Characterizing and Characterizing and Detecting Extrasolar Detecting Extrasolar

PlanetsPlanetsDavid SpergelDavid Spergel

February 2004February 2004

PREDICTIONPREDICTIONSome time in the next decade, Some time in the next decade, SIM, Kepler, Eclipse, JPF, or SIM, Kepler, Eclipse, JPF, or some other telescope will be some other telescope will be

detect an Earth-like planet. This detect an Earth-like planet. This will revolutionize astronomywill revolutionize astronomy

Will We Find Life?Will We Find Life? The necessary ingredients of life are widespread The necessary ingredients of life are widespread

Observation reveals uniformity of physical and chemical Observation reveals uniformity of physical and chemical laws laws

Origin of the elements and their dispersal is well understoodOrigin of the elements and their dispersal is well understood Life on Earth can inhabit harsh environmentsLife on Earth can inhabit harsh environments

Micro- and environmental biology reveal life in extremes of Micro- and environmental biology reveal life in extremes of temperature, chemistry, humiditytemperature, chemistry, humidity

Life affects a planetary environment in a detectable wayLife affects a planetary environment in a detectable way Our own atmosphere reflects the presence of primitive Our own atmosphere reflects the presence of primitive

through advanced lifethrough advanced life Planets are a common outcome of star formationPlanets are a common outcome of star formation

Modern theory of Modern theory of starstar formation makes formation makes planetplanet formation formation likelylikely

10 -30 m Optical Telescope as 10 -30 m Optical Telescope as Life FinderLife Finder

TPF will likely be a 4-8 TPF will likely be a 4-8 meter class optical meter class optical telescope or a “small” telescope or a “small” mid-IR interferometermid-IR interferometer

It will be capable of It will be capable of detecting Earths out to detecting Earths out to 10-15 pc10-15 pc

10-30 m telescope 10-30 m telescope would be the next step: would be the next step: Characterize planets Characterize planets and detect a large and detect a large samplesample

Direct Planet Imaging: Good Direct Planet Imaging: Good NewsNews

•Much faster detections

•Immediate detection of entire system

•Enormous additional science

•Size and Albedo

•Spectroscopy

•Biomarkers

Ford/Seager/Turner ModelFord/Seager/Turner Model•See FST, See FST, Nature, Nature, 412412, 885 (2001)., 885 (2001). 180x360 deg resolution map of surface180x360 deg resolution map of surfacePixel auto-classification by satellite imageryPixel auto-classification by satellite imageryBDRFs - in 4 bands for 6 pixel typesBDRFs - in 4 bands for 6 pixel typesSingle scattering, no elevation variationsSingle scattering, no elevation variationsGray cummulus clouds onlyGray cummulus clouds onlyMonte Carlo to 1% accuracy: B, G, R, NIRMonte Carlo to 1% accuracy: B, G, R, NIR•Water with waves (specular & isotropic components)

•Permanent ice (strong backscattering)

•Seasonal/sea ice (80% dirty ice, 20% dirt)

•Bare ground (90% sand, 10% clay)

•Grass/brush land (67% dirt, 33% clover)

•Forested land (75% leaves, 25% peat)

Scattered LightScattered Light The scattered light comes from a small part of the planet surface

Viewing GeometryViewing Geometry No Clouds: high contrast between land and ocean

Ford, Seager,Turner, Nature, 2001

CloudsCloudsClouds: bright, variable, correlated in space and time.

Ford, Seager,Turner, Nature, 2001

Extrasolar PlanetsExtrasolar Planets

Ford, Seager,Turner, Nature, 2001

TIME (Days)

Plants in Plants in visiblevisible versus versus near infrarednear infrared lightlight

Optical Plant Signature as a Optical Plant Signature as a BiomarkerBiomarker •Chlorophyll causes

strong absorption blueward of 0.7 m.

•The high reflectance red-ward of 0.7 m is from light scattering in the gaps between plant cells.

•This “red edge” is an evolutionary adaptation which helps plants stay cool enough to allow efficient photosynthesis.

0

10

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200 700 1200 1700 2200 2700wavelength(nm)

reflective index(%)

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??????

