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Astronomy&Astrophysicsmanuscriptno.55Cnc˙vf (cid:13)c ESO2012 January16,2012 Improved precision on the radius of the nearby super-Earth ⋆ 55Cnce Michae¨lGillon1,Brice-OlivierDemory2,Bjo¨rnBenneke2,DianaValencia2,DrakeDeming3,SaraSeager2,Christophe Lovis4,MichelMayor4,FrancescoPepe4,DidierQueloz4,DamienSe´gransan4,Ste´phaneUdry4 1Institutd’AstrophysiqueetdeGeóphysique,Universite´deLie`ge,Alleédu6Aouˆt17,Bat.B5C,4000Lie`ge,Belgium 2 Department of Earth, Atmospheric and Planetary Sciences, Department of Physics, Massachusetts Institute of Technology, 77 MassachusettsAve.,Cambridge,MA02139,USA 3DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742-2421,USA 2 4ObservatoiredeGene`ve,Universite´deGene`ve,51ChemindesMaillettes,1290Sauverny,Switzerland 1 0 2 Receiveddate/accepteddate n ABSTRACT a J Wereportonnewtransitphotometryforthesuper-Earth55CnceobtainedwithWarmSpitzer/IRACat4.5µm.Anindividualanalysis 3 ofthesenewdataleadstoaplanetradiusof2.21+0.15 R ,whichagreeswellwiththevaluespreviouslyderivedfromtheMOSTand 1 −0.16 ⊕ Spitzertransitdiscoverydata.AglobalanalysisofbothSpitzertransittime-seriesimprovestheprecisionontheradiusoftheplanetat 4.5µmto2.20±0.12R⊕.WealsoperformedanindependentanalysisoftheMOSTdata,payingparticularattentiontotheinfluence ] ofthesystematiceffectsofinstrumentaloriginonthederivedparametersanderrorsbyincludingtheminaglobalmodelinsteadof P performingapreliminarydetrending-filteringprocessing.Wededuceanopticalplanetradiusof2.04±0.15R fromthisreanalysisof E ⊕ MOSTdata,whichisconsistentwiththepreviousMOSTresultandwithourSpitzerinfraredradius.Assumingtheachromaticityof h. thetransitdepth,weperformedaglobalanalysiscombiningSpitzerandMOSTdatathatresultsinaplanetradiusof2.17±0.10R⊕ p (13,820±620km).Theseresultspointto55Cncehavingagaseousenvelopeoverlyingarockynucleus,inagreementwithprevious - works.Aplausiblecompositionfortheenvelopeiswaterwhichwouldbeinsuper-criticalformgiventheequilibriumtemperatureof o theplanet. r t Keywords.binaries:eclipsing–planetarysystems–stars:individual:55Cnc-techniques:photometric s a [ 1. Introduction eraltransitsof55CncewiththeMOSTsatelliteandreporteda 2 v planetradiusconsistentwithours,2.00±0.14R⊕ (W11).Soon Transitingplanetsareoffundamentalinterestforthefieldofexo- 3 afterthisdoubletransitdetection,athirdteamreportedimpres- planetaryscience.Theiradvantageousgeometricalconfiguration 8 sivelyprecisevaluesforthehoststar’sparametersbasedonnew relative to Earth enables the thorough study of their physical, 7 interferometric observations (von Braun et al. 2011, see Table orbital and atmospheric properties, provided they orbit around 4 1).Thankstotheselastresults,madepossiblebythebrightness 0. starsbrightenoughtopermithighsignal-to-noisefollow-upob- of the star, our knowledge of the mass and size of 55Cnce is servations. For a given stellar type, this last condition is dras- 1 onlylimitedbytheprecisionofthe radialvelocitiesandtransit tically morestringentfor terrestrialplanets than for gasgiants, 1 photometrygatheredsofar. leadingto the conclusionthat onlythe handfulof solid planets 1 Aiming to pursue the characterization of this fascinating thatshouldtransitstarsoftheclosestsolarneighborhoodwould : v besuitableforathoroughcharacterizationwithexistingorfuture planet, we monitoredanother of its transits with Spitzer in our Xi instruments(e.g.Seageretal.2009). program dedicated to the search of nearby transiting low-mass planets (ID 60027). In the next two sections, we present these Inthiscontext,twoteams,includingours,independentlyan- r newdataandtheiranalysis,includingaglobalanalysisofMOST a nounced the first transit detection for a solid planet orbiting a andSpitzerphotometrictime-seriesaimingtodeterminethesize nearbystarvisibletothenakedeye(Winnetal.2011,hereafter