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EPJWebofConferenceswillbesetbythepublisher DOI:willbesetbythepublisher (cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2013 3 1 0 2 n Hot Moons and Cool Stars a J 2 RenéHeller1,a and RoryBarnes2,3,b ] P 1Leibniz-InstitutfürAstrophysikPotsdam(AIP),AnderSternwarte16,14482Potsdam,Germany E 2UniversityofWashington,Dept.ofAstronomy,Seattle,WA98195 . 3VirtualPlanetaryLaboratory,USA h p - o Abstract. Theexquisite photometric precision of the Kepler space telescope now puts r t thedetectionofextrasolar moonsat thehorizon. Here, wefirstlyreview observational s andanalyticaltechniquesthathaverecentlybeenproposedtofindexomoons. Secondly, a [ wediscusstheprospectsofcharacterizingpotentiallyhabitableextrasolarsatellites.With moonsbeingmuchmorenumerousthanplanetsinthesolarsystemandwithmostexo- 1 planetsfoundinthestellarhabitablezonebeinggasgiants,habitablemoonscouldbeas v abundantashabitableplanets. However,satellitesorbitingplanetsinthehabitablezones 5 ofcoolstarswillencounterstrongtidalheatingandlikelyappearashotmoons. 3 2 0 . 1 0 1 Introduction 3 1 : v Theadventofhigh-precisionphotometryfromspacewiththeCoRoT andKeplertelescopeshasdra- i maticallyincreasedthenumberofconfirmedandputativeextrasolarplanets.Beyondthesheernumber X ofdetections,smallerandsmallerexoplanetswerefoundwiththetodayrecordbeingabout0.5Earth- r a radii[1]. Thisachievementisofparamountimportanceforastrobiologicalinvestigations,asroughly Earth-sizedplanetsmaybeinhabited–providedmanyotherrequirementsaremet,ofcourse. Habitabilityofterrestrialplanetscanformallybedefinedasa planet’sabilitytoallow forliquid surfacewater. Thepresenceofliquidsurfacewaterwilldependontheplanet’sdistancetoitshoststar, amongstothers, with the adequatedistancerangespanningthe stellar habitablezone, dependingon theplanet’satmosphericcompositionandsurfacepressure[2,3]. Asoftoday,roughly50extrasolar planetcandidateshavebeenconfirmedintheKeplerdata,mostofwhicharemuchbiggerthanEarth. Theseplanetsarelikelyto begaseousandtoresembleNeptuneorJupiter,ratherthanEarth. While theyarenotlikelytobehabitable,theirmoonsmightbe. Referredtoexomoons,MartinStill,DirectoroftheKeplerGuestObserverOffice,saidinhistalk on the extendedKepler project during this meeting: “They are gonna come.” So how can massive extrasolar moonsbe detected, providedthey exist in the first place? And to which extentwill they possiblybecharacterized? ae-mail:[email protected] be-mail:[email protected] EPJWebofConferences Figure1. ThetectonicallymostactivebodyinthesolarsystemisJupiter’smoonIo. Itsenhancedvolcanismis drivenbytidaldissipationinsidethesatellite.Thetwoinletsintheleftimage,takenbytheGalileoOrbiter,show asulfuricplumeoveravolcanicdepressionnamedPillanPatera(upperphotograph)andanothereruptioncalled thePrometheusplume(lowerphotograph).Therightimageshowsthestrongorangeinfraredemissionofflowing lavainTvashtarCatena,achainofcalderasonIo. Inextrasolarmoons,tidalheatingmaybemuchstrongerand evenmakethemdetectableviadirectimaging.(Imagecredits:NASA/JPL) 2 Detection methods 2.1 Transittiminganddurationvariationsoftheplanet Two of the most promising techniques proposedfor finding exomoonsare transit timing variations (TTVs) and transit duration variations (TDVs) of the host planet. Combination of TTV and TDV measurementscanprovideinformationaboutasatellite’smass,itssemi-majoraxisaroundtheplanet [4–6],andpossiblyabouttheinclinationofthesatellite’sorbitwithrespecttotheorbitaroundthestar [7]. ThefirstdedicatedhuntforexomoonsintheKeplerdataisnowunderway[8]andcouldpossibly detect exomoonswith masses down to 20% the mass of Earth [9]. This correspondsto roughly10 timesthemassofthetwomostmassivemoonsinthesolarsystem,GanymedeandTitan. 2.2 Directobservationsofthemoon Observationsofanexomoontransititself[10–13]aswellasplanet-satellitemutualeclipses[14,15] canprovideinformationaboutthesatellite’sradius.Spectroscopicinvestigationsofamoon’sRossiter- McLaughlineffectcanyieldinformationaboutitsorbitalgeometry[16,17],althoughrelevanteffects requireaccuraciesinstellarradialvelocityoftheorderofafewcm/s. Moonsinthestellarhabitablezoneoflow-massstarsmustorbittheirhostplanetverycloselyto remaingravitationallybound[18,19]. Thiswilltriggerenhancedtidalheatingonthosehypothetical moonsandcouldmakethemuninhabitable.Whileathreattolife,enormoustidalheatinginterrestrial moonsaboutgiantplanetscouldbe strongenoughto make themdetectableby directimaging[20]. Tidal heating in moons has been observed in the solar system, with Jupiter’s moon Io serving as the most prominent example (see Fig. 1). As tidal heating in a satellite is proportionalto the host HotPlanetsandCoolStars 7 6 0.1 001 1 3 5 4 .3 =0. 