The Earth as a Distant Planet: A Rosetta Stone for the Search of Earth-Like Worlds PDF
Preview The Earth as a Distant Planet: A Rosetta Stone for the Search of Earth-Like Worlds
ASTRONOMY AND ASTROPHYSICS LIBRARY SeriesEditors: G.Bo¨rner,Garching,Germany A.Burkert,Mu¨nchen,Germany W.B.Burton,Charlottesville,VA,USAand Leiden,TheNetherlands A.Coustenis,MeudonCedex,France M.A.Dopita,Canberra,Australia A.Eckart,Ko¨ln,Germany E.K.Grebel,Heidelberg,Germany B.Leibundgut,Garching,Germany A.Maeder,Sauverny,Switzerland V.Trimble,CollegePark,MD,andIrvine,CA,USA Forfurthervolumes: http://www.springer.com/series/848 M. Va´zquez E. Palle´ P. Montan˜e´s Rodr´ıguez (cid:129) (cid:129) The Earth as a Distant Planet A Rosetta Stone for the Search of Earth-Like Worlds 123 M.Va´zquez P.Montan˜e´sRodr´ıguez InstitutodeAstrof´ısicade InstitutodeAstrof´ısicade Canarias,Tenerife Canarias,Tenerife Spain Spain [email protected] [email protected] E.Palle´ InstitutodeAstrof´ısicade Canarias,Tenerife Spain [email protected] ISSN0941-7834 ISBN978-1-4419-1683-9 e-ISBN978-1-4419-1684-6 DOI10.1007/978-1-4419-1684-6 SpringerNewYorkDordrechtHeidelbergLondon LibraryofCongressControlNumber:2010922052 (cid:2)c SpringerScience+BusinessMedia,LLC2010 Allrightsreserved.Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY10013, USA),except forbrief excerpts inconnection with reviews orscholarly analysis. Usein connectionwithanyformofinformationstorageandretrieval,electronicadaptation,computersoftware, orbysimilarordissimilarmethodologynowknownorhereafterdevelopedisforbidden. Theuseinthispublicationoftradenames,trademarks,servicemarks,andsimilarterms,eveniftheyare notidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyaresubject toproprietaryrights. Coverillustration:G.Pe´rez(IAC) Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Preface Since the discovery of the first planet outside the Solar System (or exoplanet) in 1992, detection of the number of planets is increasing exponentially. This planet searchisgeneratingoneofthemostactiveandexcitingfieldsinastrophysicsforthe nextdecades.Althoughwearenotcapable ofdetectingandexploringplanetslike ourownyet,ambitiousgroundandspace-basedprojectsarealreadybeingplanned forthenextdecades,andthediscoveryofEarth-likeplanetsisonlyamatteroftime. The theory of stellar evolution has been tested and developed by observations of several stellar types at different times of their evolution. In the 1980s, the ob- servations of ‘The Sun as a Star’ provided the role of our star as the Rosetta stone in interpreting the observations of sun-like stars with different mass, age and level of magnetic activity. This solar–stellar connection had a double avenue, because the stellar observations also contributed to a better understanding of the solarmagnetism. Although we are probably setfor some surprises,the example of the Earth and the rest of the rocky planets of the solar system will be our guidance to classify- ingandunderstandingthemultiplicityofplanetarysystemsthatmightexistinour galaxy. In a similar way to that of the Sun as a star, it is reasonable to expect that the future observed population of planets in the galaxy will exhibit a wide range of planet types and evolutionary stages. Observations of ‘The Earth as a Planet’ will provide the key to understanding future observational spectra of such bodies. However,theEarth-Exoplanetsconnectionwillalsoworkinbothdirections.When a substantial database of exoplanets becomes available, statistics of planetary for- mation and evolution will become possible. This will provide vital information in solvingsomeofthequestionsabouttheformationandevolutionofourownplanet andthesolarsystem,forwhichwestillhavenoanswers. Thecurrentviewonstellarevolutionisverydeterministic.Thefutureandevolu- tionofastardependontwobasicproperties:itsmassanditsmetallicity.Ifthesetwo quantitiesareknown,wecanestablishwhetherthestarwillexplodeasasupernova inafewmillionyearsorifitwillenditsdaysasaredgiant.Forplanets,thepicture isalittlemorecomplicated.Atfirstinstance,themassoftheplanet,itscomposition and its distance from the parent star will determine its habitability and evolution. But other factors, such as the presence of gas giant planets, can also play a major roleinitsevolution.Theparentstarwillalsoinfluencetheevolutionoftheplanet. v vi Preface Toestablishthesolar–stellarconnection,weneededonlytocomparethestars;simi- larly,toestablishtheEarth–exoplanetsparallelism,weneedtocomparenotonlythe planetsbutalsothephysicalpropertiesandevolutionoftheirplanetarysystemsas