oftheplanetaspreciselyaspossible.Wediscussourresultsand W11;Demoryetal.2011,hereafterD11).Thistransiting‘super- theirimplicationsinthelastsectionofthepaper. Earth’, 55Cnce, is the inner most of the five planets currently knowntoorbitaround55Cancri,aG8-K0dwarfstarlocatedat only41light-yearsfromEarth(seeD11andreferencestherein). We detected one of its transits with Spitzer, allowing us to de- ducearadiusof2.08+0.17R andamassof7.81+0.58 M forthe 2. NewWarmSpitzer transitphotometry −0.16 ⊕ −0.53 ⊕ planet(D11).Together,these valuesfavora solid planetwitha Wemonitored55CncwithSpitzeron20June2011from09h08 significantfractionofice.Ontheirside,Winnetal.detectedsev- to 15h02 UT, corresponding to a transit window of 55Cnce Sendoffprintrequeststo:[email protected] as computedfromourtransitephemerispresentedin D11.The ⋆ The photometric time series used in this work are available in IRACdetectoracquired6230sets of64subarrayimagesat4.5 electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr µmwithanintegrationtimeof0.01s.These6230setswerecal- (130.79.128.5)orviahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ ibrated by the Spitzer pipeline version S18.18.0 and are avail- 1 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce phase’ effect (see Fig. 1), which we added as needed to one or several functions of time. At the end, more than 30 models weretested,theonecorrespondingtothehighestmarginallike- lihoodhasasbaselineasecond-orderpositionpolynomialadded toafourth-ordertimepolynomialandtoasinusoid.Becausethe Bayesfactor(e.g.Carlin&Louis2008)betweenthismodeland the second most likely model is about 100, we selected it for sampling the posterior probability density distributions of the transitparameters.WeperformedanewMCMCanalysisforthis purposecomposedoftwochainsof100,000steps,andchecked their convergenceusing the statistical test of Gelman & Rubin (1999).Table 2 providesthe resultingvaluesand errorsfor the transitparameters,whileFig.2showsthebest-fitglobalmodel superimposedonthedataandthebest-fittransitmodelsuperim- posedonthedatadividedbythebest-fitbaselinemodel. ComparingtheresultingtransitparametersshowninTable2 tothosederivedinD11,wenoticethatthederivedtransitdepths agree at better than 1σ (463+64 ppm here vs 410±63 ppm for −57 D11) but that the agreementfor the derivedimpact parameters is only of ∼2.8 σ (0.509+0.056 here vs 0.16+0.10 for D11). We −0.074 −0.13 suspectthatthisdiscrepancycomesmostlyfromtheinstrumen- tal effect that affected our first transit data. Indeed, in D11 we hadtomodelasharpincreaseoftheeffectivegainofthedetec- tor during the run that was correlated to a strange behavior of Fig.1.Topleft:rawlightcurveobtainedfor55Cnc withWarm thebackgroundcounts.Weencounteredthissystematiceffectin Spitzer at 4.5 µm. Top right: corresponding background time- other Warm Spitzer time-series acquired in our program60027 series. Bottom: correspondingtime-series for the x (left) and y (Gillon et al., in prep.). Unfortunately,this effect occurredjust (right) positions of the stellar center. The correlation between duringtheegressofthetransit,andwesuspectthatitcouldhave measuredstellarcountsandthestellarimagepositionsisclearly biased the derived marginalized posterior distribution function noticeable.This‘pixel-phase’effectiswell-knownfortheInSb forthetransitimpactparameter. Spitzer/IRACdetectors(e.g.Knutsonetal.2008). The derivedperiodand amplitude forthe sinusoid function ofthebaselinemodelare59±2minand107±24ppm,consistent ableontheSpitzerHeritageArchivedatabase1undertheformof withthevaluesderivedinD11forthefirstSpitzertransit,51±3 minand115±27ppm.Theoriginofthislow-amplitudeperiodic Basic CalibratedData (BCD) files. Our reductionof these data effectisstillunclear(seeD11). wasidenticaltotheonepresentedinD11andwereferthereader