Quantifying and Predicting the Presence of Clouds in Exoplanet Atmospheres

0.2 MB·

by  Kevin B. Stevenson

PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
QUANTIFYINGANDPREDICTINGTHEPRESENCEOFCLOUDSINEXOPLANETATMOSPHERES
KEVINB.STEVENSON1,2
1DepartmentofAstronomyandAstrophysics,UniversityofChicago,5640SEllisAve,Chicago,IL60637,USAand
2NASASaganFellow
ABSTRACT
Oneofthemostoutstandingissuesinexoplanetcharacterizationisunderstandingtheprevalenceofobscur-
6
ing cloudsand hazesin their atmospheres. The ability to predictthe presence of clouds/hazesa priori is an
1
importantgoalwhenfacedwithlimitedtelescoperesourcesandadvancementsinatmosphericcharacterization
0
thatrely on the detection of spectroscopicfeatures. As a meansto identifyfavorable targetsforfuture stud-
2
ieswithHSTandJWST,weusepublishedHST/WFC3transmissionspectratodeterminethestrengthofeach
n planet’swaterfeature,asdefinedbytheH O–Jindex. Byexpressingthisparameterinunitsofatmospheric
a 2
scaleheight,weprovideameanstoefficientlycomparethesizeofspectralfeaturesoveraphysicallydiverse
J
sample of exoplanets. We find the H O – J index to be strongly correlated with planet temperature when
2
4 T <750+90Kandweaklycorrelatedwithsurfacegravityforplanetswithlogg<3.2+0.3dex. Otherwise,the
1 eq - 60 - 0.2
medianvalueoftheH O–Jindexis1.8±0.3H.Usingthesetwophysicalparameters,weidentifyadivision
2
] between“classes”ofexoplanets,suchthatobjectsaboveTeq=700Kandlogg=2.8dexaremorelikelytohave
P cleareratmosphereswithstrongerspectralfeatures(H O–J>1)andthosebelowatleastoneofthesethresh-
2
E oldsareincreasinglylikelytohavepredominantlycloudyatmosphereswithmutedspectralfeatures(H O–J
2
. <1). Additionalhigh-precisionmeasurementsareneededtocorroboratethereportedtrends.
h
p Subjectheadings:planetarysystems:planetsandsatellites:atmospheres—methods:analytical—techniques:
- spectroscopic
o
r
t
s 1. INTRODUCTION tures (thus inferring the presence of clouds/hazes) in the
a near-infrared. For example, the correlation between clouds
In the last few years, there has been a surge of pub-
[ and temperature is well known in brown dwarfs (e.g.,
lications reporting reliable constraints on the transmission
Ackerman&Marley 2001; Lodders 2002; Burgasseretal.
1 spectra of extrasolar planets. Using HST/WFC3, many
2006). There is also evidence for a cloud-gravity depen-
v of these publications have reported the detection of H O,
2 thus revealing insight into their chemical makeup. Ho2w- dence near the L-T transition (e.g., Burrowsetal. 2006;
9 ever, a substantial fraction have also reported statistically Metchev&Hillenbrand2006;Saumon&Marley2008). Us-
4 flat transmission spectra, which inhibit further characteriza- inghigh-precision,space-basedtransmissionspectra,welook
3 tion and understanding. The strength of the measured wa- forsimilarcorrelationsinoursampleexoplanetpopulation.
0 Thelayoutofthispaperisasfollows. Section2describes
ter feature can be reduced by the presence of obscuring
. ourprocedureforquantifyingthestrengthofthewaterfeature
1 clouds/hazes,highmetallicitiescausinghighmeanmolecular
usingtheH O–Jindex.InSection3,weinvestigatepossible
0 weightatmospheres,and/oranextremelylowH Oabundance 2
2 trendsbetweenoursampleplanetpopulation’sreportedH O–
6 (e.g., Seager&Sasselov 2000; Charbonneauetal. 2002; 2
Jindexvaluesandtheirphysicalproperties. Finally,Section
1 Miller-Riccietal. 2009; Morleyetal. 2013; Mosesetal.
4discusseswhatfutureobservationsareneededtorefineour
: 2013;Madhusudhanetal.2014). However,theproductionof
v constraintsandfurtherourunderstandingofexoplanetatmo-
clouds and hazes is generally believed to be the most com-
i spheres.
