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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DETECTION PROTOCOL

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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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NUMBER OF TERRESTRIAL PLANETS IN HZ

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NUMBER OF PLANETS FOUND OUTSIDE HZ

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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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END

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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

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Transit Signal vs. Solar Noise

Time Scale, days

Energy

&

Power

Density

8-hour transit10-hour transitSolar MinSolar Max