AstroTalk: Behind the news headlines of September 2017 Richard de Grijs (何锐思) (Kavli Institute for Astronomy and Astrophysics, Peking University) The hunt is on for chemical analysis of atmospheres outside our solar system September was an interesting month for exoplanet watchers. Current instrumentation on the best telescopes on Earth and in space is now used with ever increasing frequency to detect and characterize the atmospheres of alien worlds orbiting distant stars. And it turns out that those alien worlds are sometimes anything but ‘normal.’ Astronomers reported this month that the exoplanet WASP-12b reflects almost no light, making it appear essentially pitch black. Using the Space Telescope Imaging Spectrograph (STIS) on the NASA/ESA Hubble Space Telescope, an international team of astronomers measured how much light WASP-12b reflects—a quantity known as its ‘albedo’—in order to learn more about the composition of the exoplanet’s atmosphere. The results were surprising, explains Taylor Bell from McGill University (Canada): “The measured albedo of WASP-12b is 0.064 at most. This is an extremely low value, making the planet darker than fresh asphalt!” Indeed, this makes WASP-12b two times less reflective than our own Moon, which has an albedo of 0.12. Bell adds: “The low albedo shows we still have a lot to learn about WASP-12b and other, similar exoplanets.” WASP-12b orbits the Sun-like star WASP-12a, located some 1400 light-years away. With a radius of almost twice that of Jupiter and a year that lasts only just over one Earth day, WASP-12b is categorized as a ‘hot Jupiter,’ a giant gas planet. Because it is so close to its parent star, the gravitational pull of the star has stretched the exoplanet into an egg shape and raised the surface temperature of its daylight side to 2600 degrees Celsius. This high temperature is also the most likely explanation for WASP-12b’s low albedo. “There are other hot Jupiters that have been found to be remarkably black, but they are much cooler than WASP-12b. For those planets, it is suggested that things like clouds and alkali metals are the reason for the absorption of light, but those don’t work for WASP-12b because it is so incredibly hot,” explains Bell. In fact, the daylight side of WASP-12b is so hot that clouds cannot form and alkali metals are ionized, that is, some of their outermost electrons have been stripped off the atoms because of the intense radiation from the star. It is even hot enough
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AstroTalk:BehindthenewsheadlinesofSeptember2017RicharddeGrijs(何锐思)(KavliInstituteforAstronomyandAstrophysics,PekingUniversity)ThehuntisonforchemicalanalysisofatmospheresoutsideoursolarsystemSeptemberwasaninterestingmonthforexoplanetwatchers.CurrentinstrumentationonthebesttelescopesonEarthandinspaceisnowusedwitheverincreasingfrequencytodetectandcharacterizetheatmospheresofalienworldsorbitingdistantstars.Anditturnsoutthatthosealienworldsaresometimesanythingbut‘normal.’AstronomersreportedthismonththattheexoplanetWASP-12breflectsalmostnolight,makingitappearessentiallypitchblack.UsingtheSpaceTelescopeImagingSpectrograph(STIS)ontheNASA/ESAHubbleSpaceTelescope,aninternationalteamofastronomersmeasuredhowmuchlightWASP-12breflects—aquantityknownasits‘albedo’—inordertolearnmoreaboutthecompositionoftheexoplanet’satmosphere.Theresultsweresurprising,explainsTaylorBellfromMcGillUniversity(Canada):
“ThemeasuredalbedoofWASP-12bis0.064atmost.Thisisanextremelylowvalue,makingtheplanetdarkerthanfreshasphalt!”
Indeed,thismakesWASP-12btwotimeslessreflectivethanourownMoon,whichhasanalbedoof0.12.Belladds:
“ThelowalbedoshowswestillhavealottolearnaboutWASP-12bandother,similarexoplanets.”
WASP-12borbitstheSun-likestarWASP-12a,locatedsome1400light-yearsaway.WitharadiusofalmosttwicethatofJupiterandayearthatlastsonlyjustoveroneEarthday,WASP-12biscategorizedasa‘hotJupiter,’agiantgasplanet.Becauseitissoclosetoitsparentstar,thegravitationalpullofthestarhasstretchedtheexoplanetintoaneggshapeandraisedthesurfacetemperatureofitsdaylightsideto2600degreesCelsius.ThishightemperatureisalsothemostlikelyexplanationforWASP-12b’slowalbedo.
“ThereareotherhotJupitersthathavebeenfoundtoberemarkablyblack,buttheyaremuchcoolerthanWASP-12b.Forthoseplanets,itissuggestedthatthingslikecloudsandalkalimetalsarethereasonfortheabsorptionoflight,butthosedon’tworkforWASP-12bbecauseitissoincrediblyhot,”explainsBell.