X monmechanismforcausingmutedspectroscopicfeaturesin
exoplanet spectra that have been obtained so far (Singetal.
r
a 2015a). 2. PROCEDURE
It is important to understand why some planets have In taking our first step, we look to select a sample popu-
predominantly cloud-free atmospheres, thus revealing in- lation of transiting exoplanetswith well-characterizedatmo-
sight into their compositions, while others appear to con- spheres. AlthoughthereisasignificantcumulationofSpitzer
tain high-altitude clouds or hazes that obscure our mea- Space Telescope transit data and increasing efforts to obtain
surements. The ability to predict the presence of ob- ground-basedtransmissionspectra ofexoplanets,forthisin-
scuring clouds/hazes a priori can increase telescope effi- vestigation we limit our sample population to only include
ciencybyallowingfutureinvestigationstoconcentrateontar- HST/WFC3dataacquiredwiththeG141grism. Thesehigh-
gets most likely to contain relatively clear atmospheres and precisionspectrahavebeenshowntoreliablyandrepeatably
strong spectroscopic signals. This is especially true for the resolve the water feature centered at 1.4 µm without con-
JamesWebb SpaceTelescope (JWST),whichis scheduledto fusion from other absorption features or variability caused
launch in 2018 (Beichmanetal. 2014; Barstowetal. 2015; by Earth’s atmosphere. Furthermore, the HST/WFC3 sam-
Cowanetal.2015). ple populationis both significantin size and diverse in their
The goal of this investigation is to use exoplanets’ phys- physicalproperties.
ical properties to predict the strength of spectroscopic fea- Next, we needto establish wavelengthregionsbothinside
andoutsideofourselectedspectroscopicfeature. We define
E-mail:[email protected] two spectral regions: (1) the peak of the broad water fea-
2 K.B.Stevenson
TABLE1
PLANETPARAMETERSANDSOURCES
Planet Teq Log(g) H2O–J H2O J Referencea
(K) (cgs) (H) (µm) (µm)
GJ436b 649 3.10 0.54±0.46 1.230–1.289 1.362–1.438 Knutsonetal.(2014a)
GJ1214b 559 2.94 0.02±0.10 1.228–1.297 1.366–1.435 Kreidbergetal.(2014b)
HAT-P-1b 1303 2.93 2.13±0.61 1.223–1.300 1.376–1.434 Wakefordetal.(2013)
HAT-P-11b 870 3.06 3.47±0.72 1.228–1.303 1.360–1.435 Fraineetal.(2014)
HAT-P-12b 957 2.75 0.21±0.60 1.226–1.297 1.367–1.438 Lineetal.(2013)
HD97658b 733 3.15 1.85±0.91 1.237–1.292 1.366–1.440 Knutsonetal.(2014b)
HD189733b 1199 3.34 1.86±0.36 1.222–1.297 1.372–1.447 McCulloughetal.(2014)
HD209458b 1445 2.97 1.15±0.13 1.232–1.288 1.364–1.439 Demingetal.(2013)
WASP-12b 2581 3.02 1.48±0.32 1.251–1.320 1.389–1.458 Kreidbergetal.(2015)
WASP-17b 1547 2.53 0.67±0.29 1.240–1.296 1.381–1.437 Mandelletal.(2013)
WASP-19b 2064 3.16 3.03±0.64 1.230–1.286 1.371–1.427 Mandelletal.(2013)
WASP-31b 1573 2.70 0.86±0.48 1.234–1.294 1.374–1.434 Singetal.(2015b)
WASP-43b 1374 3.70 1.08±0.50 1.228–1.297 1.366–1.435 Kreidbergetal.(2014a)
XO-1b 1206 3.19 1.66±0.55 1.234–1.290 1.365–1.422 Demingetal.(2013)
aThereferencesprovidethemeasuredtransitdepthsorradiusratiosfromwhichwecalculatetheH2O–Jindexwithinthespecifiedwavelengthregions.
1.25 versalmetric to whichplanetswith varyingphysicalproper-
Model Spectrum tiescanbecompared.Forthis,weadopttheatmosphericscale
1.20 J H2O 1.22 - 1.30 µm height,
1.36 - 1.44 µm k T
B eq
H= , (1)
%)1.15 µg
h ( where k is Boltzmann’s constant, T is the planet’s equi-
pt1.10 B eq
e librium temperature, µ is the atmospheric mean molecu-
D
sit 1.05 lar weight, and g is the planet’s surface gravity (in MKS
n units). For planets with R > 0.9 Jupiter-radii, we adopt
a
Tr1.00 Jupiter’s atmospheric mean molecular weight (µ = 2.2 u).