tothispaperfordetails.Fig.1showstheresultingrawlightcurve composedof 6197flux measurements,and also the time-series 3.2.GlobalanalysisofWarmSpitzerdata for the backgroundcountsand the x and y positionsof the tar- get’spoint-spreadfunction(PSF)onthedetectorarray.We can Aiming to minimize the impact of Warm Spitzer instrumental notice from Fig. 1 that the backgroundcounts remained stable effectsandtoobtainthestrongestconstraintsontheplanet’sin- duringthewholerun,unlikeduringourfirsttransitobservation fraredradius,wethenperformedaglobalMCMCanalysisusing (seeD11,Fig.1). asinputdatathetwoSpitzerlightcurves.Weassumedforbothof themthesamebaselinemodelasusedfortheirindividualanaly- sis,withtheexceptionthatwedidnotfixthestarttimeoftheef- 3. Dataanalysis fectivegainincreaseforthefirsttransit,butinsteadletitbeafree 3.1.IndividualanalysisofthenewWarmSpitzerdata parametertotakeintoaccountourlimitedunderstandingofthis systematiceffect.TheGaussianpriorP=0.7365437±0.000052 In a first step, we performedan individualanalysis of our new daysbased ontheRV data analysispresentedin D11wasused data. We used for this purpose our adaptative Markov-Chain fortheorbitalperiodofthe planet.TheMCMC wascomposed MonteCarlo(MCMC)code(seeD11andreferencesthereinfor oftwochainsof100,000steps. details). Gaussian priorsassumed for the stellar parametersare Thebest-fittransitmodelandthedetrendedlightcurvesare shown in Table 1. Uniform priors were assumed for the other visibleinFig.3.Table2presentstheresultingvaluesanderrors parametersofthesystem.First,weperformedathoroughmodel forthetransitparameters.Wenoticefromthistablethatinclud- comparison,performingforeachtestedmodelaMCMCanalysis ing the first transit as input data improves the global precision composedofonechainof10,000steps andderivingthe model on the planet’s size, but not on the transit duration/impact pa- marginallikelihoodfromtheMCMCoutputsusingthemethod rameter,becauseofthedegeneracybetweenthetransitduration described by Chib & Jeliazkov (2001). Each model was com- andtheincreaseoftheeffectivegainthataffectedthefirsttime- posedofabaselinerepresentingthelow-frequencyinstrumental series. andstellareffectsmultipliedbyatransitmodelcomputedunder the formalism of Mandel & Agol (2002) assuming a quadratic limb-darkeninglaw.Thebaselinemodelsincludedan x- andy- 3.3.IndependentanalysisoftheMOSTdata positionpolynomial(D11,eq.1)tomodelWarmSpitzer‘pixel- To assess the consistency of our Spitzer data with the MOST 1 http://sha.ipac.caltech.edu/applications/Spitzer/SHA photometry,weperformedourownanalysisoftheMOSTtime- 2 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce fitted it to the data, we analyzed the residuals and performed a 10-sigmaclippingto reject outliers.The resultinglightcurve wasthenbinnedwithintervalsof5minforthesakeofcompu- tationalspeed,resultingin3,232fluxmeasurements. We thenbeganto iterate on modelsto representthe data in thebestway.Wenoticedthatthecorrelationofthefluxesandthe positionscouldbesatisfactorilymodeledbyanx-andy-position polynomial. We also noticed a strong dependence of the mea- suredfluxeswiththebackgroundcountsandthemagneticfield which we were also able to model with polynomial functions oftheseexternalparameters.Oncedetrendedfromtheposition, magneticfieldandbackgroundeffects,thedatarevealedaftera Lomb-Scargleperiodogramanalysis (Press et al. 1992)signifi- cantpowerpeaksat∼1.7hour,∼0.94daysand∼0.72days,with false-alarmprobabilities< 10−11 forthethreeperiods.Thefirst period correspondsto the orbital period of the MOST satellite (W11),whilethesecondandthirdareclosetotherotationperiod oftheEarthand,interestingly,totheorbitalperiodof55Cnce, respectively.Afterfilteringtheresidualsfromtheseperiodicsig- nalsbyfittingsinusoidsatthecorrespondingperiods,theresult- ing