Infact,thedaylightsideofWASP-12bissohotthatcloudscannotformandalkalimetalsareionized,thatis,someoftheiroutermostelectronshavebeenstrippedofftheatomsbecauseoftheintenseradiationfromthestar.Itisevenhotenough
tobreakuphydrogenmoleculesintoatomichydrogen,whichcausestheplanet’satmospheretoactmoreliketheatmosphereofalow-massstarthanlikeaplanetaryatmosphere.Thisleadstoitslowalbedo.TomeasurethealbedoofWASP-12bthescientistsobservedtheexoplanetduringaneclipse,whentheplanetwasnearfullphaseandpassedbehinditsparentstarforaperiodoftime.Thisisthebestmethodtodeterminethealbedoofanexoplanet,becauseitallowsdirectmeasurementsoftheamountoflightthatisreflected.However,thistechniquerequiresaprecisionthatistentimesgreaterthanthatoftraditionaltransitobservations.UsingHubble’sSpaceTelescopeImagingSpectrograph,thescientistswereabletomeasurethealbedoofWASP-12batseveraldifferentwavelengths.Thenewdataindicatethatitsatmosphereiscomposedofatomichydrogenandhelium.WASP-12bisonlythesecondplanettohavespectrallyresolvedalbedomeasurements,afterHD189733b,anotherhotJupiter.Theirdataallowedtheastronomerstodeterminewhethertheplanetreflectsmorelighttowardstheblueortheredendofthespectrum.WhileHD189733bissuggestedtohaveadeepbluecolour,WASP-12bdoesnotreflectlightatanywavelength.Itdoes,however,emitlightbecauseofitshightemperature,givingitaredhuesimilartoahotglowingmetal.
“ThefactthatthefirsttwoexoplanetswithmeasuredspectralalbedoexhibitsignificantdifferencesdemonstratestheimportanceofthesetypesofspectralobservationsandhighlightsthegreatdiversityamonghotJupiters,”concludesBell.
Aroundthesametime,ateamofastronomersledbyElyarSedaghatiattheEuropeanSouthernObservatory(ESO)examinedtheatmosphereofanotherexoplanet,WASP-19b.WASP-19bhasaboutthesamemassasJupiter,but—likeWASP-12b—itissoclosetoitsparentstarthatitcompletesanorbitinjust19hoursanditsatmosphereisestimatedtohaveatemperatureofabout2000degreesCelsius.AsWASP-19bpassesinfrontofitsparentstar,someofthestarlightpassesthroughtheplanet’satmosphereandleavessubtlefingerprintsinthelightthateventuallyreachesEarth.ByemployingtheFORS2instrumentontheVeryLargeTelescopeatESO’sParanalObservatoryinChile,theteamwasabletocarefullyanalysethislightanddeducethattheatmospherecontainedsmallamountsoftitaniumoxide,water,andtracesofsodium,alongsideastronglyscatteringglobalhaze.
“Detectingsuchmoleculesis,however,nosimplefeat,”explainsSedaghati.“Notonlydoweneeddataofexceptionalquality,butwealsoneedtoperformasophisticatedanalysis.Weusedanalgorithmthatexploresmanymillionsofspectraspanningawiderangeofchemicalcompositions,temperatures,andcloudorhazepropertiesinordertodrawourconclusions.”
TitaniumoxideisrarelyseenonEarth.Itisknowntoexistintheatmospheresofcoolstars.IntheatmospheresofhotplanetslikeWASP-19b,itactsasaheatabsorber.Ifpresentinlargeenoughquantities,thesemoleculespreventheatfromenteringorescapingthroughtheatmosphere,leadingtoathermalinversion—thetemperatureishigherintheupperatmosphereandlowerfurtherdown,theoppositeofthenormalsituation.OzoneplaysasimilarroleinEarth’satmosphere,whereitcausesinversioninthestratosphere.InEarth’satmosphere,thestratospheresitsabovethetroposphere—theturbulent,active-weatherregionthatreachesfromthegroundtothealtitudewherenearlyallcloudstopout.Inthetroposphere,thetemperatureiswarmeratthebottom—groundlevel—andcoolsdownathigheraltitudes.OnEarth,temperatureinversionoccursbecauseozoneinthestratosphereabsorbsmuchoftheSun’sultravioletradiation,preventingitfromreachingthesurface,protectingthebiosphere,andthereforewarmingthestratosphereinstead.Similartemperatureinversionsoccurinthestratospheresofotherplanetsinoursolarsystem,suchasJupiterandSaturn.Inthesecases,theculpritisadifferentgroupofmoleculescalledhydrocarbons;methane,forinstance,isresponsibleforheatinginthestratospheresofJupiterandSaturn’smoon,Titan,forexample.Neitherozonenorhydrocarbons,however,couldsurviveatthehightemperaturesofmostknownexoplanets.Thisleadstoadebateastowhetherstratosphereswouldexistonthematall.