ForplanetswithR<0.5Jupiter-radii,we computeanatmo-
spheric mean molecular weight of 3.8 u using the prescrip-
0.95
tion of Fortneyetal. (2013) and assuming 80×Solar metal-
licity, which is comparable to both Uranus and Neptune
0.90
1.1 1.2 1.3 1.4 1.5 1.6 1.7 (Kreidbergetal. 2014a). All of the planets in our sample
Wavelength (µm) are expectedto have H /He-dominatedatmospheres(Rogers
2
FIG.1.— Definedwavelengthregionstoestimatethestrengthofaplanet’s 2015). One advantage of adopting the scale height as our
water feature. For each planet with a published HST/WFC3 transmission metric is that it accounts for differences in our sample pop-
spectrum,wecomputethedifferenceintransitdepthbetweenthesetwospec-
ulation’sequilibriumtemperaturesandsurfacegravities,two
tralregionsandconverttounitsofatmosphericscaleheight.Thisdefinesthe
H2O–Jindex. prominentfactors thoughtto affect the productionof clouds
andhazesinexoplanetatmospheres.
For each planet, we compute the scale height using
ture from 1.36 to 1.44 µm and (2) a J-band baseline from the T and logg values listed in Table 1, which we de-
eq
1.22 to 1.30 µm (see Figure 1). Compared to brown dwarf rive from data provided by the Exoplanet Orbit Database
measurements, exoplanettransmission spectra have a higher
(www.exoplanets.org, Hanetal. 2014). OurT calculation
degree of uncertainty; therefore, we have chosen relatively eq
assumeszeroalbedoandfullheatredistribution. Foreaseof
wide spectral regions to improve the precision of our com-
calculation,weconvertthescaleheightofeachplanetintoa
parisons. Also, authors reporttransit depths or radius ratios changeintransitdepth,∆D,usingthefollowingrelation:
using spectrophotometric bins of varying widths; therefore,
utilizingwidespectralwindowsminimizestheeffectsofhav- 2HR
∆D∼ P, (2)
ing slightly offset spectral regions for different planets (see R2
Table 1). Establishing new transit depths over our specified S
spectralregionswouldrequirereanalyzingalloftheavailable where R and R are the planet and stellar radii, respec-
P S
HST/WFC3 data and is outside the scope of this investiga- tively. Therefore,∆Disthechangeintransitdepththatcor-
tion.Futureanalyses,however,areencouragedtoreporttran- respondsto a one-scale-heightchange in altitude and serves
sit depths and uncertainties over these two spectral regions. as a normalizingfactor between planetary systems with dif-
As a cautionary note, using a dual-bandmeasure of the wa- ferent physical properties. Next, we compute the difference
terfeaturestrengthhasitslimitations. Inherentlynoisyspec- in transit depth between our two spectral regions, ensuring
tra(e.g.,HD97658b, Knutsonetal.2014b)maynotexhibit topropagateuncertainties,thendivideby∆Dforeachplanet.
distinct water features, but can still produce H O – J index ThisdefinestheH O–Jindex,whichquantifiesthefractional
2 2
values>1ifthemeasuredtransitdepthsexhibitwavelength- changeinatmosphericscaleheightbetweenourtwospectral
dependenttrends. regions. For the purposes of this investigation, we discard
Forourthirdstep,weareconcernedwithestablishingauni- planetswhoseH O–Jindexuncertaintiesarelargerthanone
2
QuantifyingandPredictingthePresenceofCloudsinExoplanetAtmospheres 3
scaleheight;thisincludesGJ3470b,TrES-2b,TrES-4b,and Surface Gravity (m/s2)
CoRoT-1b. Due to their large uncertainties, these measure- 4.0 6.3 10.0 15.8 25.1 39.8 63.1 100.0
2.0
ments are uninformative and only serve to obfuscate trends