awhole.Forexample,wemaybeabletodeterminehowmanyofthe‘rocky’plan- etsthatwedetecthaveexperiencedarunawaygreenhouseeffect,suchasVenus,or howmanyhavelosttheiratmosphereasMarshas.Byobservingplanetsofdifferent ages,wealsolearnaboutthestateofourownplanetindifferentepochs. Undoubtedly,oneofthemainconcernsforastrophysicsinthecomingyearswill bethesearchforlife.Ifaplanethasallthesuitableoriginalconditionstodevelop and sustain life, does life necessarily occur? And if it does, what are the average time scales for the development of bacteria, plants or intelligence? Are we alone? ...During itsevolution, some of the most dramatic changes suffered by our planet affected the composition of the atmosphere. Extraterrestrial observers would ob- taintwodifferentspectraofourplanetdependingontheepochoftheobservation. Early on the Earth’s history, the major atmospheric signatures were those of CO 2 andwatervapour,butinrecenttimes(intermsofmillionsofyears),togetherwith suchspectralfeatures,thebandsofmolecularoxygen(O )andozone(O )arealso 2 3 present.Thisdramaticchange,theriseofoxygencontent,wastriggeredbytheap- pearance of life. In the future, we may be able to infer whether life is common or notintheuniversebyobservingtheevolutionarystagesofmillionsofplanets.From thetinybacteriatotechnologicalcivilizationswecanexpecttoseelifesignaturesin theatmosphereofexoplanets. In summary, this book will focus on observations of the Earth as a model for thesearchofexoplanetsandontheinformationthatwewillbeabletoextractfrom theirobservation.Weputourselvesinthepositionofanexternalobserverlooking at the solar system from an astronomical distance, and we try to answer how we couldconcludethatthisparticularplanet,thethirdindistancetothecentralstar,is essentiallydifferentfromtheothersandcapableofsustaininglife.Then,weapply whatwelearnfromthischangeofperspectivetothesearchforexoplanetssimilar toEarth. Thefirstchapterofthebookprovidesfirstahistoricalbriefingontheprogressive knowledge of our planet. Then, we start with a sort of space travel. Starting with firstobservationsfromthealtitude,usingballoonsandrockets,wecontinuewiththe viewsofourblueplanetfromtheMoonandthedifferentplanetsofoursolarSystem. One of the most important achievements of space research was the capability to observetheEarthfromoutside,floatinginspace. Thisconceptwillbecomplementedwiththesecondchapter,wherewedescribe themainpropertiesofourplanet.AdescriptionofthepresentEarthfromitsinterior totheatmosphereisgiven,followedbyareviewofthedifferentperiodsoftheEarth history. The third chapter shows how the Earth should be observed from space as Sagans’sbluedot.Thephotometric,spectroscopicandpolarimetricpropertiesofthe globallyintegratedlightreflected/emittedbyEartharediscussed.Specialemphasis is given to the Earthshine observations, the sunlight reflected by Earth toward the darksideoftheMoon,asaproxyforsuchglobalobservations. Preface vii TheouterlayersoftheEartharediscussedinChap.4.Manyinterestingprocesses resultingfromtheinteractionoftheatmospherewiththehighenergysolarradiation, solar wind and cosmic rays occur at high altitude. Observed from space the Earth glowsindiscretespectrallines(airglow),enhancedduringtransitoryeventssuchas theauroras.TheUVandX-raysareexcellentdiagnostictoolsforinvestigatingthese regionsoftheatmosphere. Theexistenceoflifeisoneofthemostrelevantpropertiesofourplanet,andthe Earthwouldlookcompletelydifferentwithoutit.However,detectingitunambigu- ously from vast distances is no trivial matter and ingenious techniques should be usedforthispurpose.ThismatterwillbehandledinChap.5.Towardtheendofthe chapter, the main features of our technological society, as reflected in the Earth’s electromagneticspectrum,aredescribed,providingahintastohowtodetectother extraterrestrialcivilizations. Detection of other Earths is the essential requirement to apply the Earth– Exoplanetsconnection.Chapter6isdedicatedtoareviewofthecurrentandfuture projects, explaining in detail the current limitations in detecting the less massive planets. A research field with a brief history, starting in the 1990s, but which has undergonerapiddevelopment andisbecomingoneofthemostimportantfieldsin currentAstrophysics. Themassofanastronomicalbody,togetherwiththechemicalcompositionofthe environmentwhereitwasformed,determinesitsfuture.Starsaremassiveenoughto reachtemperaturesthatpermitthermonuclearreactionsintheirinteriors.Lessmas- sivebrowndwarfscanmakethisprocessonlybyburningdeuterium.Planetswere recently defined by an international committee and come mainly in two classes: Giants and terrestrial. For the latter, the Earth, Mercury, Mars and Venus are our references;however,thepossibilitiesarelarger.Lackingobservationsofterrestrial