“ThepresenceoftitaniumoxideintheatmosphereofWASP-19bcanhavesubstantialeffectsontheatmospherictemperaturestructureandcirculation,”explainsRyanMacDonaldattheUniversityofCambridge(UK).“Tobeabletoexamineexoplanetsatthislevelofdetailispromisingandveryexciting,”addsNikkuMadhusudhan(UniversityofCambridge),whooversawthetheoreticalinterpretationoftheobservations.
TheastronomerscollectedobservationsofWASP-19boveraperiodofmorethanayear.Bymeasuringtherelativevariationsintheplanet’sradiusatdifferentwavelengthsoflightthatpassedthroughitsatmosphereandcomparingtheobservationstoatmosphericmodels,theycouldextrapolatedifferentatmosphericproperties,suchasitschemicalcontent.Thisnewinformationaboutthepresenceofmetaloxidesandothersubstanceswillallowmuchbettermodellingofexoplanetatmospheres.Lookingtothefuture,onceastronomersareabletoobserveatmospheresofpossiblyhabitableplanets,theimprovedmodelswillgivethemamuchbetterideaofhowtointerpretthoseobservations.
“ThisimportantdiscoveryistheoutcomeofarefurbishmentoftheFORS2instrumentthatwasdoneexactlyforthispurpose,”addsESOteammemberHenriBoffin,wholedtherefurbishmentproject.“Sincethen,FORS2hasbecomethebestinstrumenttoperformthiskindofstudyfromtheground.”
Meanwhile,inAugust2017,anotherinternationalteamofscientistsfoundthestrongestevidencetodateforastratosphereonanenormousplanetoutsideoursolarsystem,withanatmospherehotenoughtoboiliron.TheteammadethediscoverybyobservingglowingwatermoleculesintheatmosphereofWASP-121bwiththeHubbleSpaceTelescope.Scientistsusedspectroscopytoanalysehowtheplanet’sbrightnesschangedatdifferentwavelengthsoflight.Watervapourintheplanet’satmosphere,forexample,behavesinpredictablewaysinresponsetodifferentwavelengthsoflight,dependingonitstemperature.Atcoolertemperatures,watervapourintheupperatmosphereblockslightofspecificwavelengthsradiatingfromdeeperlayerstowardsspace.Butathighertemperatures,thewatermoleculesintheupperatmosphereglowatthesewavelengthsinstead.Thephenomenonissimilartowhathappenswithfireworks,whichgettheircoloursfromchemicalsemittinglight.Whenmetallicsubstancesareheatedandvaporized,theirelectronsmoveintohigherenergystates.Dependingonthematerial,theseelectronswillemitlightatspecificwavelengthsastheyloseenergy:sodiumproducesorange-yellowandstrontiumproducesred,forexample.ThewatermoleculesintheatmosphereofWASP-121bsimilarlygiveoffradiationastheyloseenergy,butitisintheformofinfraredlight,whichthehumaneyeisunabletodetect.
“Theoreticalmodelshavesuggestedthatstratospheresmaydefineaspecialclassofultra-hotexoplanets,withimportantimplicationsfortheatmosphericphysicsandchemistry,”saidTomEvansattheUniversityofExeter(UK).“WhenwepointedHubbleatWASP-121b,wesawglowingwatermolecules,implyingthattheplanethasastrongstratosphere.”
WASP-121b,locatedapproximately900light-yearsfromEarth,isanotherhotJupiter,althoughwithagreatermassandradiusthanJupiter,makingitmuchpuffier.LikeWASP-12BandWASP-19b,itorbitsitsparentstarevery1.3days,andisalsoaroundtheclosestdistanceitcouldbebeforethestar’sgravitywouldstartrippingitapart.Thiscloseproximityalsomeansthatthetopoftheatmosphereisheatedtoablazinghot2500degreesCelsius—thetemperatureatwhichironexistsingasratherthansolidform.Insolarsystemplanets,thechangeintemperaturewithinastratosphereistypicallylessthan100degreesCelsius.However,onWASP-121b,thetemperatureinthestratosphererisesby1000Celsius.
“We’vemeasuredastrongriseinthetemperatureofWASP-121b’satmosphereathigheraltitudes,butwedon’tyetknowwhat’scausingthisdramaticheating,”saysNikolayNikolov(UniversityofExeter).“Wehopetoaddressthismysterywithupcomingobservationsatotherwavelengths.”
