ExtraExtra --solarsolar PlanetsPlanets ((ExoplanetsExoplanets )) The search for planets around other stars David Wood Oct 7, 2009 OutlineOutline WhyWhy dodo wewe care?care? OverviewOverview ofof ourour knowledgeknowledge DiscoveryDiscovery techniquestechniques SpaceSpace --basedbased observationsobservations ResultsResults soso farfar ExtraExtra --solarsolar lifelife SummarySummary WhyWhy DoDo WeWe Care?Care? StudiesStudies ofof otherother planetaryplanetary systemssystems greatlygreatly enhanceenhance ourour understandingunderstanding ofof ourour ownown planetaryplanetary systemsystem –– especiallyespecially itsits originorigin andand evolutionevolution WeWe wouldwould likelike toto findfind EarthEarth --likelike planetsplanets andand looklook therethere forfor extraextra --solarsolar lifelife ExoplanetsExoplanets BackgroundBackground TheThe conceptconcept usedused toto bebe theoreticaltheoretical –– therethere OUGHTOUGHT toto bebe otherother planetsplanets FirstFirst discoverydiscovery ofof aa presumedpresumed exoplanetexoplanet inin 19881988 waswas notnot believedbelieved (but(but waswas true)true) SinceSince thethe firstfirst confirmedconfirmed discoverydiscovery inin 1995,1995, thethe raterate ofof discoverydiscovery hashas explodedexploded MoreMore thanthan 370370 exoplanetsexoplanets areare nownow knownknown ExoplanetsExoplanets MostMost ofof thethe knownknown exoplanetsexoplanets areare JupiterJupiter -- likelike (giant(giant gasgas planets)planets) ThisThis isis aa selectionselection effect,effect, sincesince allall discoverydiscovery techniquestechniques usedused favorfavor planetsplanets thatthat areare eithereither massivemassive oror largelarge SelectionSelection effecteffect alsoalso forfor closeclose --in,in, rapidlyrapidly movingmoving planetsplanets TheThe realreal interestinterest is,is, ofof course,course, inin detectingdetecting EarthEarth --likelike planetsplanets ExoplanetExoplanet DiscoveryDiscovery TechniquesTechniques Astrometry Direct Imaging Radial Velocity Speckle Interferometry Pulsar Timing Eclipsing binary timing Planetary Transit Orbital phase of reflected Gravitational Microlensing light Circumstellar Disks Polarization Stellar Atmospheric Contamination All observational techniques suffer from the problem of detecting a true, very weak signal in a very noisy environment AstrometricAstrometric DiscoveryDiscovery A planet can be inferred by observing a “wiggle ” in the position of the host star. We must be observing the system nearly pole -on. Two equal-mass stars orbiting about their barycenter (common center of gravity) A planet orbiting a star about their barycenter – the host star moves very little RadialRadial VelocityVelocity DiscoveryDiscovery WhenWhen wewe observeobserve nearnear thethe orbitalorbital planeplane ofof thethe system,system, wewe seesee velocityvelocity changeschanges alongalong thethe lineline --ofof --sightsight (radial(radial velocity)velocity) A star-planet system observed edge-on We observe a change in the velocity of the star along the line of site as it orbits about the barycenter of the system. Here we see the star HD164922 oscillating by about 8 meters/second in about 4 years. PulsarPulsar TimingTiming InIn thethe casecase ofof pulsars,pulsars, thethe pulsespulses areare veryvery preciselyprecisely timed,timed, soso motionmotion aboutabout aa barycenterbarycenter cancan bebe detecteddetected Pulsars,Pulsars, however,however, areare ofof littlelittle interestinterest ifif wewe aa searchingsearching forfor habitablehabitable planetsplanets PlanetaryPlanetary TransitTransit IfIf thethe planetplanet crossescrosses inin frontfront ofof thethe primaryprimary star,star, therethere willwill bebe aa slightslight diminutiondiminution inin thethe lightlight (eclipse)(eclipse) AsAs withwith radialradial velocityvelocity measures,measures, wewe mustmust observeobserve nearnear thethe planetplanet ’’ss orbitalorbital planeplane AA veryvery promisingpromising methodmethod toto pursuepursue onon spacespace PlanetaryPlanetary TransitTransit As the planet transits the star, we measure the light curve and observe a reduction in light from the star PlanetaryPlanetary TransitTransit TheThe HATNetHATNet project,project, runrun byby thethe HarvardHarvard -- SmithsonianSmithsonian CenterCenter forfor AstrophysicsAstrophysics isis systematicallysystematically lookinglooking forfor transitstransits AnAn automatedautomated networknetwork ofof sixsix 1111 --cmcm telescopestelescopes (Hawaii,(Hawaii, Arizona,Arizona, Israel)Israel) FollowFollow --upup observationsobservations withwith TopHATTopHAT ,, aa 2525 --cmcm telescopetelescope (Arizona)(Arizona) GravitationalGravitational MicrolensingMicrolensing Possible if a dense star with a