light curve showed power excess at lower frequencies. We averagedthembydividingthelightcurveintofivelightcurves coveringnearlyequaldurations,whichweanalyzedglobally,as- sumingadifferentbaselineforeachofthem. Fig.2. Top: raw light curve for the second transit of 55Cnce We then performed a thorough model comparison as de- observedbyWarmSpitzer,withthebest-fitglobalmodelsuper- scribed above for Spitzer data to select the best model to rep- imposed. Middle: same light curve after division by the best- resentthefiveresultinglightcurves.Thebaselinemodelthatwe fitbaselinemodel,withthebest-fittransitmodelsuperimposed. selected contained a third-order x- and y-position polynomial, Bottom:idemafterbinningperintervalsof5min. afourth-orderbackgroundpolynomial,afourth-ordermagnetic fieldpolynomial,afourth-ordertimepolynomial,onesinusoidat thesatelliteorbitalperiodandoneataperiodof∼0.94days.The seriesbasedonadifferentstrategyfromthatusedbyW11.These transitsof55CnceweremodeledwiththeformalismofMandel authors analyzed the MOST data in two steps. First, they cor- & Agol (2002) assuming a quadratic limb-darkening law and rectedtheirphotometryfromseveralinstrumentaleffectsknown using the Gaussian priors on the coefficients u and u shown 1 2 to affect MOST photometry. In a second step, they folded the inTable1.Inaddition,wealso introduceda modelfortheflux correctedlightcurveontheorbitalperiodof55Cnce,binnedit modulationattheorbitalperiodof55Cnce.Werepresentedthis totimeintervalsof2min,andperformedaMCMCanalysisof ‘phasecurve’withasimplemodeldividingtheplanetintofour the resulting light curve. To be consistent with our analysis of equalslicesandassumingforeachofthemauniformluminosity the Spitzer data, we decided on our side to use the raw MOST anda1:1spin-orbitresonancefortheplanet.Foreachslice,the lightcurve(J.Winn,privatecom.)asinputdataandtoincludea fluxmodulationwasmodeledbyasimplesinusoidwithitsmax- modelforthesystematiceffectsaffectingMOSTphotometryin imum corresponding to the center of the slice pointing toward our globalmodeling.This strategy hasthe advantageto ensure Earth. We emphasize that our goal here was to select a model thepropererrorpropagationfromthebaselinemodelparameters that satisfactorily represents the flux modulation without wor- tothetransitparameterswhileavoidinganybiasofapreliminary ryingaboutits physicalrelevance.Indeed,theamplitudeofthe detrending/pre-whiteningprocessonthederivedresults. observedfluxmodulationismuchtoolargetobeattributedtothe Wethusstartedfromthe27,950photometricmeasurements variableilluminatedfractionoftheplanetortoitsthermalemis- gatheredbyMOSTbetween07Feb201100h15to22Feb2011 sion,asoutlinedbyW11.We tested introducinganoccultation 00h05UT.Foreachphotometricmeasurement,wealsohadthe inourglobalmodel,butitdidnotimprovethemodelmarginal corresponding background counts, PSF center position on the likelihood,consequentlywe discarded it from our final model- MOST CCD, a calculatedmagneticfield,andseveralotherex- ing. ternal parameters. Following W11, we rejected measurements Inourcomplexmodel,thebest-fitresidualsofthefivelight obtainedwithadifferentexposuretimefromthebulkofthedata, curvesstillshowasmallamountofcorrelatednoisethatwetook 41.82s.Wealsorejectedmeasurementswithanx-ory-position intoaccountinthesamewayasforSpitzerdatabyrescalingthe off by more than 0.5 pixelfrom the median of the correspond- measurementserrors.Atthisstage,weperformedaglobalanal- ingdistributions.Weindeednoticedastrongdependanceofthe ysisofourfivelightcurvesandperformedforthispurposetwo measuredfluxestothePSFcenterpositions,sowechosetodis- MCMCchainsof100,000steps,whoseconvergencewesuccess- card discrepant measurements in terms of position to improve fullycheckedforthetransitparametersusingthestatisticaltest ourchancestomodelthepositioneffectwithasimpleanalytical