Hot Moons and Cool Stars

0.46 MB·English

by  René Heller

EPJWebofConferenceswillbesetbythepublisher
DOI:willbesetbythepublisher
(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2013
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Hot Moons and Cool Stars
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RenéHeller1,a and RoryBarnes2,3,b

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P 1Leibniz-InstitutfürAstrophysikPotsdam(AIP),AnderSternwarte16,14482Potsdam,Germany
E 2UniversityofWashington,Dept.ofAstronomy,Seattle,WA98195
. 3VirtualPlanetaryLaboratory,USA
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Abstract. Theexquisite photometric precision of the Kepler space telescope now puts
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t thedetectionofextrasolar moonsat thehorizon. Here, wefirstlyreview observational
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andanalyticaltechniquesthathaverecentlybeenproposedtofindexomoons. Secondly,
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[ wediscusstheprospectsofcharacterizingpotentiallyhabitableextrasolarsatellites.With
moonsbeingmuchmorenumerousthanplanetsinthesolarsystemandwithmostexo-
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planetsfoundinthestellarhabitablezonebeinggasgiants,habitablemoonscouldbeas
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abundantashabitableplanets. However,satellitesorbitingplanetsinthehabitablezones
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ofcoolstarswillencounterstrongtidalheatingandlikelyappearashotmoons.
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1 Introduction
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v Theadventofhigh-precisionphotometryfromspacewiththeCoRoT andKeplertelescopeshasdra-
i maticallyincreasedthenumberofconfirmedandputativeextrasolarplanets.Beyondthesheernumber
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ofdetections,smallerandsmallerexoplanetswerefoundwiththetodayrecordbeingabout0.5Earth-
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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]
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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
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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
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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].
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