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John Southworth
Astrophysics
Group
Keele University
TRANSITING PLANETS AND
DEFOCUSSED PHOTOMETRY
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Extrasolar planets – a history
• 1940s and 1950s: false alarms
• 1992: debris discovered around a pulsar
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Extrasolar planets – a history
• 1940s and 1950s: false alarms
• 1992: debris discovered around a pulsar
• 1995: first extrasolar planet (51 Peg)
– Mayor & Queloz (1995, Nature, 378, 355)
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Extrasolar planets – a history
• 1940s and 1950s: false alarms
• 1992: debris discovered around a pulsar
• 1995: first extrasolar planet (51 Peg)
– Mayor & Queloz (1995, Nature, 378, 355)
• 1999: First transiting planet (HD209458)
– Charbonneau et al. (2000, ApJ, 529, L45)
– Henry et al. (2000, ApJ, 529, L41)
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Extrasolar planets – a history
• 1940s and 1950s: false alarms
• 1992: debris discovered around a pulsar
• 1995: first extrasolar planet (51 Peg)
– Mayor & Queloz (1995, Nature, 378, 355)
• 1999: First transiting planet (HD209458)
– Charbonneau et al. (2000, ApJ, 529, L45)
– Henry et al. (2000, ApJ, 529, L41)
• 2002: First planet discovered because it istransiting (OGLE-TR-56)
– Konacki et al. (2003, Nature, 421, 507)
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Extrasolar planets – a history
• 1940s and 1950s: false alarms
• 1992: debris discovered around a pulsar
• 1995: first extrasolar planet (51 Peg)
– Mayor & Queloz (1995, Nature, 378, 355)
• 1999: First transiting planet (HD209458)
– Charbonneau et al. (2000, ApJ, 529, L45)
– Henry et al. (2000, ApJ, 529, L41)
• 2002: First planet discovered because it istransiting (OGLE-TR-56)
– Konacki et al. (2003, Nature, 421, 507)
• Current census: 420+ (www.exoplanet.eu)
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Transiting planets – discovery surveys
WASP-South installation (South Africa)
• Monitor millions of stars for months
• Detect periodic transit events
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Transiting planets – discovery surveys
WASP-South installation (South Africa)
• Monitor millions of stars for months
• Detect periodic transit events
• Follow up candidates
– Radial velocity confirmation
– High-quality transit light curves
– High-resolution imaging
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Transiting planets – discovery surveys
WASP-South installation (South Africa)
• Monitor millions of stars for months
• Detect periodic transit events
• Follow up candidates
– Radial velocity confirmation
– High-quality transit light curves
– High-resolution imaging
• Successful surveys:
– SuperWASP (UK)
– HAT (US/Hungary)
– OGLE (myriad)
– XO (US)
– TrES (US/Spain)
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Transiting planets – current status
WASP-South installation (South Africa)
• Roughly 60 known transitingextrasolar planets
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Transiting planets – current status
WASP-South installation (South Africa)
• Roughly 60 known transitingextrasolar planets
• Dedicated ground-based transitingplanet surveys:
– SuperWASP: 17 published
– HAT: 12
– OGLE: 8
– XO: 5 (survey stalled)
– TrES: 4 (survey finished?)
• Dedicated space-based surveys:
– CoRoT (France): 7
– Kepler (US): 5
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The end of the main sequence. . . and beyond
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Anatomy of a transit light curve
Light curve gives: Porb orbital period
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Anatomy of a transit light curve
Light curve gives: Porb orbital periodrA = RA/a fractional radius of star
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Anatomy of a transit light curve
Light curve gives: Porb orbital periodrA = RA/a fractional radius of stark = rb/rA ratio of planet radius of star radius
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Anatomy of a transit light curve
Light curve gives: Porb orbital periodrA = RA/a fractional radius of stark = rb/rA ratio of planet radius of star radiusi inclination of the orbit
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Defocussed photometry
• Look at a bright star with a large telescope:
– long-ish exposure times: 120 s maximum
– defocus PSF to cover thousands of pixels
Focussed PSF for WASP-4
Defocussed PSF for WASP-5
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Defocussed photometry
• Look at a bright star with a large telescope:
– long-ish exposure times: 120 s maximum
– defocus PSF to cover thousands of pixels
• Advantages:
– large PSF ⇒ low flatfielding noise
– long exposure times ⇒ less time lost toCCD readout
– less CCD readout ⇒ more photons,less scintillation
Focussed PSF for WASP-4
Defocussed PSF for WASP-5
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Defocussed photometry
• Look at a bright star with a large telescope:
– long-ish exposure times: 120 s maximum
– defocus PSF to cover thousands of pixels
• Advantages:
– large PSF ⇒ low flatfielding noise
– long exposure times ⇒ less time lost toCCD readout
– less CCD readout ⇒ more photons,less scintillation
• Disadvantages:
– lower time resolution ⇒ 120 s is goodenough for planets
– higher background ⇒ not importantfor bright stars
– still need good weather. . .
