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Astronomy 110 Announcements: • Extra Credit due tomorrow • Review session tomorrow during class • Final Exam this Friday – No calculators, cell phones, etc. – Mostly short answer, some multiple choice • Sunday July 3rd, Deep Impact (Encounter with Comet Temple 1) – Events on all islands – Viewing at Bishop Museum ($3 admission) Three Methods for Detecting Extrasolar Planets 1. Doppler effect due to “wobble” of star 2. Transiting planets (eclipses) 3. Microlensing Cannot currently detect extrasolar planets through direct imaging. Planets are too faint compared to their Sun. Improved technology like interferometry may help in the future. Deep Impact website: http://astroday.net/DeepImpactHawaii.html 2. Transiting Planets 1. Doppler Effect Doppler effect due to “wobble” of star • • • Kepler’s 3rd law gives us orbital distance Size of wobble gives us mass of planet Useful only for large planets close to their Sun Transiting planets (eclipses) • • Light from star dims as planet passes in front Useful only for large planets 3. Microlensing 7.5 Earth as a Living Planet Microlensing • • • Foreground star with planets acts as a lens against distant star. As lens passes in front, light from star and is amplified. Useful for low mass planets (Earth-like) But you only get one shot at it • Our Goals for Learning • What unique features on Earth are important for human life? • How might human activity change our planet? • What makes a planet habitable? What unique features of Earth are important for life? 1) 2) 3) 4) Surface liquid water Atmospheric oxygen Plate tectonics Climate stability What unique features of Earth are important to human life? 1) 2) 3) 4) Surface liquid water Atmospheric oxygen Plate tectonics Climate stability Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible What unique features of Earth are important to human life? 1) 2) 3) 4) Surface liquid water Atmospheric oxygen Plate tectonics Climate stability PHOTOSYNTHESIS (plant life) is required to make high concentrations of O2, which produces the protective layer of O3. Plate Tectonics • Pieces (plates) of the Earth’s surface move around over geologic time (few centimeters/yr) • Internal heat needed to maintain this process (heat also drives volcanism) What unique features of Earth are important to human life? 1) 2) 3) 4) Surface liquid water Atmospheric oxygen Plate tectonics Climate stability Plate tectonics are an important step in the carbon dioxide cycle. The Carbon Dioxide Cycle • Atmospheric CO2 dissolves in the oceans. • Rainfall erodes rocks and sends minerals into the ocean • Minerals + CO2 combine to form carbonate rocks (e.g., limestone) – depends on temperature • Plate tectonics carries rocks into subduction zones • Carbonate rock melts and releases CO2 back into the atmosphere. What unique features of Earth are important to human life? 1) 2) 3) 4) Surface liquid water Atmospheric oxygen Plate tectonics Climate stability The CO2 cycle acts like a thermostat for the Earth’s temperature. These unique features are intertwined: • plate tectonics creates climate stability • climate stability allows liquid water • liquid water is necessary for life • life is necessary for atmospheric oxygen Earth’s ice ages end as oceans freeze over and volcanoes release CO2 into the atmosphere How might human activity affect Earth’s climate? Chapter 18 Life in the Universe Human activity is increasing the concentration of greenhouse gases in the atmosphere, which may strengthen the greenhouse effect and lead to global warming. We, this people, on a small and lonely planet Travelling through casual space Past aloof stars, across the way of indifferent suns To a destination where all signs tell us It is possible and imperative that we learn A brave and startling truth. — Maya Angelou When did life arise on Earth? • Probably before 3.85 billion years ago. • Shortly after end of heavy bombardment, 4.2-3.9 billion years ago. • Evidence from fossils, carbon isotopes. 2 billion years… How did life arise on Earth? • Life evolves through time. • All life on Earth shares a common ancestry. • We may never know exactly how the first organism arose, but laboratory experiments suggest plausible scenarios. Laboratory experiments allow us to investigate possible pathways to the origin of life. Miller-Urey experiment (and more recent experiments): • Building blocks of life form easily and spontaneously under conditions of early Earth. Chemicals to Life? Microscopic, enclosed membranes or “pre-cells” have been created in the lab. Could life have migrated to Earth? • Venus, Earth, Mars have exchanged tons of rock (blasted into orbit by impacts) • Some microbes can survive years in space... Maybe this is how it happened… What makes the Earth habitable? • Large size: retains heat longer. – Leads to plate tectonics which drive the CO2 cycle, which regulates the climate – Volcanism (outgassing). Leads to our oceans and atmosphere. • Orbital distance: just the right distance to retain liquid water. Are habitable planets likely? Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. • It does not necessarily have life. Caveat: Telescopically we can search only for planets with habitable surfaces — not for worlds with Europa-like subsurface oceans. What are the necessities of life? • Nutrient source • Energy (sunlight, chemical reactions, internal heat) • Liquid water (or possibly some other liquid) Hardest to find on other planets Constraints on star systems: 1) Old enough to allow time for evolution (rules out high-mass stars - 1%) 2) Need to have stable orbits (might rule out binary/multiple star systems - 50%) 3) Size of “habitable zone”: region in which a planet of the right size could have liquid water on its surface. Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds. Finding them will be hard Recall our scale model solar system: The more massive the star, the larger the habitable zone — higher probability of a planet in this zone. • Kepler (2007 launch) will monitor 100,000 stars for transit events for 4 years. • Looking for an Earthlike planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast — with a VERY bright grapefruit nearby. • But new technologies should soon show the way… Spectral signatures of life Venus oxygen/ozone Earth Later: SIM (2009?), TPF (2015?): interferometers to obtain spectra and crude images of Earthsize planets. Mars Are Earth-like planets rare or common? • Galactic “habitable zone”: minimum limits on heavy element abundance, distance from galactic center? • Jupiter protection from frequent impacts? • Climate stabilized by a large Moon and plate tectonics? We don’t yet know how important or negligible these concerns are. How many civilizations are out there? The Drake Equation Number of civilizations with whom we could potentially communicate = NHP ! flife ! fciv ! fnow NHP = total # of habitable planets in galaxy flife = fraction of habitable planets with life fciv = fraction of life-bearing planets w/ civilization at some time fnow = fraction of civilizations around now. Are we “off the chart” smart? We do not know the values for the Drake Equation NHP : probably billions. flife : ??? Hard to say (near 0 or near 1) fciv : ??? It took 4 billion years on Earth fnow : ??? Can civilizations survive long-term? How does SETI work? Looking for deliberate signals from E.T. Your computer can help! SETI @ Home: a screensaver with a purpose. We’ve even sent a few signals ourselves… Earth to globular cluster M13: Hoping we’ll hear back in about 42,000 years!