of Gelman& Rubin(1999).Table 2providestheresultingval- function.We also rejected outlier measurementswith a magni- ues and 1-σ error bars for the transit and physical parameters, tudeoffbymorethan0.01magcomparedtothemedianvalue. whileFig.4displaysthebest-fitmodels(global,phasecurve+ We then tested several models to represent the data satisfacto- transits,transit)superimposedonthecorrespondinglightcurves. rily, our goal being at this stage not to find an optimum model WenoticethatourresultsareconsistentwithW11’sresults, buttoidentifydiscrepantmeasurementscausedbytransientef- theagreementbeingatbetterthan1-σforthetransitdepthsand fects(e.g.cosmicrays).Oncewehadselectedsuchamodeland ∼1.5-σforthetransitimpactparameters.Consideringthelarge 3 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce Mass[M ] 0.905±0.0151 sition that yields the smallest radius and a magnesium-silicate ⊙ Radius[R ] 0.943±0.0101 oxide composition (devoid of iron) that yields the largerst ra- ⊙ dius. Owing to the fact that iron, magnesium, and silicate are T [K] 5196±241 eff allrefractoryelementswith similar condensationtemperatures, Metallicity[Fe/H][dex] +0.31±0.042 planetsareunlikelytoformwitheitheroftheseextremecompo- Limb-darkeninglinearparameteru 0.0711±0.00093 sitions.TwoplausiblecompositionswelookedatareEarth-like 1,4.5µm Limb-darkeningquadraticparameteru 0.1478±0.00203 composition(33%ironcoreaboveasilicatemantlewith10%of 2,4.5µm ironand90%ofsilicatebymol)andaniron-enrichedcomposi- Limb-darkeninglinearparameteru 0.657±0.0904 1,MOST tion(63%ironcoreaboveaMg-Simantle,withnoiron). Limb-darkeningquadraticparameteru 0.115±0.0454 2,MOST We find that 55Cnce is too large to be made out of just Table 1. Gaussian priors assumed in this work for the stellar rocks despite its relative high bulk density of ρ = 4.0+−00..53 g parameters. 1von Braun et al. 2011, 2Valenti & Fischer 2005, cm−3 (e.g. Earth’s bulk density is ρ⊕ = 5.5 g cm−3) obtained 3Claret&Bloemen2011,4W11. with the radius reportedin this study. Therefore,it has to have anenvelopeofvolatiles.We considertwo compositionsforthe gaseous envelope: a hydrogen and helium mixture, and a pure watervaporcomposition(whichatthesetemperaturesissuper- differencebetweenbothanalyses,notablythedifferenttreatment critical). We added different amounts of envelope to an Earth- ofsystematiceffects,thisagreementisreassuringfortherobust- likenucleus.Theresultsshow(seeFig.5)thatthedatamaybe nessoftheresultinginferences.Asdescribedabove,wedivided fitted with a H-He envelope of ∼0.1% by mass or a H O en- 2 theMOST dataintofivelightcurves,whichweretreatedsepa- velope of ∼20% by mass. As described in D11, based on sim- ratelyfromtheothersinourglobalMCMCanalysis,exceptfor pleatmosphericescapecalculationsdescribedinValenciaetal. the transit parameters.The free parametersfor the phase curve (2010), this low-mass envelope of H-He would escape in Myr model, i.e. the amplitudes of the sinusoid of each planet slice, timescales, whereas a water-vapor envelope would escape in werethusdifferentforthefivelightcurves.Interestingly,Fig.4 billions of years timescales. Thus, the favored composition for showsthatthefivebest-fitphasecurvemodelsshowsomevari- 55Cnce is a volatile planet with a water dominated envelope ability(seediscussioninSec.4.2). comprisingtensofpercentofthetotalmassoftheplanet. Theradiusobtainedinthisstudyislargerthanthatreported 3.4.GlobalanalysisMOST+Spitzerdata by D11 and above a one-sigma level of W11 value, ruling out a rocky composition for 55Cnce. An interesting characteristic AscanbenoticedinTable2,ourindependentanalysisofSpitzer of this planet is that it has an intermediate composition (see and MOST data led to consistentresults andsimilar precisions Fig. 6). While most of its mass is bound in a