2500 1.8
in certain figures. Table 1 lists the remainingH O – J index
2
valuesinunitsofscaleheight. 1.6
Finally, to allow for a more intuitive understanding, we K)2000 1.4
qsfuuunmacliitntiaogtnitvhoeaflytccldoheuasndcgrcieobsveientrh.tehPeslatHante2eOtsow–f iaJthipntldhaneexelta’vrsaglaeutsmetHaors2epOhsoe–rleeJlyainsa-- erature (1500 O - J (H) 11..02
dexvalues(>2)arelikelytohavecloud-freeatmospheresat mp H2 0.8
the depths probed by the WFC3 measurements. Those with e
T1000 0.6
valuesbetweenoneandtwomaybethoughtofashavingpar-
tiallycloudytomostlyclearskies,orathinhazelayer. Planet 0.4
atmosphereswith H2O – J index valuesless than unity have 500 0.2
theweakestwaterfeatures(ifany)thatarelikelycausedbyin-
0.0
creasedcloudcoverorhazethicknessatorabovetheprobed 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
depths. Log Gravity (dex)
Surface Gravity (m/s2)
3. ANALYSESANDRESULTS 4.0 6.3 10.0 15.8 25.1 39.8 63.1 100.0
2.0
After exploringhow the water-feature strength varies as a 2500 II I 1.8
functionoftheplanets’physicalproperties(i.e. mass,radius,
1.6
temperature,rotationrate, etc.), we findthatthe H O – J in-
2
dex is strongly correlated with equilibrium temperature and K)2000 1.4
wldoievgaigdk)il.nyTgchtohererteoslpaamtpeapdnlweeliptoohfpsFuuilgrafutaircoeen2ginirlatlovuistttwyrao(tte“yscpltiahcsiassleldyse”epxoepfnrdeexesnoscpeedlabanys- erature (1500 O - J (H) 11..02
etsusingH2O–J=1asaborderline. ThosewithH2O–J> mp H2 0.8
1areboundedinphasespacebyT >700Kandlogg>2.8. e
eq T1000 0.6
Recallthatfortheseplanets,anyputativeatmosphericclouds
0.4
would play only a minor role in altering the strength of the
measured water features. Conversely, for the sample popu- 500 0.2
III IV
lation with H2O – J < 1, cloudsand/or hazes likely play an 0.0
importantroleinmutingtheirspectroscopicfeatures. 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
Log Gravity (dex)
In the bottom panel of Figure 2, we include a best-fit 2D
modelthatsimultaneouslyfitsthetemperatureandgravityde- FIG.2.— Waterfeaturestrengthasafunctionofequilibriumtemperature
andsurfacegravity. Thenineplanets withstrongmeasuredwaterfeatures
pendence.Thismodelconsistsoffourquadrantsseparatedby
(H2O–J>1,bluesquares)haveTeq>700Kandlogg>2.8(dottedlines).
freely-varyingtransition(orknot)values.Abovethetempera-
TheremainingfiveplanetswithH2O–J<1(redsquares)havemutedwa-
tureandgravityknotvalues(QuadrantI),weadoptaconstant terfeatures. Thesizeofthesquaresisinverselyproportional totheuncer-
H O–Jindexvalue(onefreeparameter,c)becausetheavail- tainty intheH2O–Jindex, thus largersymbolscarrymoreweight. Like
2 I thedottedlines,thedasheddiagonallinealsodelineatesthesetwo“classes”
abledataexhibitnosignificanttrend.QuadrantsIIandIVas-
ofexoplanets. Withinthetoppanel,thegraycirclesdepictconfirmedtran-
sumelineardependencesingravityandtemperature,respec- sitingexoplanetswithknownmassesandradii. Thecoloringinthebottom
tively (two free parameters each, m , b , m , and b ). To paneldepicts thebest-fit2Dmodelsegmented intofourquadrants (yellow
II II IV IV
completethemodel,weapplybilinearinterpolationinQuad- linesandnumerals). Additionalmeasurementsareneededtomoreprecisely
determinethetransitionregionforatmosphereswithandwithoutobscuring
rantIII.Thefinalmodelcontainsfivefreeparametersandthe
clouds/hazesatthedepthsprobedbyWFC3.
knotvaluesare computedat each step of our Markov-Chain
MonteCarloanalysis(MCMC, Ford2005).
