Part I---Introduction: planets, and habitable planetsstar--about 1011 in our galaxy. Average separation is a few lightyears. (Compare with size of Galaxy: about 100,000 light years)planet--indirect arguments from theory as well as directobservations of extrasolar planets suggest giant planets may be verycommon. But Earth-like (much smaller, rocky) planets?habitable planet--requires liquiad? Liquid water? Nearlyeveryone agrees this is fundamental (we’ll discuss why later).Requires special temperature range, and so only certain range ofdistances from star.Probably additional factors for habitability, like planetary mass (foratmosphere), …life--How probable or improbable? Need to understand how lifearose and developed on Earth (our only example). We will spendthe 2nd part of this course discussing the many theories,experiments, and types of evidence related to this.intelligence--What does this mean? Are there different “types”?Why think that extraterrestrials would share our forms ofcognition? Compare cross-cultural, cross-species, cross-historicalcognition.communication—representation, language. How likely? Otherforms?length of time spent communicating—we expect any nearbycivilizations to have had a long lifetime. (We’ll see why this is soshortly.)Another way to look at the problem is: What is the likelihood or probabilitythat the following sequence of events will occur, ending up with the ratherpeculiar group of phenomena listed at the bottom branching? And are theremany other possible branchings that we haven’t thought of, or are incapable(for, say, biological reasons) of thinking of?“rational thought”languagetechnologyscienceplanetstarartsincreasing complexitylifeself-consciousnessliteracymental statesbig bang????The “Drake equation”Main purposes of discussing this equation are:1. To organize the topics that we need to discuss in detail.2. To demonstrate that in order for Galactic civilizations to be closeenough for communication to be feasible, the average lifetimes ofcivilizations must be VERY large.You will never be asked to do calculations using this equation; it is just ahandy symbolic tool for discussion.Multiply together the separate probabilities for:star, planet, habitable planet, life, intelligence, technologyto obtain an estimate of the number of communicating civilizations inour galaxy N:N = N* · fp · ne · f l· f i · fc · (L/LGalaxy)where N* is the number of stars in our Galaxy (~1011), L is the averagelifetime spent in the phase in question (e.g. technological andcommunicative) and LGalaxy is the age of our Galaxy (~1010 yr).(The formula is written slightly differently in different sources,but the idea is the same. BSJ [pp. 274-277] use “NHP” for the productN* · fp · ne above (number of habitable planets) and “fnow” in place of(L/LGalaxy), but they mean the same thing. The “original” Drakeequation that is usually used is given on p. 276, and uses the rate ofstar formation R* and L instead of N* and L/LGalaxy but the product isequivalent. Thinking about this will sharpen your understanding ofthe equation, but if it makes sense to you and you remember that weare really only using it to make a couple of points, you’ll be ok.)Notice that the ratio in the last factor gives the fraction of timethat the civilization is “on”. (A “light switch” analogy will bediscussed in class).[We could also include factors for fraction of planets with oceans, or large moons, orenough metals for communicating technology, or other effects that might be important,but we’ll come back to that later in the course.]It is important to understand the relation between the number ofcivilizations N and their average separation, since that is what determineswhether any communication (at the speed of light) is possible. We discussthis further below. First, let’s see that if N is large enough for two-waycommunication, the lifetimes L of these civilizations must be (on average)extremely large.Example: Number of planets with life If we leave out fi and fc (i.e. assume they are unity—all life formsdevelop our kind of intelligence and technology and try to communicate),we are calculating the number of life-bearing planets in our Galaxy at anygiven time (like now). We know there has been life on our planet for 3billion years, so take L = 3 billion. Let’s be optimistic about fp (0.1), np (1),and fl = (0.1). ThenNlife ~ 1011 x 0.1 x 1 x 0.1 x (3 billion/10 billion) = 300 million300 million planets with life in our Galaxy! That’s roughly1 out of 1000stars. This means that the nearest life-bearing planet might only be 10-100light years away, close enough that in the future we may be able to detectsuch planets and obtain their spectra (that is the primary goal ofastrobiology space missions for the next decade).This result is a major reason for exerting most of our efforttoward detecting signatures of biochemistry in the spectra of planetsorbiting nearby stars. You will be reading and hearing a lot about“biosignatures” in this class soon! But if we are interested in planets with communicating life,even if fi and fc = 0.1 (optimistic!), we only getNcomm ~ 1011 x 0.1 x 1 x 0.1 x 0.1 x 0.1 x Lcomm / 10 billion = Lcomm If Lcomm is only 1000 yr (roughly 10 times our age), then Ncomm ~ 1(unlikely to be any others: we are essentially alone). And this is the“optimistic case” with large values of the various Drake equationprobabilities, where we are getting N = L. Many people think it is verylikely that N ~ 10-4 L or even much smaller. (See later pictures.)⇒ So for communicating civilizations to be numerous, L must be verylarge, e.g. !>~ 106 years!What would such a civilization with a very long lifetime be like?e.g. genetically-engineered photosynthetic, disease-free humanoids with regeneratingbrains and extremely long lifespan?or replicating conscious bio-computers?or beings that have transcended the level of ideas, concepts, etc.? etc.In any case, the point is that if the Galaxy is populated enough sothat we can communicate with them, we had better be prepared toencounter civilizations that have been around much longer than us.Significance of N: Distance to our nearest neighborsIn most respects the Drake equation is merely a nice way to organizethe categories of questions we must consider in developing strategies forSETI. But it is still interesting to consider the implications of the number“N” of
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