1. Terrestrial Planet Finder - Coronagraph
Wesley A. Traub
Jet Propulsion Laboratory, California Institute of Technology
NAI PSFFG Meeting
AAS, Long Beach, CA, 7 January 2009

2. Yes, we now have images of exoplanets….
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3. but the Big Questions remain:
Are there Earth-like planets around nearby stars?
Are there signs of life on these planets?
For answers, we need these missions:
Astrometric
Mission
Large Strategic
Coronagraph/Occulter
Mission
Large Strategic
Interferometer
Mission
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4. We know that the physics of direct imaging is formidable…
10-10
visible
10-6
infrared
Contrast is ≤ 10-10
Separation is ≤ 0.1 arcsec
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5. Therefore, a large strategic coronagraph mission is needed…
before
Off-axis
secondary
mirror
V-shaped
thermal shields
after
Coronagraph
section
off-axis
primary mirror
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6. … or a large strategic starshade mission
NWD Starshade
JWST
Target Star
Planet
Big telescope (planet is faint!)
Big occultor (few times size of telescope)
Big separation (to see close to star)
See talk by Web Cash
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7. Coronagraph Technology Progress
Images from
3 Types of Coronagraph
Coronagraph comparison
Useful throughput
Telescope diameter
Angle from Star (λ/D)
Message: PIAA & Optical Vortex
are more efficient than VNI/BL4 or 8,
and make better-quality images
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8. Proof of principle: lab demo of (1 – sinc2)2 mask
Star image (no mask)
Star image (with mask)
Focal-plane mask
Pupil-plane stop
Deformable mirror
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Trauger & Traub, Nature, April 2007

9. Color can give our first impression of a planet …
Color-color diagram:
blue= , green= , red= m
Spectra:
Solar system planets have colors
that label them by type.
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10. … & astrometric, visible, & IR can characterize its habitability …
Molecular
column
Mass
IR flux
IR color
Vis flux
Vis & IR
spectra
Eff temp.
Radius
Albedo
Greenhouse
warming
Density
of planet
Surface
gravity
Surface & cloud
reflectances
pressure
Scale height
of atmos.
Lapse rate
Temp.
Visible IR
Astrometry
measured
derived
Type of
planet
Likelihood of
plate tectonics &
atmos retention
Presence
of H2O
Cumulus,
cirrus, ice,
rock, sand,
water
implied
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11. … & also characterization its variability.
Visible
spectra
Near-IR
Infrared
Vegetation
variation IR
derived
implied
Water, oxygen,
carbon dioxide
variations
Cloud
Surface
spectrum
Temperature
Mass of
atmosphere
observed
Orbital
eccentricity
Cloud height
Large-scale
weather
patterns
Obliquity
Thermal
time const
of atmos
Continents,
oceans, ice
areas
Seasons
Length
of day
Astrometry
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12. Spectra can reveal a planet’s geologic stage of development.
Detectable phases:
1 High CO2 compensates for faint Sun
2 First life consumes CO2
3 Methanogens start
4 Oxygen-producing bacteria start
5 Global ice
6 Modern Earth
Refs: Kasting, Scientific American;
Kaltenegger et al, 2006; Holland 2006;
Des Marais et al 2002
Kaltenegger et al 2005 in prep.
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13. But, astrobiology characterization requires a large telescope …
Signal (photon/sec) from Earth-twin,
10 pc, 10% bandwidth, 100% efficiency
Dashed line = local zodi.
Integration time (hr), S/N = 10,
1% bandwidth, 25% efficiency
Signal = Earth-twin,
Noise = local zodi
Ref: Beckwith, ApJ 684, 1404, 2008
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14. … and a large sample of targets requires a large telescope.
Number of target stars
for Earth-twin in HZ,
100% efficiency
24 hr integration
S/N = 10
1% bandwidth
Examples:
8-m telescope, 512 stars
4-m telescope, 41 stars
Ref: Beckwith, ApJ 684, 1404, 2008
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15. plus stellar conditions for an Earth-twin,
Conclusions:
The physics of imaging,
plus stellar conditions for an Earth-twin,
require large telescopes for astrobiology characterization.
So a medium strategic mission will be of value,
but ultimately a large strategic mission will be needed.
Traub