versely,theplanetsinQuadrantsIandIIhaver=0.06,which
We derivethefollowingmedianparametervalueswith1σ
isconsistentwithnocorrelation. Next,werepeatthetestus-
uncertainties: c = 1.8±0.3 H, m = 1.9±0.7 H/dex, b =
I II II ing surface gravity as the correlating parameter. The eight
- 4.3±1.9 H, mIV = 0.0074+- 00..00003244 H/K, and bIV = - 3.7-+11..84 H. planetsinQuadrantIIexhibitamoderatepositivecorrelation
Interestingly, Quadrant IV from the best-fit model does not (r=0.61)with a relatively small p-value(0.053). The three
contain any planets from our sample, but the free param- planetsinQuadrantIhaveasurprisinglystrongnegativecor-
eters are tightly constrained by the population in Quad- relation (r=- 0.85) with surface gravity, but they also have
rant III. Nonetheless, the model would benefit from having largeuncertaintiesontheir H O– Jindexvaluesandalarge
2
HST/WFC3 measurements within this region. We derive a p-value(0.18). So, we repeat this test while including three
mediantransitiontemperatureof750+90Kandamediantran- additionalplanetsfromQuadrantII with logg>3.0(the 1σ
- 60
sition logg of 3.2+0.3 dex. With 14 data points and five free lower limit on the transition surface gravity). In this case,
parameters,ourbe-s0t.-2fitmodelachievesaχ2valueof19.8. wefindamuchweakercorrelation(r=- 0.32)thatsuggestsa
largersamplesizeisneededinQuadrantIbeforeanyconclu-
We test ourproposedtemperaturedependencebycomput-
sionscanbedrawn.
ingthevariance-weightedexperimentallinear-correlationco-
efficient,r,whichrangesfrom-1to1(Bevington&Robinson
3.1. TemperatureDependence
2003). ThethreeplanetsinQuadrantIIIhaver=0.96,indi-
cating a near-total positive correlation between planet tem- HerewefurtherexaminetheH O–Jindexdependenceon
2
peratureandwaterfeaturesize,andap-valueof0.092. Con- planettemperature. ThetoppanelofFigure3plotsthewater
4 K.B.Stevenson
tenseUVradiationmayfacilitatetheproductionofobscuring
4.0
3.60 aerosols.Therefore,asacriticalnextstep,werecommendad-
3.5 ditionalhigh-precisionHST/WFC3measurementsofJupiter-
3.45 sizeexoplanetswithT =500–700KandorbitingF,G,or
eq
3.0
Kstarstovalidatethereportedtemperaturedependence.
H)2.5 ex) 3.30 uaTlshferobmottooumrpbaenset-lfiotfmFiogduerlev3eprsluostspthlaenHet2Otem–pJeirnadteuxrer.esWide-
HO - J (212..50 avity (d 33..0105 annodte8th6a%t5a7re%woifthoiunr2sσam. pTlheeposcpautltaetrioinnathreewreisthidinua1lsσmofayzebroe
r reducedbyapplyingmoresophisticatedmodelsorexamining
G
1.0 g correlationsbetweentheH O–Jindexandotherphysicalpa-
o 2.85 2
L rameters not considered in this analysis. Most WFC3 light
0.5
curves do not exhibit time-correlated (red) noise and model
2.70
0.0 log(g)=3.3 fitstendtoberepeatableandnearthephotonlimit;therefore,
log(g)=2.7 the H O – J index uncertainties are not likely to be signifi-
-0.5 2.55 2
500 1000 1500 2000 2500 cantlyunderestimated.
Temperature (K) Thetemperaturedependenceofourbest-fitmodel,includ-
ingthe750+90Ktransitiontemperature,maybeexplainedby
2.5 - 60
3.60 the productionof photochemicalhazes (Morleyetal. 2015).
2.0 Attemperaturesbelow∼1100K,CH4becomesthedominant
3.45 carbon-bearingmoleculeforanatmosphereinthermochemi-
1.5 cal equilibrium. Methane photochemistry,which is initiated
H) x) 3.30 bytheabsorptionofincidentUVradiation,resultsinthefor-
esiduals (01..50 avity (de 33..0105 mdcaraontgitoehnnenocylfeasanodiodtteopt(rhHeecCpuNrros)odirunscsttihuoecnhuopafpsheaarczaeettsmy.lMoensoperhl(eeCryi2cHetl2aa)yl.aen(r2sd0th1hy5a)-t
R0.0 Gr findthatasplanettemperaturesdecreasetowards500K,soot-
g precursor production peaks and does so at higher altitudes,
o 2.85
−0.5 L thus muting the size of spectroscopic features and reducing
H O – J index values. Theyalso predictsootprecursorpro-
2.70 2
−1.0 duction to slow below ∼500 K; therefore, more temperate
−1.5 2.55 exoplanet atmospheres could return to clearer skies and the
500 1000 1500 2000 2500 H O – J index may rebound to values greater than unity.