rocky nucleus, on the planet radius. For both instruments, the limiting factor it has a non-negligible (most likely high-molecular) envelope. on the transit depth precision is not the white noise associated This lies between the composition of GJ1214b, which proba- withthefluxmeasurementsbutthehighlevelofsystematicef- bly consistsmostlyof water (see Valenciaet al. 2011),andthe fectsthataffectthephotometry.Aimingtominimizetheimpact terrestrial planets in our solar system and exoplanets such as ofthesesystematicsandtoimprovetheprecisiononthetransit’s ‘super-Mercuries’ CoRoT-7b (Hatzes et al. 2011) and Kepler- shape and the planet’s size even more, we performed a global 10b(Batalhaetal.2011). analysis of MOST and Spitzer data, assuming that the transit depth is exactly the same in both channels, i.e. that chromatic atmospherictransmissioneffectsarenotsignificantatthislevel 4.2.Potentialatmosphericstudiesof55Cnce ofphotometricprecision.Thisisanentirelyreasonableassump- Follow-up observations made to detect the spectral signature tion,consideringtheexpectedsmallatmosphericscaleheightof of the planetary atmosphere may bring new constraints on the theplanet(seeSec.4.2formoredetails).Theusedpriors,base- chemical composition of 55Cnce. The planet is suitable for linemodelsandanalysisdetailswerethesameasintheseparate theseobservations,becauseitorbitsaroundaverybright,nearby analysis described above. Table 2 presents the resulting values staratanextremelyclosedistance.Still,dependingonthenature and1-σerrorbarsforthetransitandphysicalparameters. oftheplanetaryatmosphere,follow-upobservationscanbe ex- Finally,weassessedthevalidityoftheassumptionthatboth tremelychallengingduetotheshallowtransitdepth. channels probe the same transit depth by performing a new global analysis of MOST and Spitzer data, this time adding as Atmospheric modeling (Fig. 7) suggests that transmission freeparameteradifferenceintransitdepthbetweenthetwoin- signaturesofanatmosphereinhydrostaticequilibriumcouldbe struments.ThederivedtransitdepthdifferenceSpitzer-MOST ontheorderof100ppm,iftheplanethasbeenabletoaccreteand was94±80ppm,i.e.theinfraredradiusoftheplanetisconsis- retainahydrogen-dominatedatmosphere.Suchsignaturescould tentwiththeopticalradius,asalreadydeducedfromtheindivid- bedetectedorexcludedwithcurrentlyavailablespace-basedin- ualanalysisofthedataofbothinstruments.Theotherdeduced strumentation. However, the short evaporation timescale for a transitparameterswereverysimilartothoseshowninTable2. H-Heenvelopestronglydisfavorsthisscenario,asstatedabove. The most probable scenario implies an envelope mostly com- posedofwaterandothericeswhichwouldresultinmuchweaker 4. Discussion transmission signatures on the level of tens of ppm due to the highermeanmolecularmass(Fig.7).Adirectdetectionofsuch 4.1.Thecompositionof55Cnce a water/ices dominated atmosphere on 55Cnce will probably Toinferthecompositionoftheplanetweusedtheinternalstruc- have to wait until next generation instruments on-board JWST turemodelbyValenciaetal.(2006,2010)andconsideredrepre- becomeavailable. sentativecompositionsforrockyandvolatileplanets.Therange Stronger transmission signatures are plausible, if one hy- in radii for rocky planets are delimited by a pure iron compo- pothesizes that the planet is surrounded by a low-density halo 4 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce Fig.3. Left: Warm Spitzer 55Cnc light curves (1 and 2: individual transits, 1+2: combined light curve) divided by their best- fit baseline models deduced from their global MCMC analysis, binned to intervals of 5 minutes, with the best-fit transit model superimposed.Right:residualsofthefitbinnedtointervalsof5 minutes.For bothpanels,two time-serieswereshiftedalongthe y-axisforthesakeofclarity. Parameter Spitzertransit2 