ExoplanetsSaturdayPhysicsforEveryone

JonThalerOctober27,2012

Credit:NASA/KeplerMission/DanaBerry

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Outline •  Whatisanexoplanet?

•  WhyaretheyinteresKng?

•  Howcanwefindthem?

•  Exolife??•  Thefuture...

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What is an Exoplanet? Mostoftheobjectsthatweseearestars.Exoplanetsarethingsthatorbitstars.

Howdoweknowthatwearenotseeingbinarystars,whicharequitecommon?

•Theminimumstellarmassisabout0.075*MS(=75MJ).Otherwisethereisnofusionatthecore.

•Planetsdon’temitlight.

LowmassandnolightarebothusedtoidenKfyplanets.

Tcore

GravitaKonalpressureheatsthecore.Tcore~15milliondegreesforfusion.

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Why Are They Interesting? •Stars(includingbinaries)andplanetsarethoughttoformfromaprimordialblobofgas,butthedetailsarecontroversial.Iwon’tdiscussthistopic.

•Planetscomeintwotypes:° Gasgiants (e.g.,Jupiter)° Rocks (e.g.,Earth)We’dliketoknowifthereareotherEarth-likeplanetsthatcouldsupportlife:“Goldilocks”planets.Nottoohotandnottoocold

ThediskshapeisaresultoftherotaKon.

Thegreenregionsarethe“habitablezones”.

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How Can We Find Exoplanets?

Therearetwoprimarymethods:• Thestarwobblesastheplanetorbitsit.• Thestarbecomes(slightly)dimmeriftheplanetpassesinfrontofit(“transits”thestar).

Ideally,we’dliketoobserveaplanetbothways.

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The Wobble method WhenobjectsareorbiKngeachother,neitherisstaKonary.Thecenterofmassremainsfixed,andeachobjectmovesaroundit.TheraKoofspeedsequalstheraKoofmasses.Thismeansthatwecanmeasurethemassoftheplanetbymeasuringthespeedofthestar.TheSunismuchheavierthantheplanets:

MJ/MS=0.001andME/MS=0.000003(3×10-6)

ThismeansthattheSun’sspeedisquitesmall:

12.5m/sduetoJupiterand8.4cm/sduetotheEarth.

ThesespeedscanbemeasuredusingtheDopplereffect.

Radialvelocity

Star&planet

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The Doppler Effect You’veallheardthechangeofpitchasacarorambulancepassesby.ThesizeoftheeffectdependsontheraKoofthecar’sspeedtothespeedofsound.Inthisvideo,thepitchdropsfromabout475Hzto425Hz(BtoG),indicaKngthatthecarisgoingabout19m/s(43mph).

Lightisawave,justlikesound,sowecanusetheopKcalDopplereffecttomeasurespeeds.Thedifficultyisthatthespeedoflightishuge(300,000,000m/s)andthestarsarenotmovingveryfast(1m/s,orso).

Thefrequencyneedstobemeasuredtoanaccuracyof1/300,000,000!!(threepartsperbillion)

Carhorndoppler

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Some Doppler Data Fortunately,opKcaltechniquesarecapableofexquisiteaccuracy.Let’slookatsomedata.

Notethescarerofthedatapoints.Themeasurementaccuracyisabout±2m/s.Measurementaccuracylimitsourabilitytoobservesmalleffects.

Thisisa“Jupiter”(M~0.96MJ)orbiKnga“Sol”(M=0.88MS).Theorbitalperiodis9years(Jupiter’sis12),becausetheorbitisabitsmaller(4.2AUinsteadof5.2).Thisishowastronomers60light-yearsawaycoulddiscoverJupiter.

Radialvelocity

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More Doppler Data MostplanetobservaKonsinvolvemuchshorterorbitalperiods:

Notethattheplothasmarginalsignificance.Kepler10bwasnotdiscoveredbythewobblemethod.Thisdataservedonlytoconfirmthediscovery.

Thisisan“Earth”(M~4.6ME)orbiKnga“Sol”(M=0.90MS).Theperiodis0.84days(20hours),becausetheorbitisonly0.016AU).ItiscalledKepler10b(moreaboutitlater).It’stoohot(1800K)tobehabitable.

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Can We Discover a Goldilocks? Here’saplotofplanetsdiscoveredbythewobblemethod.Oursolarsystemislabeledinblue.OnlyJupitercouldbediscoveredwiththewobblemethod,unlessmeasurementaccuracyimproves.

Speed

Earth

Jupiter

TheSunmoves8.4cm/sduetoEarth’sorbit.

1m/sresoluKonmakesitundetectable.

Kepler10b

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Planetary Transits Whenaplanetpassesinfrontofastar,thestarappearsdimmer.ThefracKondependsontheraKooftheareas:

RadiusofSun: 7.0×105km Dimming:RadiusofJupiter: 7.1×104km 0.01(i.e.,1%)RadiusofEarth: 6.4×103km 0.0001(i.e.,0.01%)

VenusandMercurytransittheSun,butthedimmingisminiscule.Here’saninteresKngvideo:

1882transit!!

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Planetary Transits (2) Here’swhatoneexpectstoobserve:

Jupiter:10-3brightnessaccuracyrequired

Earth:10-5brightnessaccuracyrequired

10-3accuracyisachievabledirectly,but10-5canonlybeachievedbyaveragingmanymeasurements(requiresorbitswithshortperiods).

TransitSimulator

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NASA’s Kepler Mission “Asearchforhabitableplanets”hrp://www.nasa.gov/mission_pages/kepler/main/index.html

Itusesan85Megapixelcameratomonitormorethan100,000starsfortransiKngplanets.Launchedin2009,itisinsolar(notEarth)orbit.

