Planet DetectionEstimating fpCan We See Them?• Not yet, but there are plans…– 3 recent claims, but planets very far fromstar, so some doubts• Problem is separating planet light from starlight– Star is 109 times brighter in visible light– “Only” 106 times brighter in infraredPlanet is Much Fainter than StarIndirect DetectionWobbling starDetect effect of planet on star (both orbit around center of mass)M1M2M1M2M1M2M =1M2M >>1 M 2Large planet will make a star “wobble”In plane of sky observeposition shiftAlong our line ofsightObserve DopplerShiftStar and Planet Orbit Center of MassThe Astrometric TechniqueMeasure stellar position (angle) accurately - see wobble compared to moredistant starsHow far does the star wobble?M*R*Center of massMplrR =*M rplM*We measure angle; for small angles,Θ =R*DΘ =M rplM*in radiansR*ΘDso1DBig planet, big orbitsmall star, close to sunCurrent limit: 1 mas = 10-3 arcsec = 2.8 × 10-6 degrees = 4.9 × 10-8 radianse.g. M = M , M = M , D = 15 ly ⇒ Θ = 1 maspl Jupiter*1 mas-1 masThe Sun as viewed from 10 pc (~30 ly)30 lyThe Spectroscopic Method• Relies on Doppler Effect• Motion of star towards and away from us• Almost all planets around other stars foundby this method so farThe Doppler ShiftLight is a wavewavelength(λ)moving starwavelength seen bywavelength seen byBLUESHIFTREDSHIFTλ observedλ emitted= 1 + vcDoppler Shift Magnitude and direction of velocityBut only along line-of-sightThe Spectroscopic TechniqueMeasure velocity, not position, of starUse spectrometer to get Doppler Shift ofspectral lineshiftλlightM rV ∝Mpl**1/2 1/2Shift ∝Big planet, small orbitSmall starDistance doesn’t matter (except for brightness)Edge - On30 lyWhat We Can Learn1. There is a planet(If not a mistake)2. The orbital period (P)(The time for pattern to repeat)3. The orbital radius r3 ∝ M* P2(Kepler’s Third Law)4. Lower limit to planet mass (Mplanet )Conservation of momentumMpl>M V P**2π r= if we see orbit edge-on> if tiltedComparison of Search MethodsAstrometricBig PlanetBig OrbitSmall StarNearby StarSpectroscopicBig PlanetSmall OrbitSmall Star--Edge-on OrbitAdvantagesAstrometric Technique (1 mas)M (in Jupiter Masses)pl0.10.31310301000.010.1110100Spectroscopic Technique (50 - 100 m s-1)(at ~ 15 ly)JupiterSaturnSeparate StarsStarsOrbital Radius (AU)Other MethodsTransits: Planet passes in front of a starstartransitLightfromstarTimeusOnly about 0.5% of stars withplanets will line upFirst planet found with this method in January 2003; 9 detected as ofJanuary 2006Microlensing: Light from more distant star is focused by gravity of nearer star passing infrontusnearer stardistant starplanetplanetlightfromdistantstarFortuitous alignment ⇒ brightensThree planets found this way as of January 2006Planets from the Transit MethodOGLE-TR-10Light curveStar field, shows starPlanet Detected by Microlensing OGLE 2005-BLG-235Lb, announced 1/25/06Sharp spike indicates second lens.Mass of second lens only 8 x 10–5as massive as star. Most likelymass of planet is 5.5 Mearth andseparation from star is 2.6 AU.Most likely star is low mass (0.22Msun).This method can detect very lowmass planets, but they are one-time events. Cannot follow up.http://www.eso.org/outreach/press-rel/pr-2006/pr-03-06.htmlCurrent Statistics (Jan. 2006)• Based on Extrasolar Planets Encyclopedia– http://www.obspm.fr/encycl/encycl.html• 170 Planets in 147 systems• 17 with multiple planets• Most planets in one system is 4 (55 Cancri)• Least massive– M = 0.023 MJup = 7 Mearth (Gliese 876)– Claim of 5.5 Mearth (Microlens 1/25/06)Estimating fp• Maximum? fp ~ 1– All young stars may have disks• Binaries?– Can have disks, but planet formation?– Even if form planets, orbits may not bestable– If reject binaries, fp < 0.3Estimating fp• Minimum?– Based on success rate of searches (nfound/nsearched)– Estimates now up to 5% (fp > 0.05)• Note larger than 0.02 given in book– Extrapolate trends to finding• Smaller planets, larger orbits, …– Estimates range from 0.11 to 0.25• Allowed range: fp = 0.05 to 1.0– Explain your choice!– Include/exclude binaries?Artist’s conception of the view from the outmost planet of three in UpsilonAndromedaeCopyright Lynette Cookused with permissionhttp://www.extrasolar.spaceart.orgCopyright Lynette Cookused with permissionhttp://www.extrasolar.spaceart.orgArtist’s conception of Transit of HD209458Copyright Lynette Cookused with permissionhttp://www.extrasolar.spaceart.orgArtist’s conception of 47 U ma “view” from Moon of the Second PlanetImplications of New PlanetsPlanets more massive than Jupiter can form around stars like the Sun.Large Planets can form much closer to a star than Jupiter (or move there)Does this mean we are unusual and our ideas about other planetarysystems are just “solar system chauvinism”?Not necessarily.The ones found so far are the “easy” ones. (Big planets close to a star)Now there are many more with lower masses than higher masses.Too early to say that we are unusual.Number of planets for different massesFuture ProspectsDirect detection (and study) of Earth-like planets~ 2015 Terrestrial Planet Finder (TPF)Darwin (Europe)Astrometric Method GAIA ~ 2010MJ Planets out to 600 ly.Further Spectroscopic SearchesTransits Kepler (~ 2007)Monitor 100,000 stars for 4 years“Hundreds of Terrestrial Planets”Direct Detection in Future• Terrestrial Planet Finder (TPF)/Darwin– TPF-C Visible light coronagraph (~2014)– TPF-I Infrared interferometer (~2020)• Goal is to detect earth-mass planets• And to see what gases in atmosphere– Suitable for life?• http://planetquest.jpl.nasa.gov/TPF/tpf_index.htmlTPF ConceptsTPF-C Visible light coronagraph (2014)TPF-I Infrared Interferometer (2020)Spectroscopy of
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