# Basis of science's belief that extraterrestrial life is likely? **URL:** \ **Category:** Factual Questions\ **Created:** [November 9, 2010, 7:59pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078 "2010-11-09T19:59:16Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Diogenes\_the\_Cynic](https://avatars.discourse-cdn.com/v4/letter/d/e99b99/32.png) [@Diogenes\_the\_Cynic](https://boards.straightdope.com/u/Diogenes_the_Cynic)\ **Post date:** [November 9, 2010, 10:52pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/21 "2010-11-09T22:52:22Z")
> [@Blake](#): > > What are you talking about? I’m talking about your assertion that reaosning the probability of life arising twice if it can happen once as an example of the anthropic principle. The anthropic principle is a non-sequitur in that regard. > [@](#): > > The Drake equation can produce any answer Not correct. The Drake equation is based on the ubiquity of earth-like conditions and the universe and the extreme improbabibilty that billions of such conditions could only lead to abiogenesis once. thereis nothing magical about the earth. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 9, 2010, 11:00pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/22 "2010-11-09T23:00:44Z")
> [@Lemur866](#): > > But the problem with the lottery analogy is that we know in advance that there will be only one winner of a lottery, because that’s the way the game is structured. 😕 Where do you live? My local lottery draws a winner every single week. My analogy even specified that the prize was drawn every week. > [@](#): > > But life existing on Earth doesn’t decrease the probability of life existing on another planet, whereas if person X wins the lottery that means everyone not-X has to lose. No it doesn’t. The odds of person X winning next week are exactly the same as his chance of winning this week regardless of who won last week, or even if nobody won last week. I’m not sure where your confusion is here, but maybe it will help if I draw the analogy with two lottery winners: I surveyed all the lottery winners in my town. There were only _two_, and one only bought one ticket each week for a year before he won a million and the other bought a ticket every week for 10 years before he won a million. Therefore the average time that a person has to play the lottery before winning a million is just 5 years. Do you believe that reasoning is correct? Or do you think that selecting just the winners prevents us from drawing any conclusions at all about the probability of winning? I mean, there could be billion of people who play every week for 70 years and never win, right? As far as I can see, no matter how improbable the odds of an event are, if you only sample the winners you are going to conclude that it is likely. When you only have a single, self-selected sample that is a winner, you can conclude absolutely nothing. > [@](#): > > It’s tempting to conclude that there isn’t anything extraordinary about our planet. But as you say, we’ve got a sample size of 1. All that tells us for sure is that the generation of life doesn’t violate the laws of physics in this universe. Exactly. Yet people are somehow concluding that the probability of life arising here is closer to one figure than another. I ma interested in seeing how they arrived at such conclusions with a self-selected sample of one. ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 9, 2010, 11:03pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/23 "2010-11-09T23:03:40Z")
I have a vague recollection that Carl Sagan and some of his fellow cosmologists put together a list of some forty or fifty physical criteria that were required for life to exist on a planet. These criteria included such things as the location of the star in the galaxy, the type of star, proximity to that star, proximity to other stars, physical composition of the planet, size of the planet, age of the planet, etc etc. Given the extreme improbability of any planet meeting all of the specified criteria, the conclusion drawn was that life anywhere other than on earth is extremely improbable. I understand that the implications of this list drove Sagan to start thinking seriously about religion. I don’t have a cite for this, and I acknowledge that my recollection may be wrong. But in that it has great relevance to the discussion, does anyone else recall this list? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 9, 2010, 11:06pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/24 "2010-11-09T23:06:21Z")