Spitzertransit1&2 MOST Spitzer+MOST Unit TransittimingTtr 5733.0094+−00..00001121 5733.0085+−00..00001114 5607.0584+−00..00001167 5733.0087+−00..00001131 BJDTDB−2450000 OrbitalperiodP 0.7365437(fixed) 0.7365460+0.0000049 0.7365437±0.0000052 0.7365449+0.0000046 days −0.0000046 −0.0000050 Transitdepth(Rp/R∗)2 463+−5574 458±47 394+−6511 447+−4308 ppm Planet-to-starradiusratio(Rp/R∗) 0.0215±0.0013 0.0214±0.0011 0.0198+−00..00001153 0.02113+−00..0000009931 Transitcircularimpactparameterb 0.509+−00..005764 0.500+−00..005875 0.44+−00..1116 0.459+−00..007864 R∗ TransitdurationW 0.0589+0.0026 0.0593+0.0029 0.0612±0.0039 0.0607+0.025 days −0.0023 −0.0023 −0.028 Orbitalinclinationi 81.7+1.2 81.8+1.4 82.8+2.6 82.5+1.4 deg −1.0 −1.0 −1.8 −1.3 PlanetradiusRp 2.21+−00..1156 2.20±0.12 2.04+−00..1154 2.173+−00..009978 R⊕ Table2.Medianand1-σlimitsofthemarginalizedposteriordistributionsobtainedfortheparametersof55CncefromourMCMC analysisoftheWarmSpitzerdata. ofatomicgasresultingfromatmosphericescape.Boththesmall lifetime of these evaporated gases would modulate the opacity Rochelobeandthehighequilibriumtemperaturefavorastrong of this cloud along the orbit. Furthermore, the produced ionic atmosphericescapeandthelostatmosphericmasscouldberead- species would rotate with the star magnetic field, leading to a ilyreplenishedbyevaporationoftheplanet’ssurfaceoroceans modulationoftheresulting‘phasecurve’attherotationalperiod (Yelle et al. 2008, Schaefer & Fegley 2009, Ehrenreich 2010). ofthestar,42.7±2.5days(Fischeretal.2008),whichisconsis- The evaporated gas should be readily dissociated in the high- tentwiththeapparentvariabilityofthefittedphasecurvemodel temperaturehalo.TheresultingatomicspecieslikeC,H,O,Na, in our analysis (Fig. 4, middle panel). More work is needed to MgandCacouldbedetectableintransmissionbecauseoftheir assesstheplausibilityofthishypothesis. strong absorption cross sections near their electronic transition Because of its extreme proximity to its host star and the lines and the large extent of the halo (e.g. Mura et al. 2011). resulting incident flux of ∼3.3 109 erg s−1 cm−2, 55Cnce’s The exosphereof 55Cnce could also explain the flux modula- thermal emission could be measured through infrared occulta- tionattheplanet’sorbitalperioddetectedbyW11andourown tionphotometry.WithrealisticassumptionsfortheBondalbedo analysisofMOSTdata.Becausetheamplitudeofthismodula- and heat distribution efficiency of the atmosphere, occultation tion is too large for the planet’s thermal emission or reflected depthsrangingbetween90and150ppmareexpectedat4.5µm. light, W11 hypothesizedthe inductionby the planetof a patch Thephotometricprecisiondemonstratedhereshowsthatalow- of enhancedmagnetic activity on the star. Anotherexplanation amplitudeeclipselikethiscouldbedetectedbySpitzer,provided isthatapartofthegasesescapedfromtheplanet’satmosphere several events are monitored. This is the goal of our accepted formsacircumstellardiskalongtheplanet’sorbitsimilartoIo’s Spitzerprogram80231,andwearewaitingeagerlyforthesefu- donut-shapedcloud(e.g.Schneider&Bagenal2007).Theshort tureobservationstolearnmoreaboutthisfascinatingplanet. 5 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce Fig.4.Top:55Cnc MOSTraw photometrywith ourbest-fitglobalmodelsuperimposed. Middle:MOST photometrydividedby thebest-fitbaselinemodel,andwiththebest-fittransit+phase-curvemodelsuperimposed.Bottom:MOSTphotometrydividedby thebest-fitbaseline+phasecurvemodel,foldedwiththebest-fitorbitalperiodof55Cnceandbinnedper5minintervals,withthe best-fittransitmodelsuperimposed. Acknowledgements. WearegratefultoJ.Winnforhavingprovideduswiththe Chib,S.&Jeliazkov,I.2011,JASA,96,453 MOSTphotometry.Thisworkisbasedinpartonobservations madewiththe Claret,A.&Bloemen,S.2011,A&A,529,A75 Spitzer Space Telescope, which is operated bythe Jet Propulsion Laboratory, Demory,B.