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?

Red

Ed

ge

chlorophyll

absorption

water absorptio

n

wavelength （ nm ）

rosechinquapin

Benjamin

Reflection by plantsReflection by plants

hibiscus pothos

baseline

APO 3.5m Earthshine Spectra (Feb 2002)

watervapor

watervapor

watervapor oxygen

(A)oxygen

(B)oxygen

()

REDEDGE?

Bad NewsBad News Detecting light from Detecting light from

planets beyond solar planets beyond solar system is hard:system is hard: Planet signal is weak Planet signal is weak

but detectable (few but detectable (few photons/sec/mphotons/sec/m2)2)

Star emits million to Star emits million to billion more than planetbillion more than planet

Planet within 1 AU of Planet within 1 AU of starstar

Dust in target solar Dust in target solar system system 300 brighter 300 brighter than planetthan planet

Finding a firefly next to a Finding a firefly next to a searchlight on a foggy nightsearchlight on a foggy night

>109>106

The Diffraction Problem The Diffraction Problem (Visible)(Visible)

Focal plane

Diameter (D)

The image in the focal plane is the spatial Fourier transform of the entrance field

Wavelength

Entrance Pupil

0 5 10 1510 -12

10 -10

10 -8

10 -6

10 -4

10 -2

100

Angle (lambda/D)

1

1e-10

Airy RingsAiry Rings

Linear Scale

Log Scale(1e-10 is black)

The Angular Resolution The Angular Resolution ChallengeChallenge

+

• Coronagraphs at >3/D• Interferometers at > 1 /B

10 m, 28 mCoronagraph

Cost ($$), L

aun

ch D

ate

Control of Control of Star LightStar Light

Control Control diffracteddiffracted light with light with various apodizing pupil various apodizing pupil and/or coronagraph and/or coronagraph masksmasks Square masksSquare masks Graded apertureGraded aperture Multiple Gaussian Multiple Gaussian

masksmasks Band limited masksBand limited masks

-6

-5.5

-5

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-2Circular pupil PSF

-200 -150 -100 -50 0 50 100 150 200

-200

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Circular pupil

-6 -4 -2 0 2 4 6

-6

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Pupil position (m)

Pupil position (m)

Pupil

-6 -4 -2 0 2 4 6

-6

-4

-2

0

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6 -12

-11.5

-11

-10.5

-10

-9.5

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-8.5

-8

Sky Angle (mas)

Sky Angle (mas)

Final PSF (max=100)

-200 -150 -100 -50 0 50 100 150 200

-200

-150

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-50

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• Control scattered light–Deformable mirror with 10,000 actuators for final /3000 wavefront (<1 Å)

Wavefront Sensing Wavefront Sensing and Controland Control

What is the biggest problem?What is the biggest problem?

Wavefront Error!

Phase Aberrations Amplitude Aberrations

Coronagraph StatusCoronagraph Status

• Rapid progress in past year• HCIT (Trauger at JPL) is now achieving

contasts of ~ few 10-9 with band-limited masks and active optics

• Significant progress in understanding control and imaging

Can a VLST be Life Finder?Can a VLST be Life Finder?

Life Finder will have special requirementsLife Finder will have special requirements Stability (or sensing)Stability (or sensing) Uniform amplitude or controlUniform amplitude or control

Need to evaluate 10-30 m in space versus 100m Need to evaluate 10-30 m in space versus 100m on groundon ground

Requirements:Requirements: Ability to do 5-10% photometry within an hourAbility to do 5-10% photometry within an hour

(size depends on distance to Earth-like planet)(size depends on distance to Earth-like planet) Small field, spectral resolution improves ability to Small field, spectral resolution improves ability to

remove speckleremove speckle Wave front sensing and controlWave front sensing and control

Planet Finding Is Planet Finding Is A Decades-Long A Decades-Long

UndertakingUndertaking Like cosmology, the Like cosmology, the

search for planets and life search for planets and life will motivate broad will motivate broad research areas and utilize research areas and utilize many telescopes for many telescopes for decadesdecades to come to come