KEPLER: The Search for Earth-size Planets in the Habitable Zone of Solar-like Stars
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Transcript of KEPLER: The Search for Earth-size Planets in the Habitable Zone of Solar-like Stars
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•Gibor Basri, U. Cal., Berkeley• Alan Boss, Carneige Inst. W. •Timothy Brown, HAO, UCAR •Donald Brownlee, U. Wash.•John Caldwell, York U. •Christensen-Dalsgaard, Arhus U.•William Cochran, U. Texas•Edna Devore, SETI Institute•Edward Dunham, Lowell Obs.•Andrea Dupree, CfA, SAO•T. Nick Gautier, JPL•John Geary, CfA, SAO
• Ronald Gilliland, STScI• Alan Gould, Lawrence Hall of Sci.• Steve Howell, U. Ariz • Jon M. Jenkins, SETI Institute• Yoji Kondo, NASA GSFC• David Latham, CfA, SAO• Jack Lissauer, NASA Ames• Geoff Marcy, U. Cal., Berkeley• David Monet, U.S. Naval Obs.• David Morrison, NASA Ames• Tobias Owen, U. of Hawaii• Harold Reitsema, Ball Aerospace• Dmiter Sasselov, CfA, SAO• Jill Tarter, SETI Institute
William J. Borucki, PI, and David Koch, Deputy PI
SCIENCE TEAM
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CHARACTERISTICS OF EXO PLANETS
Most planets are inside of 2 AU
Many planets heavier than Jupiter
15% of systems have > 2 Jupiters
Jupiters must form readily
Most orbits have high eccentricity
Giant planet migration & scatteringremove small planets
Lineweaver claims >10% of starshave giant planets
Earths might be rare
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KEY QUESTIONS:
• Are terrestrial planets common or rare?
• What are their sizes & distances?
• How often are they in the habitable zone?
• What is their dependence on stellar properties?
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SCIENTIFIC OBJECTIVES
• Frequency of terrestrial and larger planets in & near the habitable zone of a wide variety of stellar spectral types
• Distribution of sizes & semi-major axes of these planets
• Characterisitics of additional members of each planetary system by using other techniques
• Distributions of semi-major axis, albedo, size, and density of short-period giant planets
• Occurrence frequency and orbital distribution of planets orbiting multiple star systems
• Characteristics of stars that harbor planetary systems
Determine the:
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REQUIRED SENSITIVITY
• ∆L/L = area Earth/area Sun = 1/12,000 = 8x10-5
• Require total noise <2x10-5 for 4- detection in 6.5 hours
• Three sources of noise and their contributions:
- Stellar variability:<1x10-5, Sun on timescale of ~1/2 day
- Shot noise:1.4x10-5, in 6.5 hr for mv=12 solar-like star and 0.95-meter aperture
- Instr. noise: <1x10-5, includes dark current, read noise, thermal effects, pointing jitter, & shutterless operation.
• Detectors: 42 2k x1k format CCDs with dual readout
- Thinned, back-illuminated, & anti-reflection coated
• Brightest mv=9 and full-well depth of 106 e- requires:
- Soft image and readout every 6 seconds.
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MEASUREMENT TECHNIQUE• Use differential photometry (common mode rejection):
- Stellar flux is re-normalized to the ensemble of thousands of stars in each half of each CCD & readout with a single amplifier;
• Transits only last several hours: - Long term photometric stability is not necessary;
• Star image covers 25 pixels: - Mitigates saturation (109 e-/hr) and sensitivity to motion;
• Control pointing to 3 millipixels (0.01 arc sec); - Images remain on the same pixels;
• Operate CCDs near full-well capacity at low temperature: - Dark current and read-noise effects negligible; - Minimizes damage by GCR & solar proton events;
• Photometer in a heliocentric orbit (like Spitzer): - Provides stable thermal and stray light environment.
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Use transit photometry to detect Earth-size planets 0.95 meter aperture provides enough photons Observe for several years to detect transit patterns Monitor a single FOV continuously to avoid missing transits Use heliocentric
orbit
MISSION APPROACHKepler: A Wide FOV Photometer to Monitor 100,000 Stars for 4 yrs that can Detect Earth-size Planets in the HZ
Get statistically valid results by monitoring 100,000 dwarf stars • Wide FOV telescope • Large array of CCD detectors
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MISSION FEATURES
Designed to find hundreds of Earth-size and larger planets
Stellar classification for targets
Ground-based observations rule out false positives
Single science instrument:
Photometer: 0.95m aperture, 42 CCDs, 420-890nm, passive cooling, focusable primary
FOV: 100 sq deg. centered & fixed at 19h23m, 44º 30’
Launch Vehicle: Delta 2925-10L
Launch date: June 2008
Operational life: 4 years with expendables for 6 years
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MERIT FUNCTION (MF)Quantifies science value as f(instrument & mission
properties)
Mission chosen for the science it could perform =>MF score based on currently predicted science
performance
MF properties• Models of planetary systems, instrument specs., detection
approach• Score is 100 based on currently predicted instrument perform.
