Slide 1

Kaltenegger et al. 2010 Presented by Craig Malamut December 4, 2012 Slide 2 Background: Astronomical spectroscopy Blackbody Radiation Exoplanet detection methods Characterizing a habitable planet Biosignatures Geological Evolution, Cryptic Worlds, Abiotic Sources, and Host Stars Future Missions Slide 3 lumenistics.com/ Slide 4 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 Slide 5 Segura et al. 2005 T eff,F ~ 6,000 - 7,500 K T eff,Sun = 5777 K T eff,K ~ 3,700 5,200 K Blackbody Spectrum Carroll and Ostlie 2007 Slide 6 Smithsonian Astrophysical Observatory Slide 7 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 Slide 8 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 Slide 9 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 Slide 10 Original Image credit: ESA; additional illustrations by D.K. Sing Slide 11 Favors: cooler stars, bigger planets, hotter planets (i.e. younger planets) Very Large Telescope Terrestrial Planet Finder (rejected) ESO Slide 12 phl.upr.edu Slide 13 Slide 14 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 Slide 15 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 Slide 16 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 Slide 17 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): 10 -10 Thermal IR (~10 m): 10 -7 Contrast far less for hot giant exoplanet orbiting smaller, cooler star. (Mars Global Surveyor) Slide 18 Transmission Spectrum Synthetic Reflection and Emission Spectrum Slide 19 Surface conditions on habitable planet within HZ Kaltenegger et al. 2010; adapted from Kasting et al. 1993 Inner edge of HZ: loss of water by photolysis and hydrogen escape Outer edge of HZ: formation of CO 2 clouds, increase albedo Slide 20 Kaltenegger et al. 2010 Slide 21 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 Slide 22 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. Slide 23 Has to be produced rapidly to be detectable on Earth because oxygen likes to oxidize things! Kaltenegger and Selsis (2007) Slide 24 Where there is O 3, there is a lot of O 2. O 2 + O + O O + O 2 + M O 3 + M O 3 + 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 isnt as frequent Slide 25 N 2 O (nitrous oxide) CH 4 (methane) NH 3 (ammonia) Chlorofluorocarbons: CCl 2 F 2 CCl 3 F Slide 26 H 2 O CO 2 (strong spectral feature) Not biosignatures, but raw materials for life Slide 27 Visible to near IRMid-IR H 2 O O 2 (0.76 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 Slide 28 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 Slide 29 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 Slide 30 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 Slide 31 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 Slide 32 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) Slide 33 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) Slide 34 Slide 35 Carroll, Bradley W., and Dale A. Ostlie. An Introduction to Modern Astrophysics. 2nd ed. San Francisco: Pearson Addison-Wesley, 2007. Print. Christensen, P.R. and Pearl, J.C. (1997) Initial data from the Mars Global Surveyor thermal emission spectrometer ex- periment: observations of the Earth. J. Geophys. Res. 102:10875 10880. Cockell, C.S., Kaltenegger, L., and Raven, J.A. (2009) Cryptic photosynthesisextrasolar planetary oxygen without a sur- face biological signature. Astrobiology 9:623636. 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