-O.,Gillon,M.,Deming,D.,etal.2011,A&A,533,114(D11) CaliforniaInstituteofTechnologyunderacontractwithNASA.Supportforthis Ehrenreich,D.2010,EASPublicationSeries,41,12 work was provided by NASA. We thank the Spitzer Science Center staff for Fischer,D.A.,Marcy,G.W.,Butler,R.P.,etal.2008,ApJ,675,790 efficientschedulingofourobservations.M.GillonisFNRSResearchAssociate. Gelman,A.&Rubin,D.1992,StatisticalScience,7,457 Hatzes,A.P.,Fridlund,M.,Nachmani,G.,etal.2011,ApJ,743,75 Knutson,H.A.,Charbonneau,D.,Allen,L.A.,etal.2008,ApJ,673,526 References Mandel,J.&Agol.E.2002,ApJ,580,171 Mura,A.,Wurz,P.,Schneider,J.,etal.2011,Icarus,211,1 Batalha,N.M.,Borucki,W.J.,Bryson,S.T.,etal.2011,ApJ,729,27 Press, W. H., Flannery, B. P., Teukolsky, S. A. & Vetterling, W. T. 1992, Benneke,B.&Seager,S.2011,ApJ,submitted NumericalRecipesinFortran,CambridgeUniversityPress,2ndedition Carlin,B.P.&Louis,T.A.2008,BayesianMethodsforDataAnalysis,Third Schaefer,L.&Fegley,B.2009,ApJ,703,L113 Edition(Chapman&Hall/CRC) 6 M.Gillonetal.:Improvedprecisionontheradiusofthenearbysuper-Earth55Cnce 20000 m]550 p h [p500 ept450 D 55Cnc-e, this study nsit 400 a (1) (2) Tr350 km) 20% H2O 0.4 0.5 0.6 0.70.80.91Wavelength [µm] 2 3 4 5 dius (10000 C-7b m]500 Ra 86000000 suEpnpaerorut -rihrMe- loiiernrkocenury K-10b Transit Depth [pp344505000 0.4 0.5 0.6 0.70.80.91Wavelength [µm] 2 3 4 5 T=2650 at 10 bars Fig.7.BroadbandtransitdepthmeasurementsfromMOSTand 1 10 Mass (M ) Spitzer (squares) compared to models (lines + filled circles). Earth Theoretical transmission spectra for 55Cnce are displayed for Fig.5. Composition of 55Cnce. Mass-radius relationships for anatmospherecomposedof100%watervapor(bottom)andan four different rocky compositions: pure iron, super-Mercury atmospherewithsolarcomposition(top).Themodelassumesa (63% iron core above a 37% silicate mantle), Earth-like (33% gravitationally bound atmosphere in radiative-convective equi- iron core above a 67% silicate mantle with 10% iron by mol) librium(Benneke&Seager,2011,submitted). and a silicate planet with no iron. The green band depicts the range of rocky compositions of planets between 1 and 30 M . ⊕ Twofamiliesofvolatileplanetsareconsidered:envelopesofH- Valencia,D.,Ikoma,M.,Guillot,T.&Nettelmann,N.2010,A&A,516,A20 Hewith0.01,0.1and1%bymass(pink)andenvelopesofwater Valencia, D. 2011, IAU 276 Proceedings, The Astrophysics of Planetary Systems:Formation,StructureandDynamicalEvolution,arXiv:1103.3725 with20,50%water.Kepler-10b(K-10b)andCoRoT-7b(C-7b) vonBraun,K.,Boyajian,T.S.,tenBrummelaar,T.A.,etal.2011,ApJ,740,49 are shown for reference. Data for 55Cnce from this study are Winn,J.N.,Matthews,J.M.,Dawson,R.I.,etal.2011,ApJ,737,L18(W11) showninred,andaretakenfromD11(1)andW11(2). Yelle,R.,Lammer,H.&Wing-Huen,Ip2008,SSRv,139,437 20000 pure-Fe CoRoT-7b super-Mercury )10000 Kepler-10b Earth-like 3 m / no iron g E k (2) y ( 5000 V 55Cnc-e (1) t si 55Cnc-e, this study n e D Kepler-11b 2000 N GJ 1214b U 1000 2 4 6 8 10 12 14 16 Mass (M ) Earth Fig.6.Density-massrelationshipsoflow-massexoplanets.Four rocky compositions are shown (see Fig. 5 for description). 55Cnce’s radius from this study shows the planet cannot be rocky and instead is intermediate between the envelope- rich planets (eg. GJ1214b) and the rocky ones. Earth, Venus, Uranus, Neptune, Kepler-11b, GJ1214b, Kepler-10b, and CoRoT-7bareshownforreference. Schneider,N.M.&Bagenal,F.2007,IoAfterGalileo:ANewViewofJupiter’s VolcanicMoon,Springer(Germany),p.265 Seager, S., Deming, D. & Valenti, J. A. 2009, Transiting Exoplanets with JWST,AstrophysicsintheNextDecade,AstrophysicsandSpaceScience Proceedings,Springer(Netherlands),p.123 Valenti,J.A.&Fischer,D.A.2005,ApJS,159,141 Valencia,D.,O’Connell,R.J.&Sasselov,D.2006,Icarus,181,545 7