2
Temperature (K) However, the presence of water ice clouds for objects be-
FIG.3.— H2O–Jindexdependenceonplanettemperature. Thesymbol low∼350Kmayagainmutespectralfeatures(Sudarskyetal.
sizeisproportionaltoplanetradiusandsymbolcolorisproportionaltologg. 2000;Morleyetal.2014).
Thetoppaneldepictsthewaterfeaturestrengthwith1σuncertainties. The
blueandgreenlineswithcolored1σuncertaintyregionsarerepresentative
models at two different surface gravities (logg=3.2 and 2.7 dex, respec-
3.2. SurfaceGravityDependence
tively). Curvatureinthemedianmodelanduncertaintiesnear800Kresults
fromallowingtheknotvaluetovaryinourfits. Thebottompanelshowsno Relative to planet temperature, we detect a weak correla-
discerniblecorrelationbetweentheresidualsandplanettemperature.
tion between the H O – J index and surface gravity. In the
2
top panel of Figure 4, we plot the calculated H O – J index
feature strength (with uncertainties) versus equilibriumtem- 2
values(withuncertainties)asafunctionofsurfacegravityfor
peratureforall14exoplanetsinoursamplepopulation.From
ourentire samplepopulation. Thecorrelationbetweenthese
this, it is clear that the model is strongly influenced by the
two parameters is difficult to assess without accounting for
relativelysmalluncertaintiesof GJ 1214band HD 209458b.
thetemperaturedependence. Nevertheless,thetworepresen-
Fitting a linear trend to the entire dataset (not shown) re-
tativemodelsatfixedequilibriumtemperaturesclearlyshow
sults in a 8σ detection of a slope, thus suggesting a strong
adecreasingtrendtowardslower-gravityplanets.Thesignifi-
temperature dependence at face value. However, the qual-
canceofthistrendis2.9σ.Theadditionofaprecisemeasure-
ity of the fit is relativelypoor (χ2ν =4.7), thus justifying the ment from a low-surface-gravity exoplanet would improve
use of our more complexmodel(discussed above). With an thissignificance while providinganotherdata pointwithin a
improvedfit (χ2ν =2.5), we reporta correlationbetween the poorlysampled region of phase space. The bottompanel of
waterfeaturesize andplanettemperatureat3.1σ confidence Figure4displaysthesameH O–JindexresidualsasFigure
2
whenT <750K. 3,exceptplottedasafunctionofsurfacegravity.
eq
Next, we consider how planet radius might affect the wa- Onepossibleexplanationforthegradualdecreaseinwater
ter feature size because the four coolest planets are also the featuresize with decreasingsurfacegravityis an increasein
smallest. Thesub-Neptune-sizeplanetHD97658b(2.25R ) vertical mixing efficiency. This is because as surface grav-
⊕
issmallerthanGJ1214b(2.68R ),butithasalargerH O–J ity decreases, the eddy diffusion coefficient (K ) increases
⊕ 2 zz
index value (1.85±0.91vs. 0.02±0.10). Furthermore, HAT- (Barmanetal. 2011). As a result, enhanced vertical mix-
P-11b is comparable in size to GJ 436b but has the largest ingcouldloftmoresootprecursorstohigheraltitudeswithin
H O – J index value (3.47±0.72vs. 0.54±0.46). These ex- theatmospherewheretheywouldeffectivelyreducethe size
2
amplesargueagainstastrongcorrelationbetweentheH O– of spectroscopic features. However, increased photochemi-
2
J index value and planet radius. It may be relevant that the cal production does not always occur under all atmospheric
twocoolestplanetsbothorbitMdwarfstarsbecausetheirin- conditionsandrequiresthatsootprecursorscaninitiallyform
QuantifyingandPredictingthePresenceofCloudsinExoplanetAtmospheres 5
2750 (2015),andShporer&Hu(2015)reportonthreeKeplerplan-
4.0 Teq=1500 K ets(Kepler-7b,Kepler-12b,andKepler-42b)withasymmetric
2500
3.5 Teq=600 K optical-lightphase curves. Theyattribute the westward shift
2250 inpeakbrightness(relativetothesubstellarpoint)tothefor-
3.0 mation of reflective clouds. Interestingly, the lowest-gravity
2000 planets (Kepler-7b and Kepler-12b, which have logg=2.70
O - J (H)22..05 ature (K) 11570500 atfhonerdsirt2ep.r5iht7aesd(eMecxgu,rrSveeisOsp.e)cStaiilvrieeclabytoe)tshhashuviceghhthlayessreetnrflosetnacgttiietvseet(aaMsnydgmScimoOne3dt)reyannsinde
H21.5 er at high altitud2es (P4 < 0.01 bar) between 1,500 – 2,000 K
p
1.0 m 1250 (Morleyetal.2015). Thus,inadditiontotheKeplerplanets,
e
T patchysilicate cloudscan naturallyexplain the low H O – J
0.5 1000 2
indexvaluesreportedhereforWASP-17bandWASP-31b.