exoplanets,wecanfigureouttheoreticallyhowtheseexo-earthscouldbe,changing some of the basic parameters. From Super-Earths and Super-Mercuries to Carbon planets,anamplediversityofpossibleworldsispresentedinChap.7.Someselected exoplanetsdiscoveredalreadyarestudiedindetail. However,planetsarenotisolatedbodies.Theyexperiencetheinfluencefromthe parentstarandtherestofplanetarycompanionsintheplanetarysystem.Thebroad destinyofaplanetisdeterminedbyitsinitialmassandchemistry,buttheultimate fatedependsonhowtheplanetisaffectedbytheinteractionswithitscompanions. To be in the right place is a good recipe. In Chap.8 we discuss these collective processes,alongwiththedifferenttheoriesontheformationofplanetarysystems. Asusualwestartwithourownsolarsystem,whichcanbeconfrontedwiththefirst observationsofproto-planetarydisksandmultipleplanetarysystems. Thelastchapterisanecessarilyfailedattempttoanswersomefundamentalques- tionsregardingthepositionofourplanetintheUniverse:IsourSunspecial?Isour SolarSystemcommon?andfinally:IsourEarthunique?Thecompleteanswerscan onlybeprovidedbyobservations,andsearchforbio-markers,ofterrestrialexoplan- ets to be discovered in the future. In the meantime, we can, however, remark that wehaveauniqueprocessofformationandanamplediversityofplanetarysystems. Birthandenvironmentalfactorsdeterminetogetherthestructureandevolutionofa planetarysystemanditscomponents. viii Preface ManypeoplehavebeeninvolvedattheInstitutodeAstrof´ısicadeCanarias(IAC), indifferentways,inthepreparationofthisbook.R.Castroelaboratedandretouched asubstantialnumberofthefiguresandtheLibrarystaff(M.Go´mez,andL.Abella´n) provided an excellent service in tracing old publications. The computer services maintainedourinformaticstoolsinoperation,helpingwhenaproblemappeared. We thank I. Go´mez Leal and J.A. Belmonte for the task of reading critically thewholebook.F.Anguita,V.Bejar,C.Esteban,A.Garc´ıaMun˜oz,I.Ribas,J.A. Robles, J. Schneider and R.M. Zapatero-Osorio also have critically read different drafts of the book chapters and made valuable comments, advices and sugges- tions. Figures and data have also been kindly supplied by T. Bastian, C. Benn, E. Bo¨hm-Vitense, C. Bounama, R. Buser, V. Courtillot, C. Esteban, M. Harris, P. Hoffman, B.W. Jones, L. Kaltenegger, E. Kokubo, M. Kuchner, G. Laughlin, M. Livio, N. Loeb, D. Lo´pez, J.J. Lo´pez Moreno, K. Menou, R. Neumeyer, A. Oscoz, M. Perryman, E. Pilat-Lohinger, I. Ribas, J. Robles, K. Shingareva, M. Sterzik and D. Valencia. We thank the NASA ADS service, which provides a wonderfully efficient service to the scientific community. Archives of NASA and ESAhavealsobeenused. Anna Fagan helped to make this book readable in English. We alone, how- ever,beartheresponsibilityforitscontents.WethanktheSpringerstaff,especially Dr.HarryBlom,forhisconfidenceinourwork.Wealsoacknowledgetheexcellent workdonebyMsA.Srideviandherteam(SPi)duringtheproductionofthisbook. Finally,ourfamiliesshowedgreatpatienceandgaveustheirfullsupportduring thelengthyprocessofwritingthisbook,whichwededicatetothem. LaLaguna M.Va´zquez July2009 E.Palle´ P.MontanQe´sRodr´ıguez Acronyms AIM Aeronomyoficeinthemesosphere AU Astronomicalunit BCE Beforecommonera COROT Convectionrotationandplanetarytransits ELT Extremelylargetelescope E-ELT Europeanextremelylargetelescope ESA EuropeanSpaceAgency ESO EuropeanSouthernObservatory EUV Extremeultraviolet GOES GeostationaryOperationalEnvironmentalSatellite HST HubbleSpaceTelescope IAU InternationalAstronomicalUnion IMF Initialmassfunction INGRID IsaacNewtonGroupredimagingdevice IPMO Isolatedplanetary-massobjects LHB Lateheavybombardment MAHRSI MiddleHighResolutionSpectrographInvestigation NAOS Nasmythadaptiveopticssystem NASA NationalAeronauticsandSpaceAgency NEAR Nearearthasteroidrendezvous OWL Overwhelminglylargetelescope PAL Presentatmosphericlevel SeaWiFS Sea-viewingwidefield-of-viewsensor SOHO SolarandHeliosphericObservatory TIMED Thermosphereionospheremesosphereenergeticsanddynamics TIROS Televisioninfra-redobservationSatellite TP Terrestrialplanet TPF Terrestrialplanetfinder UV Ultraviolet VIRTIS VisibleandInfraRedThermalImagingSpectrometer VLT Verylargetelescope,ESO WHT WilliamHerscheltelescope ix x Acronyms Units 1(cid:2)mD1;000nmD10;000A˚ 1Astronomicalunit(AU)DMeanSun–EarthdistanceD149:60(cid:2)106km 1LightYear.ly/D9;461(cid:2)1012kmD63;241km 1Parsec.pc/D3;2616ly Ga.Gigayears/D109years Ma.Millionofyears/D106 years EarthMass.M /D5:9736(cid:2)1024kg E JupiterMass.M /D1;8996(cid:2)1027kg J SolarMass.M /D1;989(cid:2)1030kg S 1JouleD107ergs
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