planet produces gravitational microlensing Star -planet system must be quite far away The occurrence is a one -time event, never again repeated Useful for discovering an exoplanet Not practical for studying an exoplanet Follow -up studies difficult because of distance May provide statistical data on existence of exoplanets GravitationalGravitational MicrolensingMicrolensing mag4.mov CircumstellarCircumstellar DisksDisks Our theory of planet formation is that planets accreted from dust around the host star Infrared observations of circumstellar disks thus imply the existence of planetary formation, and possibly existing planets Beta Pictoris was the first dusty disk discovered Recently HD172555 has been found to have a disk of amorphous silica (glassy ash) – probably due to a massive planetary collision ContaminationContamination ofof StellarStellar AtmospheresAtmospheres IfIf heavierheavier elementselements (like(like magnesiummagnesium andand calcium)calcium) areare detecteddetected inin aa stellarstellar atmosphere,atmosphere, itit impliesimplies thethe existenceexistence ofof circumstellarcircumstellar mattermatter TheseThese elementselements dodo notnot originateoriginate inin thethe stellarstellar nuclearnuclear energyenergy generationgeneration PerhapsPerhaps asteroidsasteroids havehave crashedcrashed intointo thethe starstar DirectDirect ImagingImaging TheThe lightlight fromfrom aa planetplanet isis normallynormally totallytotally overwhelmedoverwhelmed byby thethe lightlight ofof thethe hosthost starstar CoronagraphCoronagraph --typetype instrumentsinstruments cancan maskmask thethe starlightstarlight andand possiblypossibly permitpermit observationobservation ofof aa planetplanet BestBest donedone inin spacespace toto avoidavoid scatteredscattered lightlight The Hubble telescope has observed a planet orbiting the red giant Formalhaut SpeckleSpeckle InteferometryInteferometry AA techniquetechnique usingusing aa computercomputer toto analyzeanalyze hundredshundreds ofof veryvery shortshort exposureexposure imagesimages toto eliminateeliminate thethe effecteffect ofof atmosphericatmospheric blurblur CouldCould bebe usedused toto computercomputer --blockblock thethe lightlight fromfrom thethe hosthost star,star, andand detectdetect aa planetplanet This is the speckle image of a close double star (Zeta Bootis – separation 0.8 arcseconds) EclipsingEclipsing BinaryBinary TimingTiming TheThe timetime ofof minimumminimum ofof anan eclipsingeclipsing binarybinary starstar systemsystem isis influencedinfluenced byby thethe existenceexistence ofof aa thirdthird bodybody inin thethe systemsystem ThisThis couldcould bebe usedused toto detectdetect aa planetplanet inin thethe systemsystem (orbiting(orbiting oneone ofof thethe eclipsingeclipsing starsstars oror orbitingorbiting thethe eclipsingeclipsing system)system) OrbitalOrbital PhasePhase ofof ReflectedReflected LightLight IfIf wewe couldcould observeobserve thethe reflectedreflected lightlight ofof aa planet,planet, itit wouldwould varyvary inin orbitalorbital phase,phase, asas doesdoes Venus.Venus. ThisThis techniquetechnique dependsdepends uponupon successfulsuccessful directdirect imagingimaging PolarizationPolarization LightLight reflectedreflected byby aa planetaryplanetary atmosphereatmosphere willwill bebe polarizedpolarized WeWe couldcould determinedetermine ifif thethe lightlight wewe seesee containscontains aa polarizedpolarized componentcomponent SpaceSpace ObservationsObservations All of these techniques can be greatly enhanced by space -based observations Current contribution by spacecraft Hubble Observation of Formalhaut SWEEPS survey – 16 candidates near galactic center – some with ultra -short periods Kepler Look for Earth -like planets by transit method Launched March 6, 2009 SpaceSpace ObservationsObservations (cont.)(cont.) CoRoT French/EU – astroseismology & extrasolar planet transits Spitzer Infrared studies – now limited in capability Hershel – largest space telescope (3.5 m) Infrared studies – including circumstellar disks FutureFuture SpaceSpace MissionsMissions James Webb Space Telescope 6.5 m infrared telescope Scheduled launch 2014 Space Interferometry Mission (SIM PlanetQuest ) Use interferometry (two mirrors) to find Earth -like planets Will be in heliocentric orbit Much -postponed launch – now scheduled “no earlier ” than 2015 FutureFuture SpaceSpace MissionsMissions (cont.)(cont.) TerrestrialTerrestrial PlanetPlanet FinderFinder Two complimentary observatories Multi -mirror infrared interferometer (each mirror is separate orbit) Coronograph (to block light from star) Only in very preliminary planning stage TerrestrialTerrestrial PlanetPlanet FinderFinder ExoplanetExoplanet DiscoveriesDiscoveries