Science with Transiting Planetskristen/teaching/ExoP/ExoP4.1.pdfDiscovering transiting planets ¥...
Transcript of Science with Transiting Planetskristen/teaching/ExoP/ExoP4.1.pdfDiscovering transiting planets ¥...
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Science with Transiting PlanetsScience with Transiting PlanetsTIARA Winter School onTIARA Winter School on Exoplanets Exoplanets 20082008
Eric AgolEric AgolUniversity of WashingtonUniversity of Washington
Thanks to Josh Winn for slides
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Venusian transit 2004
August 6, 2004 from Slovenia (Lajovic et al.)
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History of Exoplanetary Transits
• Rosenblatt (1971) proposed that planets aroundother stars could be found by monitoring thecolors of the star
• Borucki & Summers (1984) expanded on this idea,eventually proposal a space telescope
• With the discovery of planets via radial velocity(RV), starting with 51 Pegasi (Mayor & Queloz1995), the question remained: were these reallyplanets? Could they be other stellar phenomena?Could they be face-on brown dwarfs?
• Solution: planetary transits...
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First Transiting planet: HD 209458b
rplanet/rstar=0.1201±0.0006
Charbonneau et al. (2000), Henry et al. (2000), Brown et al. (2001),Mandel & Agol (2002)
• First discovered withradial velocity;photometric follow-uprevealed a dip rightwhen expected (upperright): confirmed RVplanets are real!
• HST data (right) gaveexquisite precision:
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What can be learned from transits?
• Confirmed planets
• Orbital period
• Planetary mass
• Planetary radius
• Alignment between orbit,stellar spin
• Effective temperature
• Hints about atmosphericcomposition
• Crude IR spectrum
• Crude surface map
• Optical albedo
• Star spots
• Moons, additional planets(via timing)
• Planetary rings
• Planetary oblateness andspin rate
• Stellar differentialrotation
!
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lati
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flu
x
Time
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Mid-infrared transit
Knutsonet al.2007
8 microntransit ofHD 189733observedwithSpitzer: nolimbdarkening!
!
"F =Rp
R*
#
$ %
&
' (
2
= 0.025
!
Rp
R*
= 0.1545 ± 0.0002
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lati
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flu
x
Time
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Re
lati
ve
flu
x
Time
tF
tT
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Re
lati
ve
flu
x
Time
tF
tTP
!F = !Rp /Rs"2
!
"*
=24
# 2
P$F 3 / 4
G(tT
2 %tF
2)3 / 2
Seager &Mallen-Ornelas2003
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Re
lati
ve
flu
x
Time
tM
!
"*
=3
# 2
P
GtM
3
2R*
v
!
v 3=2"GM
P
Kepler’s laws+ geometry:
Derive:
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Re
lati
ve
flu
x
Time
tF
tTP
!
gp =8
"
KPtT2 #tF
2( )$F #
4" 2
P 2tT2 #tF
2( ) $F # 2$F( )%
& '
(
) *
#1/ 2
Winn et al. (2007); Southworth et al. (2007); Beatty et al. (2007); Sozzetti et al. (2007)
!F = !Rp /Rs"2
+ Radial Velocity
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Other physical quantities
• Transiting planets are like single-lined eclipsingbinaries: extra information is needed tocompletely solve for the mass/radius of planet &star:
1. Assume mass-radius relation for the star, or
2. Measure stellar properties from spectrum
• From this can be derived the planet density(composition, core), inclination, semi-major axis
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Discovering transiting planets
• Two challenges: 1) transit duty cycle is "R*/#a2) probability of transiting is "R*/a
• The semi-major axis distribution of jupiter-massRV planets predicts that 0.1% of stars shouldhave transiting planets with transit depth >1%
• To discover one transiting planet, naivelymonitor ~103 stars for 2P ~ 6 days. In realityone transiting planet requires monitoring ~105
stars: giants, false positives (e.g. grazing binaries,Brown 2003), correlated noise (Pont et al. 2006),
interruptions, & metallicity bias reduce efficiency(Gaudi 2006)
semi-majoraxis
stellarradius
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17Udalski et al. (the OGLE collaboration)
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18Udalski et al. (the OGLE collaboration)
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19Winn, Holman, & Fuentes (2007)
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Effects of correlated noise
• Ground-based surveyshave errors due toatmospheric fluctuationsthat can last ~hours atthe few mmag level
• These can create falsetransit-shaped featuresin the lightcurve, sodetection threshold hasto be set higher,reducing # detectedplanets
typicalrangefromground
Pont et al. 2006
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Planetary transit surveys
• Five transiting planets have first been detectedwith RV: Doppler shifts are present all the timeat any inclination (N2K survey Fischer et al. 2004)
• Transiting planet discoveries are now dominatedby photometric surveys. Successful surveys havethus monitored lots of stars at high precision:OGLE (Konacki et al. 2003), TrES (Alonso et al. 2004), XO(McCollough et al. 2006), WASP (Collier-Cameron et al. 2006),
HAT (Bakos et al. 2007)
• Transit surveys are highly biased towards short-period, large planets (Gaudi et al. 2006)
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Transit Discoveries
29 as ofJan 2008(2 moresubmitted)
2008
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Mass-Period Correlation• Mass inversely correlates with semi-major axis,except 2 eccentric long-period planets (Mazeh et al.
2005, Torres et al. 2008) - may relate to metallicity
Mjup
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Two classes?
!
