Transcript Document
Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the monthMay 5, 2008 Dr. Uwe Trittmann Today’s Topics • How to find Life in the Universe • The Night Sky in May On the Web • To learn more about astronomy and physics at Otterbein, please visit – http://www.otterbein.edu/dept/PHYS/weitkamp.a sp (Observatory) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) How to find Life in the Universe • What is Life? • How to detect Life? • Where to look for Life? • Life vs. Intelligent Life What is Life? • • • • • Has a metabolism Uses energy Reacts to changes in the environment Reproduces … Transforms its environment Changes to environment hopefully observable What kind of Life? • Focusing on carbon – liquid water based life – Carbon as a chemically unique element – (Liquid) Water as a powerful solvent and reactant • Assuming lower life-forms, we cannot expect life to actively produce signals • Look for life on the surface of a rocky planet Water • Phase diagram of water dictates temperature and pressure range for liquid Minimal pressure: 610 Pa -18º C: life on Earth exists +123º C: life on Earth exists Where to look for Life? • Where you can find (detect) it ;-) • If carbon-liquid water based life exists on the surface of a planet – Planet must have atmosphere for liquid water to exist – Planet must receive enough energy from host star to liquify water Planet must be in the habitable zone (HZ) of its host star Definition of “Habitable Zone” • Region around a star where stellar radiation maintains liquid water over a substantial part of the surface of a rocky planet • Note: planet must be big enough to hold on to its atmosphere, and possibly replenish it through outgassings from its interior. Extend of HZ depends on time • • • • Sun gets brighter as it ages Planets orbits change over time Planets rotational axis inclination changes Early Solar system: – Venus, Earth in HZ, Mars partially • Now: – Earth, Mars in HZ Habitats beyond the HZ • Temperature, pressure too low go to interior of planets/moons – Earth: 5km under surface liquid water can exist • Tidal heating: gravitational distortions will heat interior of (soft) moons – Io Volcanism driven by Jupiter – Europa liquid water under ice? • Saturn’s Titan Not on surface of Planet – Methane/Nitrogen based geology/climate at -180°C Hard to detect! Greenhouse Effect affects Climate • Earth absorbs energy from the Sun and heats up • Earth re-radiates the absorbed energy in the form of infrared radiation • The infrared radiation is absorbed by carbon dioxide and water vapor in the atmosphere Typically happens and stabilizes climate on planet harboring life Good, because life takes at least a billion years to develop/ have effects on planet The biggest effect life had on Earth • Shakespeare? • Building the Great Wall of China? • Explosion of the Hiroshima bomb? Transforming Carbondioxide into Oxygen! (Zero oxygen 3 billion years ago, now 21%) Chances of detecting Life • How far away is the nearest Earth-like planet? 19 ly (if 3% of stars have ELPs) • Is it habitable? – – – – Atmosphere Rocky Carbon/Water available shielded from heavy bombardment How to detect Life • Life produces oxygen, methane • Can detect ozone (made from O2), Methane via its characteristic infrared radiation • Plants use photosynthesis: chlorophyll rejects colors not utilized (utilizes visible frequencies) IR red-edge • TV & Radio signals 1992: Galileo spacecraft detects life on Earth (and not on the Moon)! (Very) Intelligent Life • Looking for extraterrestrial intelligence, we can relax our assumptions (neither carbon nor waterbase necessary) • We are looking for signals of civilizations rather than signs of life Classification of Civilizations • Type I: uses energy sources of their planet including solar radiation arriving at their planet (us!) • Type II: uses a large part of the total radiation of their sun (shows up as reddening of the star’s spectrum) • Type III: uses a large part of the energy production of their galaxy (might rearrange galaxy) Galactic Exploration • John von Neumann: build self-replicating space probe that builds many replicas of itself once it finds suitable conditions, send them on their way – Can colonize galaxy in only 100 million years (less than 1% of lifespan) • O’Neill colonies Fermi Paradox • If ETI exists it must be widespread • If it’s widespread, why aren’t they among us? • • • • ETI must have had plenty time to occur Maybe they do not exist Maybe we didn’t look hard/long enough? Maybe they are among us? Signals • Probably electromagnetic waves – Easy to generate – not exceedingly absorbed by interstellar medium, planetary atmospheres – Information can be imprinted on them with minimal energy cost • Travels fast (but not fast enough?!) • We are detectable since 12. December 1901 Green Bank (or Drake) Equation • Estimated number of technological civilizations present in the Milky Way galaxy is given by the average rate of star formation fraction of stars having planetary systems average number of planets within the habitable zone for various types of star and star system fraction of habitable planets that develop life fraction of life-bearing planets on which intelligence appears fraction of intelligent life forms that develop technology average lifetime of a technological civilization • Could be 100 to 1 billion (?) Illustration of Drake Equation Time is of the Essence • A lot of things can go wrong in “cosmic instances” like a few thousand years • It is “guesstimated” that a technological civilization might last about 3000 years Extinction of the Dinosaurs • Possibly caused by impact of a large meteorite – Large amount of dust thrown into atmosphere, causing global cooling, disruption of the food chain – Evidence: • Iridium layer found in fossil record at about time of extinction of dinosaurs • Large numbers of species become extinct at about the same time • Crater in Yucatan may be “the one” • Are extinctions periodic? SETI • If average lifetime is 1 million years, then the average distance between civilizations in the galaxy is 150 ly – Thus 300 years for messages to go back and forth • Communications via radio signal – Earth has been broadcasting in RF range for most of this century – Earth is brighter than the Sun in radio – 18–21 cm wavelength range good for interstellar communication • SETI search is ongoing – SETI@Home: http://setiathome.ssl.berkeley.edu • If they exist, should we contact them? SETI with Radio Telescopes • Radio frequencies are used because – Civilizations are likely to use these frequencies – We can observe them from the ground • Biggest radio telescope is in Arecibo, Puerto Rico CETI – Talking to Aliens • How can we communicate? – Put up a big sign (?!) – Send a (radio) signal – Send a space probe with a message • Should we try to communicate? Our Messages to the Aliens • Golden plate with essential info on humans • On board Pioneer 10 space probe • Started in the 70’s • past solar system In 1974 sent radio signal from Arecibo to globular cluster M13 (300,000 stars, 21,000ly away) Brighter than the Sun “The signal, transmitted at 2380 megahertz with a duration of 169 seconds, delivered an effective power of 3 trillion watts, the strongest man-made signal ever sent.” Our EM Message to the Aliens The Night Sky in May • Nights shorter and EDT => later observing! • Spring constellations are up: Cancer, Leo, Big Dipper • Mars, Saturn dominate early evening, Jupiter early morning. Moon Phases • Today: New Moon • 5 / 11 (First quarter Moon) • 5 / 19 (Full Moon) • 5 / 27 (Last Quarter Moon) • 6 / 3 (New Moon) Today at Noon Sun at meridian, i.e. exactly south 10 PM Typical observing hour, early May Saturn Mars Zenith Big Dipper points to the north pole South • Saturn near Praesepe (M44), an open star cluster • Oops, that was last year! Now Saturn is here! South • Spring constellations: – Leo – Hydra – Crater – Sextans East • Canes Venatici: – M51 • ComaVirgo Cluster • Globular Star Clusters – M3, M5 East Virgo and Coma with the VirgoComa galaxy cluster VirgoComa Cluster • Lots of galaxies within a few degrees M87, M88 and M91 East – Hercules – Corona Borealis – Bootes Globular Star Clusters: •M3 • M 13 • M 92 M13: Globular Cluster These guys will know of our existance in 21,000 years! Mark your Calendars! • Next Starry Monday: June 2, 2008, 8 pm (this is a Monday • Observing at Prairie Oaks Metro Park: – Friday, May 9, 2008, 9:00 pm • Web pages: – http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) )