a) 60 pts for planets in HZ, 30 pts for planets outside HZ, 10 pts for p-modes
b) Small planets have higher value than bigger (40,20,5,1)c) Outer planets have higher value than inner planets (r2)
• Adjustable parameters for instrument specs & performance, mission parameters, and surprises of nature
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STELLAR CLASSIFICATION PROGRAM
• Multiband photometry of 5x106 stars in FOV• Calibrate against spectroscopic observation
of clusters• Get spectral type & luminosity class Size
estimate• Choose late type dwarfs for targets• Choose only brightest large stars and accept
dim small stars to maximize small planet detections
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VALIDATION OF DISCOVERIES
SNR > 7 to rule out statistical fluctuations
Three or more transits to confirm orbital characteristics
Light curve depth, shape, and duration
Radial velocityMedium resolution rules out stellar companionsHigh resolution measures mass of giant planets
Image subtraction to identify signals from eclipsing background stars
High spatial resolution Identifies extremely close background stars
Color change during transit identifies bkgd stars
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DETECTION OF SHORT-PERIOD GIANTS
-10
-9
-8
-7
-6
-5
-4
0.1110100
0.01 0.1 1 10
Period, Days
Frequency, Days -1
51 Peg b ReflectedLight
Envelope
mv=10
mv=12
mv=14
Log
Rad
iativ
e P
WR
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SCIENCE DRIVER
Statistically valid result for abundance of Earth-size planets in habitable zone
Expected # of planets found, assuming one planet of a given size & semi-major axis per star and random orientation of orbital planes.
# of
Pla
net
Det
ecti
ons
Orbital Semi-major Axis (AU)
10000
1000
100
10
1 0.2 10 1.40.4 1.20.80.6 1.6
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OPPORTUNITIES FOR PARTICIPATION
Guest Observer Program• Choose targets in FOV• 250 stars at 1-min cadence, 3000 stars at 15-min
cadence, • Continuous for 3 months
Data Analysis Program• Obtain observations from STScI archive• Both differential and instrument magnitudes
Participating Scientist Program• Propose exoplanet studies that complement science
team studies
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SUMMARY & PLANS
Phase C/D work is underway.
Reduced funding requires delaying the launch by several months.
24 Flight-grade CCDs have been received.
Optics are being polished.
Kepler is on track for a June 2008 launch.
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MOST STARS ARE QUIET ENOUGH
– Variability noise declines with rotation rate• Magnetic activity declines• Spot passage period increases
– Solar-type stars slowed enough by 2-3 Gyr • Rotation-activity relationship well-known• Stellar spin-down timescales well-known
– 70% of solar-type stars are slow & quiet enough• Galaxy >10 Gyr old & star formation ~constant• Detailed galactic population models confirm • Actual observations of stellar activity confirm
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STARS QUIET ENOUGH TO FIND EARTHS
• Solar behavior at Kepler timescales and precision is known– ACRIM, DIARAD, VIRGO on SMM, SOHO– Measured throughout solar cycle
• Transits can be seen despite variability– Short durations (~10 hours)– Well-defined shapes and depths– Highly periodic repetitions
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STELLAR ACTIVITY LEVELS
-3.5
-5.0
-4
-4.5
-5.50.5 0.6 0.7 0.90.8
Very Active2.6 %
Active27.1%
(B-V)
Inactive62.5%
Very Inactive 7.9%
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HST OBSERVATION OF HD209458b
Ten-minute binned data from several orbits have a precision
of 60 ppm (Brown et al. 2001).
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OPERATIONS ORGANIZATION
Science Ops Center - Ames
• Scientific direction • Select target stars • Manage Instrument • Diff. ensemble photometry • Stellar light curves • Transit search • Candidate planet list • Direct follow-up observing • Final planet determination
Complementary Observing - SAO/UA/UC-B
• Stellar classification • Elim. false positives - low resolution
spectroscopy • Stellar properties • Giant planet mass - high resolution
spectroscopy
Data Management Center - STScI
• Calibrate data • Archive data • Difference images • HST follow-up • PSP and DAP
Education Public Outreach - LHS/SETI
• Formal education • Informal education • Public outreach
DSN
Mission Ops Center - Honeywell
• Cmd/control spacecraft • DSN scheduling • Acquire all data • Engr. data archive • Anomaly response
Sustaining Engneering - Ball Aerospace
• Manage MOC • Ensure spacecraft health/safety • Anomaly resolution
Raw data
Cmd/data
Calibrated dataTarget list
Results
Stellar data
Observ- ing request
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CONFUSION DUE TO ECLIPSING BINARIES
EXPECTED NUMBER OF GRAZING TRANSITS BY TARGET STARS
50% of target stars are binaries => 50,000 targets are binaries
20% have orbital periods of order days to weeks
10,000 stars with transit probabilities near 10%
1000 stars will show stellar transits
Of these 1,000 stars,
~ 6.5% (i.e., 65) stars will show 1% deep transits
~ 1.4% (i.e., 14) stars will show 0.1% deep transits
~ 0.3% (i.e., 3) stars will show 0.01% deep transits
20% have orbital periods between a few months and a few years
10,000 stars with transit probabilities near 1%
100 stars will show stellar transits
Of these 100 stars;
~ 6.5% (i.e., 6 ) stars will show 1% deep transits
~ 1.4% (i.e., 1.4 ) stars will show 0.1% deep transits
~ 0.3% (i.e., 0.3 ) stars will show 0.01% deep transits