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Where to Look: Habitable Zones

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Where to Look: Habitable Zones The formal definition of habitable zone is the distance from the star at which temperatures allow for liquid SETI surface water. Why should we expand this definition? Karen J. Meech, Astronomer Institute for Astronomy A. Habitability can be subsurface, enabled by geothermal heat B. Liquid water is possible on bodies without substantial atmospheres if it is under pressure (sub-surface) C. There are energy sources that allow for liquid water other than the sun D. Habitable zones change with time as a star ages E. A, B and C Overview of Today: Searching for Where and How to Look? Extraterrestrial Intelligence ! Where to look for life ! How to look for ETI ! Chances of Success ! Past, current & future search programs SETI is a Search for ET Intelligence " technology Where to Look – Where to Look: Habitable Zones Stellar Lifetimes ! L/LS Region near star where Lifetime Class Temp [K] liquid H O can exist [yrs] 2 ! Hot stars – wider zone White dwarfs 7 10 O 35,000 ! Hot stars – HZ far from star T [K] 108 – 107 B 20,000 – 30,000 ! Astronomy review 109 – 108 A 10,000 – Class Temp [K] L (Lsun) H2O H2O ! HR diagram: is a T sequence 15,000 Freeze Boil ! Mass (gravity) supported by pressure B1 23,000 13,000 173.7 93 109 – 1010 F, G 6,000 – ! More Mass " more P " higher T " 10,000 A1 9,700 61 11.9 6.4 higher energy (L) 10 G2V 5,800 1 1.5 0.8 ! Massive stars " shorter lives #10 K, M 3,000 – 6,000 M5 3,000 0.015 0.19 0.1 The Galactic Habitable Zone Habitable Zone ! An extension of the concept of the HZ ! The region(s) of the Milky Way that allow complex biological life to emerge ! Evolution of the HZ • Massive stars " supernovae ! Stellar luminosity increases • Release of dangerous radiation (not habitable) – red ! • Early on – not enough heavy elements for planets – blue HZ moves outward as star’s T rises • Green – optimal conditions for life ! Planet atmosphere composition changes Some requirements for complex life in the Galaxy: ! Located in a region with few transient radiation events, ! HZ of Stars such as supernovae ! Low-mass planets in the HZ are tidally-locked to their host stars ! Depletion of ozone ! Like the moon around the Earth ! Enough metallicity, or building blocks for terrestrial planets ! One side always hot, the other always cold ! Enough time for biological evolution ! A planet orbiting a medium mass star in the HZ is not tidally locked The Galactic Habitable Zone Binary Star Systems Two predictions Stellar center of mass ! Lineweaver et al. (2004) Majority of habitable pla- nets between 7-9 kpc, expanding outwards with time. ! Gowanlock et al. (2011) Majority of habitable pla- Schematic of a planet in a binary star system nets in the inner Galaxy, a) The planet orbits one component of the binary expanding with time, but b) The planet is in a circumbinary orbit about both stellar components entire disk suitable. Image: Astronomy Now magazine, Dec. 2011 From: Perryman 2011 Where to Look – Where to Look – Binaries? Binaries?? ! Planets have been detected orbiting in binary star systems ! ~50% of stars in the Milky Way are binary systems ! Factors influencing habitability ! Eccentricity ! How circular are the orbits of planets? ! Temperature ! How does the temperature of planets change with time? ! Dynamic stability ! How do the orbits of planets change with time? ! ! Cephei – First binary system with a planet ! Will planets be retained in binary systems over long periods? ! Free floating planets have been detected ! 1.59MSun (K1IV), 0.4MSun (M1) ! Period = 74 yr, e = 0.44 ! Planet 1.76MJup, e = 0.2 ! Stable if in resonance Class Discussion Where to Look: Extrasolar Planets ! Why do you think that planets in binary star systems could ! To date, there have been > 1000 confirmed exoplanets have habitable planets? ! Including the Kepler candidates, there are over 3000 exoplanet detections ! Statistics show that maybe 70% of stars might have planets !185 stars with planets out to 150 LY How: Manifestations of Where to Look – Summary Advanced Civilizations ! ’ ! Rule out hot stars: OBA ! Best F, G, K Stars Invention of TV – 1940 s (~70 yrs ago) ! Lifetime too short ! EM radiation travels out to 70 LY ! Rule out cool stars?? M ! 50% of power is narrow (0.1Hz) carrier wave ! HZ too small ! 50% of power is the “picture” ! But . These are the most numerous stars . ! Binaries? ! Possible, but not first choice We’ve Already Sent Messages # stars How far has our TV signal travelled? Dyson Spheres !1 AU = 1.5 LY ! Our stellar neighborhood ! Oort cloud ~ 100,000 AU = 1.5 LY ! Advanced civilization ! Local stellar density ~ 0.004 stars / cubic LY ! Builds structure !Oort Cloud ! In 2013 the signal has passed ~ 6500 stars surrounding star at HZ ! http://workshop.chromeexperiments.com/stars/ distance ! Collects all solar energy ! Only IR / radio escape ! A variant – “Ringworld” !77 LY !25 LY Types of Searches – Interstellar Travel Space Probes – Cheaper Alternative? ! Interstellar Travel ! Eavesdrop on another civilization ! Chemical rockets ! Smaller cost per probe ! v=30 km/s - to " Cen – 40,000 yr! ! Cryogenic suspension? Generations? ! Send 106 probes (to get big enough star system sample) ! Nuclear ! Launching 1/day " 3000 years! ! 