0.0 750
-0.5 500
2.6 2.8 3.0 3.2 3.4 3.6 3.8
Log Gravity (dex) 4. FUTUREWORK
2750 Inthisinvestigationintocorrelationsbetweenwaterfeature
2.5
size andplanetphysicalproperties, we findthatthe H O – J
2500 2
index is strongly correlated (r=0.96, p-value = 0.092)with
2.0
2250 planettemperaturewhenT <750+90Kandmoderatelycor-
eq - 60
1.5 related (r = 0.61, p-value = 0.053) with surface gravity for
2000
esiduals (H)01..50 erature (K) 11570500 phaptliltagernthiebe-uptustrneewdscieittsrohsiotlraonengfimdngieen<agtsh3uoe.rfs2ee-+tmh00t..ehe23snrdeetesstxhrfa.ornoHldsmoistwiapoenlnavdnerdeer,tgeswvioweenlionstp.eheIakdnemaypdoapdrrhiettiyiccosuonilcmaaarl-l,,
R0.0 p the low-gravity regime is poorly sampled and lacks a high-
m 1250
e precision constraint to definitively confirm a surface gravity
−0.5 T 1000 dependence. Additionally, HST/WFC3 measurements of a
500–800KJupiter-sizeexoplanetorbitinganF,G,orKstar
−1.0 750
areneededtodefinitivelyruleoutlinksbetweenwaterfeature
−1.5 500 sizeandplanetradiusorhoststartype.
2.6 2.8 3.0 3.2 3.4 3.6 3.8 Futurestudies,particularlythosewithJWST,willbeableto
Log Gravity (dex)
utilizeadditionalindicesoveranexpandedwavelengthrange
FIG.4.— H2O–Jindexdependenceonsurfacegravity.Thesymbolsizeis to further subdivide these two classes of exoplanets and ex-
proportionaltoplanetradiusandsymbolcolorisproportionaltoequilibrium
temperature. Thetoppaneldepicts thewaterfeature strengthwith1σ un- amineadditionalfactorsthatmightaffectthesizeofspectro-
certainties. Theblueandgreenlineswithcolored1σuncertaintyregionsare scopicfeatures(suchaspatchycloudcoverandmetallicity).
representativemodelsattwodifferenttemperatures(Teq=1500and600K, Untilthen,theseresultscanserveasaguidefortheselection
respectively). Curvatureinthemedianmodelanduncertaintiesresultsfrom of future targets while enhancing our understanding of past
allowingtheknotvaluetovaryinourfits. Thebottompanelshowsnodis-
measurements.
cerniblecorrelationbetweentheresidualsandsurfacegravity.
within the atmosphere,whichis mostlikely for planetswith
temperaturesbetween500–800K(Morleyetal.2013). Our
sampleoflow-gravityexoplanetsareallhotterthan800Kon IappreciatethehelpfulfeedbackfromJacobBean,Megan
theirdaysides,buttheirnightsidetemperaturesarelikelycool Bedell,DianaDragomir,andLauraKreidberg,aswellasthe
enoughtofavortheproductionofmethane(andhigherhydro- thoughtfulsuggestionsfrom the anonymousreferee. I thank
carbons)inthermochemcialequilibrium.Itremainstobeseen Michael Line for supplying model transmission spectra. I
whetherhigh-altitudehazescanbeproducedonalow-gravity also acknowledge the contributors to SciPy, Matplotlib, and
planet’snightsideandwhethertheycanremainopticallythick thePythonProgrammingLanguage,thefreeandopen-source
attheplanetterminatorastheyaretransportedtothedayside. community, and the NASA Astrophysics Data System for
Since photochemicalhazes are not favored at higher tem- software and services. This work was performed under
peratures,anotherpossibleexplanationisthepresenceofob- contractwith theJet PropulsionLaboratory(JPL)fundedby
scuring clouds. Low-gravity environments can hamper par- NASA through the Sagan Fellowship Program executed by
ticulate sedimentation, resulting in high-altitude cloud for- theNASAExoplanetScienceInstitute.
mation. As an example, Demoryetal. (2013), Huetal.
