetary system.Itsplanetsthen exoplanetary pen tobelookingontothe edge ofan They alwaysoccurwhenwe justhap- telescopes overthenext ten years. sphere ofanexoplanetwith existing best opportunitytostudy the atmo- searchers andengineers. nology andtestthecreativityofre- will comeupagainstthelimitsoftech- generation oftelescopes,thisproject biomarkers. Butevenwiththenext use heratmosphericmodelstoscanfor an extrasolarrockyplanet,andtothen soon getthefirstspectralfingerprintof vation. LisaKaltenegger’s dreamisto for example. the habitablezone. This appliestotheexoplanets Gliese not life aswe knowit.Favorable conditions,for instancewhere waterisliquid,existonlywithin A nichefor distancefrom life: starandnotevery Notevery itcansustain life –oratleast Regardless, therewillultimatelybe Transits offer whatisprobablythe Sun

Star mass (in solar masses) Distance from star(AU) et needsaboutayeartoorbit aSun- only around12hours–and theplan- to detecttheweakbiosignatures. one hundredhoursonaverage inorder spectrum hastoberecorded forabout tional colleagueshaveestimatedthata the Sun,Kalteneggerandotherinterna- system similartothatoftheEarthand the limitforterrestrialplanets.Fora 6.5 metersindiameter, itapproaches spite itslargemainmirror, measuring ly newpossibilitiesfrom2018.Butde- space telescope,issettooffercomplete- sphere withanappleanditspeel. compares theplanetanditsatmo- pictures ofEarth.LisaKaltenegger understand whenyoulookatsatellite rocky planetsissothin.Thiseasyto atmosphereof because theplanetary identify thebiosignaturesitcontains, light. Butitisincrediblydifficultto their spectralfingerprintinthestar’s before itreachesus. through theexoplanet’s atmosphere bit, andthestellarlightgetsfiltered or- pass infrontofthestaronceevery Mercury Yet thetransitperiodforEarth lasts This ishowthemoleculesleave Hubble’s 581 Venus d andHD successor, the Earth Habitable zone 85512 Mars b (green spheres), James Webb Jupiter

Graphic: ESO Photo: DFG 2016. Thisisimportantfor the subse- restrial planetsofnearbright starsby ect, whichisexpectedtosearch forter- Satellite(TESS)proj- Exoplanet Survey working ontheAmerican Transiting Lisa Kaltenegger. Thatiswhyshe before find oneormoreoptimumcandidates period oftransitinfrontthestar. al yeardespitetheshorterindividual more spectralacquisitionsperterrestri- more frequently, whichwouldallow bit. Asaresult,transitsoccurmuch couple ofmonthstocompleteoneor- Earth. Planetsthereforeneedonlya closer tothestarthaniscasefor greater hope.Theirhabitablezoneis more commonthantheSunoffer Parent starsthataresmaller, coolerand CERTAIN MEASUREMENTRESULTS ONLY BRIGHTSTARSPROVIDE of cially sinceitexceedstheexpectedlife quality. Asoberingcalculation,espe- to addupaspectrumofsufficient like star. Itwouldthustake10years 2012 Harvard-Smithsonian Centerfor Astrophysics. Shewasawarded theHeinzMaier-Leibnitz Prize Institute for Astronomy inHeidelberg, andissimultaneouslyaResearch Associate atthe Award-winning astronomer: LisaKalteneggerleadsanEmmy NoetherGroup attheMax Planck James Webb for herresearch intoexoplanets. “So itisextremelyimportantto James Webb . islaunched,”says quent observations with quent observations time after the nominal service lifeof time afterthenominalservice place thefirst happen before2022,whichwould projects. However, its phase togetherwithfour the cil known as within areasonableperiodoftime. portunity toobtainagoodspectrum because onlybrightstarsaffordtheop- James Webb

lation ofstars ● ● ● TOPOINT THE This isalsotheaimofmission European SpaceAgency’s selection

and radiusoftheplanet,luminosity andtemperatureofthestar. planets onthecomputer,varying thedifferentparameterssuchasmass To thisend,LisaKalteneggersimulates possibleatmospheresofextrasolar spectrum looklike? the limitsoftheirinstruments.And whatmightthesignaturein life onexoplanets.Buttheobservationofsuchbiosignaturesisapproaching Astronomers wanttousespectralanalysisonedaydetect traces of offer life-sustainingconditions. Of the800orsoexoplanetsdiscoveredfar,onlythree,atmost, PLAnetary TransitsPLAnetary andOs- hadended. PLATO ( PLATO launch resultsinthe ), other satel James Webb whichisin wouldn’t lite , PHYSICS & ASTRONOMY still alottodoanditisdifficulttask. traterrestrial spectrallines.Sothereis in separatingtheterrestrialfromex- cially dependonwhethertheysucceed sphere,” Lisaexplains.Here,itwillcru- light passesthroughtheEarth’s atmo- of course,alsoproducedwhenthestar’s detect fortheterrestrialexoplanetsare, lem here:“Thespectrallineswewantto of thisdecade. and shouldbeoperationalbytheend will beabout39metersindiameter in theChileanAndes.Itsmainmirror gave thego-aheadforitsconstruction recently pean SouthernObservatory tremely LargeTelescope, andtheEuro- cludes primarilytheEuropeanEx- er tracesoflifeonexoplanets.Thisin- will offerasecondpossibilitytodiscov- The nextgenerationoflargetelescopes tions willtheyposeatthesametime? findings? Andwhatnew, excitingques- body. Whatwillwethendowiththese first tracesoflifeonadistantcelestial be partoftheteamthatdiscovers be abletoheadaninternationalteam. ble, tohavecontactwithstudentsand search withasmuch freedomaspossi- her istheopportunitytopursuere- important toher. Thedecisivefactorfor institute. Noteventhecontinentisthat cific university, noracertainrenowned particular answer. Hergoalisn’t aspe- tination wouldbe,Kalteneggerhasno 2015. Whenaskedwhatherdreamdes- with herEmmyNoetherGroupuntil Max PlanckInstituteinHeidelberg, has foundherscientifichomeatthe search interesting,”saysKaltenegger. “But thisispreciselywhatmakesre- Who knows – maybe she will one day Who knows–maybeshewilloneday For thetimebeing,researcher But thereisonefundamentalprob- 3 | 12 MaxPlanckResearch MaxPlanckResearch _Exoplanets 53