Lecture 39: Life in the Universe Life in the Universe Extrapolating from our solar system experience... The Search for Extraterrestrial Intelligence (SETI) Is anyone else out there? How can we find out? What would it mean? Reading: Chapters 24.4-24.5 24.5 The Main Point While simple forms of life could be expected to be relatively common in the Universe, the odds of finding intelligent life out there are probably much lower. Still, we are compelled to search Astro 102/104 1 Astro 102/104 2 Simple Life vs. Complex Life... Simple organic molecules are abundant in the solar system and beyond Simple, bacteria-like life forms dominate life on Earth (Lecture 37) Focus of Mars and Europa exploration is on uncovering evidence for simple life forms So why, then, should we even consider extending the search towards more complex life forms, and ultimately ones that are intelligent? Astro 102/104 3 Why Care About Extraterrestrials? Pros: Increased awareness of our place in the Cosmos Answers the question "Are We Alone?" Practical knowledge gained over time? Cons: "Common wisdom" that complex life is rare Two-way communication takes too long We have enough trouble dealing with each other and other species on this planet... Searching for them costs money Astro 102/104 4 1
But we already care... Science (fiction) sells! Books Games Movies TV Magazines Web NASA... Billions of $$$$ industries Astro 102/104 5 The Science of SETI The Search for Extraterrestrial Intelligence (SETI) can be addressed using the methods of science But the search takes resources like money, people, time, equipment,... How much should we devote to this search?? Fundamentally, the answer depends on society s and individual people s balance between looking inward and looking outward Astro 102/104 6 Is SETI even practical to think about? Do we have valid reasons to believe that anyone else is out there? Perhaps... The Sun is a common type of star, and there are probably > 100 billion Sun-like stars in the Galaxy Planetary formation appears to be a common process around single Sun-like stars (theory and observations) So: do the math: 100 billion stars 30% in single-star systems 10 planets/star = 300 billion planets! Astro 102/104 7 Some SETI math... But how many of these putative planets contain life? Is our solar system typical? If yes: 10% Is life on Earth a freak accident? If yes: ~ 0% So out of ~300 billion planets: could be from just 1 to >30 billion in our Galaxy with life But how many planets have intelligent life? Is that intelligent life technological? And just how good are these kinds of estimates??? Astro 102/104 8 2
The Drake Equation Astronomer Frank Drake's attempt to quantitatively estimate SETI's potential success N = R * f s N p f e f L f i L Where: N = Number of civilizations in the Galaxy capable of communicating with us R * = The rate of star formation in the Galaxy (stars/year) f s = fraction of stars that are Sun-like N p = number of planets per star f e = fraction of "environmentally correct" planets f L = fraction of planets where life develops f i = fraction where intelligent & technological life develops L = lifetime of an intelligent & technological civilization (years) Astro 102/104 9 Estimating N... Factor Optimistic Pessimistic Best Estimate R * : Star formation rate 50 20 30 f S : Fraction of Sun-like stars 1 1/15 0.3 N p : Number of planets per star 20 5 10 f e : Fraction of habitable planets 1 1/1,000,000 1/40 f L : Fraction of those with life 1 1/1,000,000 0.5* f i : Fraction with intelligent life 1 1/1,000 0.75* L: Lifetime of that civilization??? = 0.85L From Goldsmith & Owen (1992) *Wild Guess N = R * f s N p f e f L f i L For reasonable estimates, N L Astro 102/104 10 Caveats... The Drake Equation is an estimate and not a true scientific derivation Call it "back of the envelope" or "handwaving" There is much uncertainty in the various factors in the equation! (N = 1 to 1,000,000! ) The general relationship that N~ L implies that: Optimism about L implies optimism about N Optimism about L implies optimism about SETI For us, L ~ 100 years (so far) as a transmitting and receiving species The number of civilizations in the galaxy is closely related to: (a) how long a technological species survives (b) how long any single technological phase lasts (c) the duration of technological phases that are compatible with our current search methods Astro 102/104 11 Evidence for intelligent life on Earth We know there is intelligent life here... Could we be detected by extraterrestrials? Directly? Images Electromagnetic "leakage" (radio, TV, military,...) Indirectly? O 2 detected in Earth's atmosphere Pollutants, man-made chemicals in the atmosphere? Astro 102/104 12 3
