Kaltenegger et al Presented by Craig Malamut December 4, 2012

 Background: Astronomical spectroscopy Blackbody Radiation Exoplanet detection methods  Characterizing a habitable planet  Biosignatures  Geological Evolution, Cryptic Worlds, Abiotic Sources, and Host Stars  Future Missions

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A hot solid or dense gas produces a continuous spectrum A cool, thin gas seen in front of a hot source produces absorption lines A hot, thin gas produces an emission spectrum Image: The Pennsylvania State University Department of Astronomy and Astrophysics

Segura et al T eff,F ~ 6, ,500 K T eff,Sun = 5777 K T eff,K ~ 3,700 – 5,200 K Blackbody Spectrum Carroll and Ostlie 2007

Smithsonian Astrophysical Observatory

 Methods: 1. Radial Velocity 2. Transit 3. Direct Imaging 4. Astrometry 5. Gravitational Lensing  Holy Grail: Earth- like planet in the habitable zone xkcd.com

High Accuracy Radial Velocity Planet Searcher (HARPS) Favors: cooler/low mass/older/high metallicity stars, massive planets, short period/semi-major axis Planet Properties: msini, period, semi-major axis

nasa.gov Copyright 2006 CNES Kepler COROT Favors: small stars, big planets, short periods Planet Properties: ratio of radius planet to star, inclination, brightness temperature

Original Image credit: ESA; additional illustrations by D.K. Sing

 Favors: cooler stars, bigger planets, hotter planets (i.e. younger planets)  Very Large Telescope  Terrestrial Planet Finder (rejected) ESO

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 Orbital elements Period Semi-major axis  Radius (transits)  Mass (or msini if RV only)  Planetary brightness temperature  Giant planetary upper atmosphere absorption Sodium, hydrogen, water, methane, CO, CO 2 So far primarily detected in hot Jupiters (distended atm)  Presence of atmosphere Low res. Spectrum Planetary day/night temperature difference

Selsis (2002) Orbital thermal IR light curve for blackbody planets in a circular orbit Mean brightness temperature can be used to estimate albedo by considering incoming stellar radiation and outgoing IR emission

 Identify compounds of planetary atmosphere  Constrain temperature and radius Mean effective temperature  Requires identification of spectral windows in which portion of the blackbody curve is seen.  Can observations be explained abiotically?  If not, biotic hypothesis considered

Visible Earthshine Near IR Earthshine Thermal Emission (Mid IR) Woolf et al. (2002) Turnbull et al. (2006) Christensen and Pearl (1997) Earth-Sun intensity ratio: Visible (~500 nm): Thermal IR (~10 μ m): Contrast far less for hot giant exoplanet orbiting smaller, cooler star. (Mars Global Surveyor)

Transmission Spectrum Synthetic Reflection and Emission Spectrum

Surface conditions on habitable planet within HZ Kaltenegger et al. 2010; adapted from Kasting et al Inner edge of HZ: loss of water by photolysis and hydrogen escape Outer edge of HZ: formation of CO 2 clouds, increase albedo

Kaltenegger et al. 2010

A “detectable atmospheric species or set of species whose presence at significant abundance strongly suggests a biological origin” (Kaltenegger et al. 2010) Pertinent characteristics: Not a natural part of a terrestrial atmosphere Not created geophysically Not produced by photochemistry Needs to have a strong spectral feature

 We are searching for life as we know it  Uses liquid water as a solvent  Carbon-based chemistry  Same input/output gases as Earth, existing out of thermodynamic equilibrium imdb.com Radically different life would produce unknown signatures.

 Has to be produced rapidly to be detectable on Earth because oxygen likes to oxidize things! Kaltenegger and Selsis (2007)

Where there is O 3, there is a lot of O 2. O 2 + γ  O + O O + O 2 + M  O 3 + M O 3 + γ  O 2 + O O + O 3  2O 2 Production of O 3 Destruction of O 3 M = third molecule needed in triple collision to take away excess energy, therefore 2 nd reaction isn’t as frequent

 N 2 O (nitrous oxide)  CH 4 (methane)  NH 3 (ammonia)  Chlorofluorocarbons: CCl 2 F 2 CCl 3 F

 H 2 O  CO 2 (strong spectral feature)  Not biosignatures, but raw materials for life

Visible to near IRMid-IR H 2 O O 2 (0.76 μ m) O 3 (strong feature in UV not shown here) CO 2 – only visible in high-CO 2 atmosphere (~10%), such as early Earth Detectable signature of biological activity in low resolution: H 2 O plus O 3 plus CO 2 CH 4 if more abundant than If O 3 highly satured – poor quantitative indicator, but excellent qualitative for O 2

 CH 4 Abundant constituent of cold planetary atmospheres in outer Solar System Produced in hydrothermal vents on Earth Without atmospheric oxygen, CH 4 could accumulate in atmosphere to detectable levels  O 2 Photolysis of CO 2 and subsequent recombination of O atoms to form O 2 (i.e. O + O + M  O 2 + M) Photolysis of H 2 O and then escape of hydrogen to space

 Photosynthetic plants reflect IR to prevent overheating  Chlorophyll a absorbs radiation at 450 nm and chlorophyll b at 680 nm  This results in a steep change in reflectivity around 700 nm (the “red edge”)  Detection of VRE requires high res. spectra and knowing the cloud coverage Reflectivity for Different Surfaces Kaltenegger et al. 2010

 Photosynthetic organisms on Earth may be driven into and under substrates where light is still sufficient for photosynthesis Exhibit no detectable surface spectral signature Cockell et al. 2009

 Reduce the relative depths, full widths, and equivalent widths of spectral features Weakens spectral lines in both thermal IR and visible  With high S/N and time resolution, possible to determine cloud contribution Kaltenegger et al. 2010

 Models of Earth orbiting different stars: K star: cooler, less massive than Sun  Thin, colder O 3 layer  Deeper O 3 feature F star: hotter, more massive than Sun  Denser, warmer O 3 layer  Weak spectral feature G and K stars better candidates for detectable O 3 signature Selsis (2002)

 James Webb Space Telescope  Ground-based telescopes: Extremely Large Telescopes (ELT)  European Extremely Large Telescope (E-ELT)  39.3 meters!  Thirty Meter Telescope (TMT)  30 meters!  Giant Magellan Telescope (GMT)  24.5 meters!  Dedicated space-based missions: Darwin (rejected), Terrestrial Planet Finder (cancelled), New World Observer (2020…)

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