Vanadiumoxideandtitaniumoxidegasesarecandidateheatsources,sincethey
stronglyabsorbstarlightatvisiblewavelengths,similartoozoneabsorbingultravioletradiation.ThesecompoundsareexpectedtobepresentinonlythehottestofhotJupiters,sincehightemperaturesarerequiredtokeeptheminthegaseousstate.Indeed,vanadiumoxideandtitaniumoxidearecommonlyseeninbrowndwarfs,‘failedstars’thathavesomecommonalitieswithexoplanets.Previousresearchspanningthepastdecadehasindicatedpossibleevidenceforstratospheresonotherexoplanets,butthisisthefirsttimethatglowingwatermoleculeshavebeendetected,theclearestsignalyetforanexoplanetstratosphere.
“Thisnewresearchisthesmokinggunevidencescientistshavebeensearchingforwhenstudyinghotexoplanets.WehavediscoveredthishotJupiterhasastratosphere,acommonfeatureseeninmostofoursolarsystemplanets,”saysDavidSing,alsoattheUniversityofExeter.“It’satrulyexcitingfindaswe’reseeingdramaticdifferencesplanet-to-planetwhichisgivingvaluablecluesinfiguringouthowplanetsbehaveunderdifferentconditions,andwe’reonlyjustscratchingthesurfaceofallthenewHubbledata.”
AnexampleofthedetectionofapossiblestratosphereelsewhereintheMilkyWaygalaxywasprovidedbyHubbleSpaceTelescopeobservationsofthemassiveandblazing-hotexoplanetknownasWASP-33bonlyafewyearsago,in2015.Atthetime,thiswashailedasanimportantdiscovery,becausethepresenceofastratospherecanprovidecluesaboutthecompositionofaplanetandhowitformed.
“Someoftheseplanetsaresohotintheirupperatmospheres,they’reessentiallyboilingoffintospace,”saidAviMandell,aplanetaryscientistatNASA’sGoddardSpaceFlightCenterinGreenbelt(USA).“Atthesetemperatures,wedon’tnecessarilyexpecttofindanatmospherethathasmoleculesthatcanleadtothesemulti-layeredstructures.”
UsingHubble’sWideFieldCamera3,whichcancaptureaspectrumofthenear-infraredregionwherethesignatureforwaterappears,theresearchersidentifiedatemperatureinversionintheatmosphereofthehotJupiterWASP-33b.TeammembersalsothinktheyknowwhichmoleculeinWASP-33b’satmospherecausedtheinversion—titaniumoxide.
“Thesetwolinesofevidencetogethermakeaveryconvincingcasethatwehavedetectedastratosphereonanexoplanet,”saidKoreyHaynesofGeorgeMasonUniversity(USA)atthetime.
“Understandingthelinksbetweenstratospheresandchemicalcompositionsiscriticaltostudyingatmosphericprocessesinexoplanets,”saidNikkuMadhusudhan.“Ourfindingmarksakeybreakthroughinthisdirection.”
Withourincreasinglysophisticatedequipmentandanalysistechniques,itisonlyamatteroftimebeforewewillbeabletostudytheatmospheresofpotentiallyhabitableplanets.Excitingtimes,indeed!
Figure1:Thisartist’simpressionshowstheexoplanetWASP-12b—analienworldasblackasfreshasphalt,orbitingastarlikeourSun.Scientistswereabletomeasureitsalbedo:theamountoflighttheplanetreflects.Theresultsshowedthattheplanetisextremelydarkatopticalwavelengths.(Credit:NASA,ESA,andG.Bacon,STScI)
Figure2:Anartist’simpressionshowingtheexoplanetWASP-19b,inwhoseatmosphereastronomersdetectedtitaniumoxideforthefirsttime.Inlargeenoughquantities,titaniumoxidecanpreventheatfromenteringorescapinganatmosphere,leadingtoathermalinversion—thetemperatureishigherintheupperatmosphereandlowerfurtherdown,theoppositeofthenormalsituation.(Credit:ESO/M.Kornmesser)
Figure3:Artist’simpressionofWASP-121b.(Credit:EngineHouseVFX,At-BristolScienceCentre,UniversityofExeter)
Figure4:Artist’simpressionofWASP-121b(Credit:NASA,ESA,andG.Bacon,STScI)
Figure5:WASP-33b'sstratospherewasdetectedbymeasuringthedropinlightastheplanetpassedbehinditsparentstar(top).Temperaturesinthelowstratosphererisebecauseofmoleculesabsorbingradiationfromthestar(right).Withoutastratosphere,temperatureswouldcooldownathigheraltitudes(left).(Credit:NASA/Goddard)
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