science with transiting planetskristen/teaching/exop/exop4.1.pdfdiscovering transiting planets ¥...

1/21/2008 Eric Agol University ofWashington

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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

!

!

!

!

!

!

!

!

!

!

!


6

Re

lati

ve

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


8

Re

lati

ve

flu

x

Time


9

Re

lati

ve

flu

x

Time

tF

tT


10

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


11

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


15


16


17Udalski et al. (the OGLE collaboration)


18Udalski et al. (the OGLE collaboration)


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

science with transiting planetskristen/teaching/exop/exop4.1.pdfdiscovering transiting planets ¥...

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