Views of Earth as it could be seen from an extra-terrestrial telescope (hundreds of km resolution) Views of Earth as seen from a lowresolution orbiting satellite platform (tens of km resolution) (Top): Global view of Earth in visible light (left), infrared light (middle), and within a water vapor absorption band (right) (Right): Global infrared view of Earth from the Galileo spacecraft during an Earth gravity assist flyby Astro 102/104 13 Views of Earth as seen from a higherresolution orbiting satellite platform (a few km resolution) No unambiguous signs of intelligent life at these scales... Astro 102/104 14 Views of Earth as seen from a very high resolution orbiting satellite platform (a few tens of meters resolution) Views of Earth as seen from an ultra high resolution orbiting satellite or an airborne platform (a few meters resolution) Spectra of Earthlike Planets Greater sensitivity to Earth-like "disequilibrium" atmospheres can be obtained by measuring the spectra of extrasolar planets Hard! Requires TPF or beyond... We start to see regular, geometric patterns (cities, farms, other Astro 102/104 15 structures) at these scales Astro 102/104 16 4
Some "messages" already sent... A few directed radio telescope messages Pioneer, Voyager plaques and records 1974 Arecibo radio signal directed at M13, 21,000 l.y. away SETI Today SETI today is primarily Passive: Listening at "magic" frequencies Assume "they" know physics and math Targeted: to nearest Sun-like stars Highest probability of Earthlike worlds? Privately funded: little NASA, government $ No Congressional support for "little green men" Lots of public skepticism... Pioneer Plaque Astro 102/104 17 Voyager Record Astro 102/104 18 http://setiathome.ssl.berkeley.edu/ Healthy skepticism is good! Much of what we have seen and learned this semester is based on theory and/or interpretation Some of what we've "learned" is no doubt wrong! Science is a self-effacing, self-correcting, incremental, and ultimately human endeavor Be critical and skeptical of "facts"!! Carl Sagan, in his book "The Demon Haunted World", advocates that everyone carry around a baloney detection kit to help maintain skepticism Astro 102/104 19 Main Parts of Carl Sagan's Baloney Detection Kit Wherever possible there must be independent confirmation of the facts. Encourage substantive debate on the evidence by knowledgeable proponents of all points of view. Arguments from authority carry little weight (in science there are no "authorities"). Spin more than one hypothesis - don't simply run with the first idea that caught your fancy. Try not to get overly attached to a hypothesis just because it's yours. Quantify, wherever possible. If there is a chain of argument every link in the chain must work. Occam's razor - if there are two hypothesis that explain the data equally well choose the simpler. Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by some unambiguous test). In other words, it is testable? Can others duplicate the experiment and get the same result? Astro 102/104 20 5
Some Final Thoughts... Astronomy matters! Daily stuff like navigation and timekeeping But also plenty of Big Picture stuff And we also learn about our planet by studying others Climate changes Impact hazards Origin and evolution of life By learning and exploring, we grow as individuals and as a species You are now armed for some awesome party conversations! Astro 102/104 21 Reminder: Final Exam! (2 hour exam) Friday May 9, 2-4 p.m., Barton Hall Closed book, independent work Final exam approximately twice as long as prelims Concepts from entire course emphasized Some additional emphasis on Part IV of course Study practice exams and old exams posted online Review chapter summaries Review the problems at ends of chapters Review your lecture notes and section notes Review your homework assignments Bring #2 pencils Calculators or other electronic devices NOT allowed No bathroom breaks for the first hour Astro 102/104 22 6