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

Yes, we now have images of exoplanets…. Traub

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 Traub

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 Traub

Therefore, a large strategic coronagraph mission is needed… before Off-axis secondary mirror V-shaped thermal shields after Coronagraph section off-axis primary mirror Traub

… 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 Traub

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 Traub

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 Traub Trauger & Traub, Nature, April 2007

Color can give our first impression of a planet … Color-color diagram: blue=0.4-0.6, green=0.6-0.8, red=0.8-1.0 m Spectra: Solar system planets have colors that label them by type. Traub

… & 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 Traub

… & 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 Traub

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. Traub

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 Traub

… 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 Traub

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