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Keplerhasfound2321planetcandidatesand2165eclipsingbinarystars.

Here’sthediscovery(in2011)ofKepler10b,thefirstrockyplanet:

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Fig. 12.— The Kepler photometry and physical models are plotted as a function of both time

(upper two panels) and phase (lower two panels). The transits of KOI-72.02 are highlighted

by blue vertical lines in the topmost panel while a cutout (defined by the yellow box) is

expanded to show the transits of Kepler-10b highlighted by red vertical lines (second from

top). The bottom two panels shows the phase-folded light curves centered on phase zero

as defined by the central transit time. The modeled light curves are shown as colored lines

(blue corresponding to KOI-72.02 and red corresponding to Kepler-10b. Also shown is a

phase cutout of the lightcurve and model (green) centered on phase= 0.5 where occulations

would occur for a circular orbit. The relative intensity scale for phase= 0 can be read o↵

the y-axis on the left-hand-side of the plot while the relative intensity scale for phase= 0.5

can be read o↵ the y-axis on the right-hand-side of the plot.

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Fig. 12.— The Kepler photometry and physical models are plotted as a function of both time

(upper two panels) and phase (lower two panels). The transits of KOI-72.02 are highlighted

by blue vertical lines in the topmost panel while a cutout (defined by the yellow box) is

expanded to show the transits of Kepler-10b highlighted by red vertical lines (second from

top). The bottom two panels shows the phase-folded light curves centered on phase zero

as defined by the central transit time. The modeled light curves are shown as colored lines

(blue corresponding to KOI-72.02 and red corresponding to Kepler-10b. Also shown is a

phase cutout of the lightcurve and model (green) centered on phase= 0.5 where occulations

would occur for a circular orbit. The relative intensity scale for phase= 0 can be read o↵

the y-axis on the left-hand-side of the plot while the relative intensity scale for phase= 0.5

can be read o↵ the y-axis on the right-hand-side of the plot.

Some Kepler Data

Thisisprobablya“Neptune”.Orbitalperiodis45days.Radiusis2.2RE.ThisisKepler10b,arockyplanet.However,it’stoohot(1800K)forGoldilocks.Orbitalperiodis20hours!Orbitalradius~0.016AU.

LightcurvewhentheplanetisinfrontofthestarLightcurvewhentheplanetisbehindthestar

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Eclipsingbinarystarsdon’tlookthesameastransiKngplanets,becausebothobjectsemitlight.

Binary Stars Look Different

Eclipsingbinary

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The Search for Exolife

SupposewefindanEarth-likeplanetinthehabitablezone.Howcanwetellifithaslife?

Weneedtoknowmoreabouttheplanetthanitsmass.Wecannotyetimageexoplanets,butitisjustnowbecomingpossibletolearnsomethingabouttheirchemicalcomposiKon.

OnecandeterminethechemicalcomposiKonofagasbymeasuringtheabsorpKonoflightthatpassesthroughit.So,thispropertyofatransiKngexoplanet’satmospherecan,inprinciple,bemeasured.

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Absorption Spectra HerearetheabsorpKonspectraofvariousgases.Whenaplanetpassesinfrontofastar,someofthewavelengthsemiredbythestarareabsorbedbytheplanet’satmosphere.Forexample,carbonmonoxideabsorbsnear5microns.

1 5 10 15Wavelength(microns)

CO

CH4

N2O

CO2

HDO

O3

H2O

Directorate of Scientific Programmes M. FridlundM. Fridlund, August 2006

Bio markersBio markers

The goal is to find rocky planets with an atmosphere out of photo-chemical balance.

(cf. Lederberg, 1965 and Lovelock, 1965)

Working definition:

Life { O3 + H2O + CO2

Ideally, detect reduced molecules (CH4, N2O) as well

HerearetheabsorpKonspectraofthreeplanetary

atmospheres.Thisisrealdata!!

Whichplanethaslife?

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Exolife Waterand,especially,oxygen(inthiscaseozone)aresmokinggunindicatorsofourkindoflife.OxygenismuchtooreacKvetoremainintheatmospherewithoutplantlifetoreplenishit.

Thisisaverydifficultmeasurementtomakeinanexoplanet,especiallyasmall,Earthlikeone.

Directorate of Scientific Programmes M. FridlundM. Fridlund, August 2006

Bio markersBio markers

The goal is to find rocky planets with an atmosphere out of photo-chemical balance.

(cf. Lederberg, 1965 and Lovelock, 1965)

Working definition:

Life { O3 + H2O + CO2

Ideally, detect reduced molecules (CH4, N2O) as well

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A Proof of the Principle Thisyear,astronomersusedtheVeryLargeTelescope(VLT,asetoffour8mtelescopesinChile)todetectcarbonmonoxideintheatmosphereofa“SuperJupiter”thatorbitsTauBooKs(avisiblestar).Gasgiantshavelotsofatmosphere,sothisisan“easy”measurement.Unfortunately,thedataanalysisisquitecomplexanddoesn’tlenditselftoanicegraphic.Trustme!

Thestar:Mass: 1.3MSDistance:51l.y.Theplanet:Mass: 4.1MJOrbit: 0.05AU

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The Future ... TheEuropeanSpaceAgency(ESA)consideredasetofsatellites(theDarwinmission)tosearchforlifeonEarthlikeexoplanets,butabandoneditasunfeasibleatthisKme.NASAconsidered,andabandoned,asimilarproject(theTerrestrialPlanetFinder).

Perhapsin10-20years...ScienKficandtechnicalprogressareintertwined,butthat’sanothertalk.