Focussed PSF for WASP-4
Defocussed PSF for WASP-5
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Example: WASP-2
• Problem: want to measure planet radius to a few percent
• Consequence: need rA and rb to a few percent
• Answer: Defocus!
Collier Cameron et al.
(2007, MNRAS, 375, 951)
20cm telephoto lens
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Example: WASP-2
• Problem: want to measure planet radius to a few percent
• Consequence: need rA and rb to a few percent
• Answer: Defocus!
Charbonneau et al.,
(2007, ApJ, 658, 1322)
1.2 metre telescope
Collier Cameron et al.
(2007, MNRAS, 375, 951)
20cm telephoto lens
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Example: WASP-2
• Problem: want to measure planet radius to a few percent
• Consequence: need rA and rb to a few percent
• Answer: Defocus!
My observations
(telescope defocussing)
1.5 metre telescope
Charbonneau et al.,
(2007, ApJ, 658, 1322)
1.2 metre telescope
Collier Cameron et al.
(2007, MNRAS, 375, 951)
20cm telephoto lens
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More examples
WASP-5
2009, MNRAS, 396, 1023
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More examples
WASP-42009, MNRAS, 399, 287
WASP-5
2009, MNRAS, 396, 1023
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More examples
WASP-182009, ApJ, 707, 167
WASP-42009, MNRAS, 399, 287
WASP-5
2009, MNRAS, 396, 1023
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More examples
WASP-2(in preparation)
WASP-182009, ApJ, 707, 167
WASP-42009, MNRAS, 399, 287
WASP-5
2009, MNRAS, 396, 1023
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Transiting planet properties
• Light curve: Porb rA k i
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Transiting planet properties
• Light curve: Porb rA k i
• Radial velocity measurements:
– stellar velocity amplitude KA
– orbital eccentricity e
– can’t observe the planet velocity amplitude (Kb)
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Transiting planet properties
• Light curve: Porb rA k i
• Radial velocity measurements:
– stellar velocity amplitude KA
– orbital eccentricity e
– can’t observe the planet velocity amplitude (Kb)
• Spectral analysis: stellar Teff and[
Fe
H
]
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Transiting planet properties
• Light curve: Porb rA k i
• Radial velocity measurements:
– stellar velocity amplitude KA
– orbital eccentricity e
– can’t observe the planet velocity amplitude (Kb)
• Spectral analysis: stellar Teff and[
Fe
H
]
• Interpolate in stellar models:
– find best-fitting mass for the star
– find most likely age for the system
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Transiting planet properties
• Light curve: Porb rA k i
• Radial velocity measurements:
– stellar velocity amplitude KA
– orbital eccentricity e
– can’t observe the planet velocity amplitude (Kb)
• Spectral analysis: stellar Teff and[
Fe
H
]
• Interpolate in stellar models:
– find best-fitting mass for the star
– find most likely age for the system
• Now we can get planet mass and radius
– get surface gravity ⇒ atmosphere studies
– get density ⇒ composition and core size
– composition and core size ⇒ formation scenario
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Transiting planet properties
• Light curve: Porb rA k i
• Radial velocity measurements:
– stellar velocity amplitude KA
– orbital eccentricity e
– can’t observe the planet velocity amplitude (Kb)
• Spectral analysis: stellar Teff and[
Fe
H
]
• Interpolate in stellar models:
– find best-fitting mass for the star
– find most likely age for the system
• Now we can get planet mass and radius
– get surface gravity ⇒ atmosphere studies
– get density ⇒ composition and core size
– composition and core size ⇒ formation scenario
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Stellar models v. observations
• Plot masses, radii and Teffs for eclipsing binary star systems
– masses and radii can be measured directly and to 1%
• Add predictions of stellar models for age 0.5Gyr and solar[
Fe
H
]
• Models disagree with observations . . . and with each other