" =1
2
Vesc
Vorb
#
$ %
&
' (
2
=aRp
Mp
M*
Hansen & Barman (2007) proposed Class I & II planets
!
Teq =Teff
R*
2a
"
# $
%
& ' 1/ 2
Torres et al. (2008)
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Jupiter
Saturn
Neptune
Earth
$=1.5g/cc
$=1
$=0.5
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The dense planet HD 149026
G. Laughlin
Sato et al.2005
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Jupiter
Saturn
Neptune
Earth
$=1.5g/cc
$=1
$=0.5
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“Bloated” planets• Early migration (Burrows et al. 2000)
• Insolation-driven, deeply penetratinggravity waves (Showman & Guillot 2002)
• Eccentricity tides (Bodenheimer et al. 2001,2003)
• Obliquity tides (Winn & Holman 2005)
• Enhanced atmospheric opacity (Burrows etal. 2007)
• Inhibition of convection of planetaryinterior (Chabrier & Baraffe 2007)
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Gaudi &
Winn (2007)
Rossiter-McLaughlineffect
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R. A. Rossiter(1896-1977)
% Lyrae: Rossiter 1924, ApJ, 60, 15
Algol: McLaughlin 1924,
ApJ, 60, 22
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Measuring spin-orbit alignment
Ohta, Taruya, & Suto 2005; Gaudi & Winn 2007
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Phase variation of HD 189733• Observed planet for
~1/2 orbit (33 hours, 0.25Mexposures) at 8 µm usingSpitzer/IRAC
• Small size of observed phasevariation indicates relativelyefficient circulationbetween day/night sides
• Secondary eclipse indicateslow (~30%) albedo
Transit
SecondaryEclipse
Knutson et al. (2007)
Phase Function
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Mapping a Hot JupiterInversion:
• Divide the planet intolongitudinal slices
• At each point in time,about half of the slicesare visible
• As the planet rotates,each slice on terminusrotates into or out of view
• Regularized linear inversion allows us to determineface-on brightness of each slice
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Mapping a Hot Jupiter
• Hot spot is ~30±10 degreesaway from substellar point(~25 mbar level) - agreeswith Fortney et al. (2006)prediction!
• Hot spot and cold spotoccur in same hemisphere
• Tb,max= 1200 K,Tb,min= 973 K
• Bond albedo ! 0.3• Pn ! 0.3
Cowan & Agol, in prep
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Steam on an extrasolar planet
Beaulieu et al. (2007), Knutson et al (2007), Tinetti et al. (2007)
Transit ofHD189733bmeasuredwithSpitzer
strongerabsorptionby water
weakerabsorptionby water
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Known transiting planet
Transit times are equally spaced.
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Perturbed by second planet
Unknown perturbing planet
Known transiting planet
Transit times are NOT equally spaced.
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Eclipse Number
Time _
=
Eclipse Number
Time
Eclipse Number
Time
Transit Times Best-Fit Orbit
Timing Residuals
Transit Timing Variations (TTV)
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Resonant libration
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Resonant libration
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Transit Time (d)
O -
C (d)
Agol & Steffen (2007)
HST observations of HD 209458
Limits on second planets in HD 209458
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Combined TTV and RV for HD 209458
TTV Analysis
TTV Theory (1)
TTV + RV (2)
RV Theory (3)
(1) Eqns. (A7-8) & (33) from Agol, Steffen, Sari, & Clarkson MNRAS 359, 567 (2005)(2) RV measurements from Laughlin et al. ApJ 629, L121 (2005)(3) Eqn. (2) from Steffen & Agol MNRAS 364, L96 (2005)
Maximum allowed mass for companion in initially circular orbit
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Future Prospects
• ESA Corot satellite: still waiting for publications
• NASA Kepler satellite: launch 2009; monitor 105
stars; should detect dozens of transiting planets
• EPOXI: 30 cm mirror on Deep Impact satellitewill be used for optical imaging of a handful oftransiting planet systems (PI Deming)
• TRACER: 60 cm infrared (0.8-1.6 µm) fordetailed studies of bright transiting systems(NASA SMEX, PI Clampin)
• Monitor 103 M dwarfs from ground: habitablezone is much closer & can detect smaller planets
arXiv:0709.2879
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Possible thesis topics:
• Which (if any) is the correct explanation for bloatedplanets? dense planets?
• Do second, short-period planets exist? are theystable?
• Do planets have moons or rings?
• Can we detect transiting super-earths/earths?
• Can we detect reflected light from planets?
• What explains the mass/period correlation oftransiting planets?
• What causes Safronov/Teq correlation?
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Selected references:
• http://www.exoplanet.eu/ Jean Schneider’swebsite - up-to-date & easy to query
• http://www.oklo.org/ Greg Laughlin’s exoplanetblog
• Charbonneau et al. “When Extrasolar PlanetsTransit Their Parent Stars” astro-ph/0603376
• Torres, Winn & Holman, 2008, arXiv:0801.1841 -uniform reanalysis of most transiting planets &catalog of the derived properties
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Exercises:
1. Derive the relation: (Mp << M*, chord across staris straight, circular orbit, no limb-darkening)
2. Derive the relation:
!
gp =8
"
KPtT2 #tF
2( ) $F1#
4" 2
P 2 $FtT2 #tF
2( ) 1# 2 $F( )%
& '
(
) *
#1/ 2
!
"*
=24
# 2
P$F 3 / 4
G(tT
2 %tF
2)3 / 2