REFERENCES
Ackerman,A.S.,&Marley,M.S.2001,ApJ,556,872 Bevington,P.R.,&Robinson,D.K.2003,Datareductionanderroranalysis
Barman,T.S.,Macintosh,B.,Konopacky,Q.M.,&Marois,C.2011,ApJ, forthephysicalsciences;3rded.(NewYork,NY:McGraw-Hill)
733,65 Burgasser,A.J.,Kirkpatrick,J.D.,Cruz,K.L.,etal.2006,ApJS,166,585
Barstow,J.K.,Aigrain,S.,Irwin,P.G.J.,Kendrew,S.,&Fletcher,L.N. Burrows,A.,Sudarsky,D.,&Hubeny,I.2006,ApJ,640,1063
2015,MNRAS,448,2546 Charbonneau,D.,Brown,T.M.,Noyes,R.W.,&Gilliland,R.L.2002,ApJ,
Beichman,C.,Benneke,B.,Knutson,H.,etal.2014,PASP,126,1134 568,377
Cowan,N.B.,Greene,T.,Angerhausen,D.,etal.2015,PASP,127,311
6 K.B.Stevenson
Deming,D.,Wilkins,A.,McCullough,P.,etal.2013,ApJ,774,95 Mandell,A.M.,Haynes,K.,Sinukoff,E.,etal.2013,ApJ,779,128
Demory,B.-O.,deWit,J.,Lewis,N.,etal.2013,ApJ,776,L25 McCullough,P.R.,Crouzet,N.,Deming,D.,&Madhusudhan,N.2014,
Ford,E.B.2005,ApJ,129,1706 ApJ,791,55
Fortney,J.J.,Mordasini,C.,Nettelmann,N.,etal.2013,ApJ,775,80 Metchev,S.A.,&Hillenbrand,L.A.2006,ApJ,651,1166
Fraine,J.,Deming,D.,Benneke,B.,etal.2014,Nature,513,526 Miller-Ricci,E.,Seager,S.,&Sasselov,D.2009,ApJ,690,1056
Han,E.,Wang,S.X.,Wright,J.T.,etal.2014,PASP,126,827 Morley,C.V.,Fortney,J.J.,Kempton,E.M.-R.,etal.2013,ApJ,775,33
Hu,R.,Demory,B.-O.,Seager,S.,Lewis,N.,&Showman,A.P.2015,ApJ, Morley,C.V.,Marley,M.S.,Fortney,J.J.,etal.2014,ApJ,787,78
802,51 Morley,C.V.,Fortney,J.J.,Marley,M.S.,etal.2015,ApJ,815,110
Knutson,H.A.,Benneke,B.,Deming,D.,&Homeier,D.2014a,Nature, Moses,J.I.,Line,M.R.,Visscher,C.,etal.2013,ApJ,777,34
505,66 Rogers,L.A.2015,ApJ,801,41
Knutson,H.A.,Dragomir,D.,Kreidberg,L.,etal.2014b,ApJ,794,155 Saumon,D.,&Marley,M.S.2008,ApJ,689,1327
Kreidberg,L.,Bean,J.L.,Désert,J.-M.,etal.2014a,ApJ,793,L27 Seager,S.,&Sasselov,D.D.2000,ApJ,537,916
—.2014b,Nature,505,69 Shporer,A.,&Hu,R.2015,AJ,150,112
Kreidberg,L.,Line,M.R.,Bean,J.L.,etal.2015,ApJ,814,66 Sing,D.K.,Fortney,J.J.,Nikolov,N.,etal.2015a,ArXive-prints,
Line,M.R.,Knutson,H.,Deming,D.,Wilkins,A.,&Desert,J.-M.2013, arXiv:1512.04341
ApJ,778,183 Sing,D.K.,Wakeford,H.R.,Showman,A.P.,etal.2015b,MNRAS,446,
Lodders,K.2002,ApJ,577,974 2428
Madhusudhan,N.,Crouzet,N.,McCullough,P.R.,Deming,D.,&Hedges, Sudarsky,D.,Burrows,A.,&Pinto,P.2000,ApJ,538,885
C.2014,ApJ,791,L9 Wakeford,H.R.,Sing,D.K.,Deming,D.,etal.2013,MNRAS,435,3481