80 years ! Cost $1013 ! Photon (matter-antimatter) ! Lack technology ! Info sent by radio " many 100m class radio telescopes ! 10 years ! Why send a probe? Just listen! ! Fuel costs ~ $1015 (106 billion = quadrillion) ! Cooling requires 2x103 km array ! Ramjets – collect fuel from ISM ! Scoop of 100 km radius ! Expensive (“millennia” of the world GNP) ! Uninvented Technologies?? ! Not invented! Characteristics of Effective Where to Search in the EM Communication Spectrum – The “Water Hole” ! Requires minimum E over background noise ! 1-10 GHz – low noise ! Travel at the speed of light ! Recall c = #f ! Not deflected by stellar ! H radiates at 1.42 GHz magnetic fields (# = 21 cm) ! Easy to generate, beam & detect ! OH at 1.6-1.72 GHz ! Not be absorbed by ISM ! Cosmic “Water Hole” EM Radiation !. But millions of frequen- No Mass cies are still possible . ! No Charge Specific Wavelengths Probability of Success? !Number of stars now The Drake Equation R = 10 yr-1 ! Nciv Number of detectable civilizations * ! N Number of stars now ! Stars form out of clouds of gas & dust ! fp Fraction forming planets ! Process ~ well understood ! np Fraction suitable for life ! We have a good observed number for local rate of star formation (R*) over time ! fl Fraction where life exists ! At least 100 billion stars in the Milky Way ! fi Fraction with intelligence ! Number of stars is related to the star ! fc Fraction with communication formation rate ! Lc Probability of survival * N* = Sum of R* over time Nciv = N x fp x np x fb x fc x fi x fc x Lc Illingworth, et al. (UCLO) Fraction Forming !How Many are Planets Habitable? ! By Star type ! M stars: small zones, but very abundant ! F stars: much larger HZ f = 0.2-0.9 ! Extrasolar Planet Searches p ! ! Rate of hot Jupiters – 0.75% Rocky planet occurrence ! 23% of stars host super Earths with P < 50 days ! Rate of Jupiter-mass objects inside 3.5 AU (Howard et al. 2010) – 7% n = 0.01-0.5 ! 17.4% of stars have planets with M < 10 Mearth ! 1-1.5% sunlike stars have gas giants p (Wittenmyer et al. 2011) ! Mostly finding Jupiter-mass to date " must have smaller planets too ! True rate must be much higher! ! 40% of stars may have low mass planets ! New Kepler discoveries " 1200 planets! !Fraction with !Fraction With Life Intelligence? ! Studies of language " markers of intelligence ! Organics in space are ubiquitous ! Information theory ! Miller-Urey Experiments ! Complexity of communication ! Organics + water + energy " amino acids ! Ability to make tools / problem solve ! Hydrothermal vents ! Brain to body mass ratio ! Oceans + internal planetary heat ! Many factors may be indicative of intelligence ! Life occurred early on the Earth fl = 0.001-0.01 fi = 0.001-0.01 Animal Communi- Fraction with Technology cation Complexity ! Homo sapiens has been around for ~ 3 million yrs (Australopithicine) Species Communication Entropy Repertoire ! Technology – 100 yrs Size ! Inevitable? Monkey Social behavior 6.9 120 Bottlenose Whistles 6.67 102 dolphin Homosapien Alphabet " 4.8 26 char (english) f = 0.1-0.9 Chinese 11.6 3000 c Survival? !Survival? ! Mean lifetime of a civilization or fraction which endures a long “The receipt of a message from an advanced civilization will show that enough time to be detected. there are advanced civilizations, that there are methods of avoiding the self-destruction that seems so real a danger of our present technological adolescence. Finding a solution to a problem is helped enormously by the certain knowledge that a solution exists. This is one of many curious connections between the existence of intelligent life elsewhere and the existence of intelligent life on Earth.” Carl Sagan -5 Lc = 10 -0.01 Meanings Evaluating the Drake Equation Variable Pessimist Optimist Description * N 100 billion 100 billion Num of stars in MW fp 0.2 0.9 Fraction of stars form planets np 0.01 0.5 Fraction w/ habitable planets f 0.001 0.01 Existence of Life l ! Optimist fi 0.001 0.01 Rise of Intelligence ! Curiosity is by product of intelligence fc 0.001 0.9 Rise of technology ! Pessimist: 106 LY ! Laws of physics same L 0.00001 0.01 Survival/duration now c between civilizations everywhere Total 0.000002 50500 Communicating civilizations now ! Pessimist ! Optimist: 16 LY " lots ! Consider development 104-109 of neighbors! yrs ago! No communication! Optimistic vs. Pessimistic Views Frank Tipler “Proof why there are ! Optimist ! Curiosity is the by product of intelligence no other civilizations” ! Laws of physics same everywhere: might give rise to intelligent life everywhere ! Tipler Argument ! Will explore galaxy in 300 x 6 ! Life occurred early on the Earth and lead to intelligent life ! Take the optimistic path for # 10 yr civilizations ! Many planets detected in the Milky Way ! No evidence on Earth ! No technology " ! Von Neumann machines ! Pessimist sophisticated ~ 100 yr ! We’ve only been able to send signals for ~100 years ! Older societies " more ! They do not exist ! It is not clear that technology does not lead to self-destruction advanced (e.g.
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