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Homogeneous studies of transiting planets
• Problems:
– People analyse transiting planet light curves in different ways
– People use a variety of stellar models to get physical properties
– Results are not consistent
– Stellar model imperfections are ignored
Southworth (2009, MNRAS, 386, 1644) Southworth (2009, MNRAS, 394, 272)
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Homogeneous studies of transiting planets
• Problems:
– People analyse transiting planet light curves in different ways
– People use a variety of stellar models to get physical properties
– Results are not consistent
– Stellar model imperfections are ignored
• Solution: homogeneous study
Southworth (2009, MNRAS, 386, 1644) Southworth (2009, MNRAS, 394, 272)
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Homogeneous studies of transiting planets
• Problems:
– People analyse transiting planet light curves in different ways
– People use a variety of stellar models to get physical properties
– Results are not consistent
– Stellar model imperfections are ignored
• Solution: homogeneous study
• Step 1: light curve analysis with jktebop code
– Uncertainties from Monte Carlo simulations
– Residual permutation algorithm to find red noise
Southworth (2009, MNRAS, 386, 1644) Southworth (2009, MNRAS, 394, 272)
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Homogeneous studies of transiting planets
• Problems:
– People analyse transiting planet light curves in different ways
– People use a variety of stellar models to get physical properties
– Results are not consistent
– Stellar model imperfections are ignored
• Solution: homogeneous study
• Step 1: light curve analysis with jktebop code
– Uncertainties from Monte Carlo simulations
– Residual permutation algorithm to find red noise
• Step 2: Physical properties
– Use measured Teff ,[
Fe
H
]
, KA
– Include five different stellar models
– Try eclipsing binary mass–radius relation
Southworth (2009, MNRAS, 386, 1644) Southworth (2009, MNRAS, 394, 272)
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The disease of systematic errors
• Left: plot of mass versus radius for transiting planet host stars
– Black diamonds: random errors
– Red diamonds: systematic errors
– Blue line: eclipsing binary mass–radius relation
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The disease of systematic errors
Southworth (2009,
MNRAS, 394, 272)
• Left: plot of mass versus radius for transiting planet host stars
– Black diamonds: random errors
– Red diamonds: systematic errors
– Blue line: eclipsing binary mass–radius relation
• Right: plot of mass versus radius for the planets
– Blue lines: density = 1.0, 0.5, 0.25 × Jupiter
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Surface gravity: immune from the disease
Southworth et al (2007,
MNRAS, 379, L11)
• Planetary surface gravity can be calculated directly:
gb = 2π
Porb
√1−e2 KA
r 2b
sin i
– e, KA from velocity measurements
– Porb, rb, i come from the light curve
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Surface gravity: immune from the disease
Southworth et al (2007,
MNRAS, 379, L11)
• Planetary surface gravity can be calculated directly:
gb = 2π
Porb
√1−e2 KA
r 2b
sin i
– e, KA from velocity measurements
– Porb, rb, i come from the light curve
• gb correlates withorbital period
– interestingphysics?
– selectioneffects?
– small-numberstatistics?
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Future part 1: secondary eclipse measurements
Spitzer Space Telescope8µm observations ofHD189733(Knutson et al., 2007,Nature, 447, 183)
• Secondary eclipses can be measured in the infrared
• Direct detection of light from planet
• Use to measure the infrared spectrum of a planet
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Future part 2: transiting Earth-likes
• CoRoT and Kepler satellites
– launched 2006/12 and2009/03
– can detect 1–2R⊕ planets
– CoRoT-7: mass 4.8M⊕,radius 1.7R⊕
• Kepler could detect somethinglike Earth, far enough fromits star to be habitable . . .
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John Southworth, Astrophysics Group, Keele University