3 0 eps x 1 e u 0 s l . u f 0 greenhosorbed e=ps =0.1 10 ay ab eps w s 2 a ’ 3 n s u h rEart 100 6 1 3 0 0 0 1 Figure2. Left: Totaltop-of-the-atmosphereflux(inlogarithmicunitsofW/m2)ofaMars-sizedmoonabouta Neptune-sizedplanetinthehabitablezoneofa0.25M star.Tidalheatingincreaseswithdecreasingsemi-major ⊙ axisa (abscissa)andincreasingeccentricitye (ordinate).Someexamplesfororbitalelementsofsolarsystem ps ps moonsareindicated.Right:Amplitudeofthetransittimingvariation(dashedlines)foraMass-sizedmoonabout arangeofhostplanets. PlanetarymassesareshowninEarthmassesontheordinate. Thehabitableedge[21]is indicatedforthreedifferentorbitaleccentricitiese ofthesatellite:0.1,0.01,and0.001. ps planet’smasscubed,massiveplanetsprovidethemostpromisingtargetsfordirectimagingdetections oftidallyheatedexomoons. 3 Characterizing exomoonsin the stellar habitable zone A massive moon in the stellar habitable zone can be subject to strong tidal heating and hence be uninhabitable. Toestimateitshabitability,wehavesetupamodelthatincludesstellarandplanetary illuminationaswellastidalheating[21].Iftheirsumisgreaterthanthecriticalfluxforthemoontobe subjecttoarunawaygreenhouseeffect[22],themoonwillloseallitswaterandbecomeuninhabitable. IntheleftpanelofFig.2weshowcontoursoftheorbit-averagedilluminationplustidalheatfluxof a hypotheticalMars-sized moonorbitinga Neptune-sizedplanetin the habitablezone of a 0.25M ⊙ star. Abscissaindicatestheplanet-satellitesemi-majoraxisinplanetaryradii,ordinateshowsorbital eccentricity.Inthereddishregionsthisprototypesatellitewillbedesiccatedanduninhabitable. Givensufficientlylongobservationalcoverageandhigh-accuracydata, thetechniquesdescribed inSect.2makeitpossibletodetectandconsiderablycharacterizeasub-Earth-sizedmoonorbitinga giantplanet. IntherightpanelofFig.2weshowtheTTVamplitudesofaMars-sizedmoonorbiting arangeofhostplanets. Itisassumedthatthemoon’sorbitiscircularandthatboththecircumstellar andthecircum-planetaryorbitareseenedge-onfromEarth[8]. Todetermineasatellite’smassandorbitviaTTVandTDV,manytransitsofthehostplanetneed tobeobserved.Thus,tofindmoonsaboutplanetsinthestellarhabitablezonewithintheKeplerduty cycleof7years, onemightbetemptedtoconcludethattheycanpreferablybedetectedin coolstar EPJWebofConferences systems(i.e. aroundMdwarfs),becausetheirhabitablezonesareclose-bywhereaplanetperforms potentiallymanytransitsinagiventimespan.However,theamplitudeofaplanet’sTTVissmallerin coolstarsystems,givenafixedsemi-majoraxis[10],andthelackofMdwarfsintheKeplersample [23]furtherdecreasesthechanceoffindinghabitablemoonsincoolstarsystems. Beyondthat,moonsofplanetsinthehabitablezonesofcoolstarsmightnotbehabitableinthefirst place[18]. Theplanet’ssphereofgravitationaldominance,i.e. itsHillsphere,isrelativelysmalldue totheclosestar. Hence,anymoonwouldhavetofollowaverytightorbitabouttheplanet.Moreover, the close star will force the satellite’s orbit to be non-circular. Both aspects, small orbital distance and an eccentric orbit, will cause any Earth-sized moon of a massive gaseous planet to experience enormoustidalheating. Ultimately, stellar irradiationandtidalheatingwill sum upto a top-of-the- atmosphereenergyfluxthatexceedsthecriticalfluxfortheinitiationoftherunawaygreenhouseeffect [21]. Moonsofplanetsinthehabitablezonesofcoolstarswillthusbehotratherthanhabitable. Tocharacterizepotentiallyhabitablemoons,thatistosay,moonsinthehabitablezonesofKand G typestars, usingthe TTV andTDV techniqueswilltake abouta decadeof observations,atleast. WiththeKeplermissionbeingscheduledforatotalmissioncycleof7years,suchadetectionmight justbeattheedgeofwhatispossible[8–10]. References [1] P.S. Muirhead, J.A. Johnson, K. Apps, J.A. Carter, T.D. Morton, D.C. Fabrycky, J.S. Pineda, M.Bottom,B.Rojas-Ayala,E.Schlawinetal.,ApJ747,144(2012),1201.2189 [2] J.F.Kasting,D.P.Whitmire,R.T.Reynolds,Icarus101,108(1993) [3] F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. 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