> [@Diogenes\_the\_Cynic](#): > > I’m talking about your assertion that reaosning the probability of life arising twice if it can happen once as an example of the anthropic principle. The anthropic principle is a non-sequitur in that regard. Still no idea what you are talking about. That first sentence isn’t even English. “I’m talking about your assertion that reaosning the probability of life arising twice if it can happen once as an example of the anthropic principle”… Is what? “… as an example of the anthropic principle _is what_”? The sentence doesn’t end. > [@](#): > > Not correct. The Drake equation is based on the ubiquity of earth-like conditions and the universe and the extreme improbabibilty that billions of such conditions could only lead to abiogenesis once. thereis nothing magical about the earth. Well if it isn’t correct then show us the equations. It’s quite simple, you made the assertion, now pony up your evidence. I want you to show us what figures you plugged into the Drake equation to get this answer, then I want to know what you picked those figures. Come on Dio, enquiring minds want to see your calculations. Can you please stop repeating the same assertions and pony up the evidence. Please. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [November 9, 2010, 11:08pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/25 "2010-11-09T23:08:08Z")
> [@Grateful-UnDead](#): > > I understand that the implications of this list drove Sagan to start thinking seriously about religion. You understand wrong. Completely wrong. Dead wrong. 100% wrong. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 9, 2010, 11:11pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/26 "2010-11-09T23:11:48Z")
Remember life does not need the narrow conditions that humans live in. Right here on earth life has filled just about every ecological niche imaginable including some places distinctly hostile to human life. From bacteria in an [incredibly isolated underground lake in Antarctica](http://www.i-fink.com/articles/articles/lonely_old_bacteria/) to an ecosystem living on the bottom of the Atlantic Ocean near volcanic vents. Pitch black, acid “water”, very high temperatures and incredible pressure and [all sorts of life is there](http://www.onr.navy.mil/focus/ocean/habitats/vents2.htm). Presumably if there is liquid water available life is possible. Certainly is so here on earth so that suggests even a planet we would deem hostile could have life thus increasing the chances it could happen elsewhere in the universe. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [November 9, 2010, 11:14pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/27 "2010-11-09T23:14:17Z")
> [@Blake](#): > > Which is just the anthropic principle at work: we arose rapidly on Earth, so it must be likely to happen elsewhere. That is like saying “I bought one lottery ticket and won a million, so it must be pretty likely that every lottery ticket wins a million”. > > I have heard several cosmologists and biologists say that exactly the opposite is true. Their position is that life is so fragile while it is developing that in the vast majority of cases it gets destroyed well before it becomes robust enough to survive the trials the universe throws at it. The reason why our single sample developed so fast is because if had developed slowly, we wouldn’t be here to notice it. Since it is rare that life would develop rapidly enough to survive, then life must be perishingly rare elsewhere. In fact it may not exist anywhere else at all, because everywhere else it keeps getting destroyed before it can develop. > > As you can see, the same single sample, Earth, can be used as evidence for the arguments both that life is common and that life only evolved in one place in the universe. > > The second point is that we have absolutely no idea what factors are needed for the origin of life. Models aside, we haven’t a clue how life arose. The theory that Earth was seeded with life from the stars is just as rigorous and accepted as the one that it originated here. That is how little idea we have. Yes, we have models, and those models propose totally contradictory things. > > Even if we did know \*how \*life arose we still wouldn’t know how common necessary conditions are. For example, many have proposed that the Earth’s anomalously large moon played a central role both in providing unpredictable tides and in reducing catastrophic meteor impacts. Yet the creation of the moon itself is a billion to one shot that may very well never have happened anywhere else. > > The problem is that if life on Earth is the result of an improbable series of long shots, and they hadn’t occurred here, we wouldn’t’ be here to realise they hadn’t. So our single sample can not possibly tell us anything about the abundance of life elsewhere. It is the ultimate self-selected sample. Even if life required a planet identical in every way to Earth and had to evolve within a few million years at odds of 1 x 10^^1, 0000, 0000 ^ 1, 000, 000 against, we would have to be the \*result \*of that improbability in order to know about it. Anthropic principle. This is the scientific summation of my thoughts exactly. The answers (up to this point I haven’t finished reading and I haven’t followed links) seem to be saying just what I’ve heard said and I’m asking about: “Well, we’re here. And there’s a gajillion other planets in the universe. So we can’t possibly be the only place where life happened.” Why not? Life is pretty astonishing. Life as it exists here is freakin’ magical it’s so astonishing. No wonder so many people believe in God! ---
**Author:** ![Max\_the\_Immortal](https://avatars.discourse-cdn.com/v4/letter/m/6de8d8/32.png) [@Max\_the\_Immortal](https://boards.straightdope.com/u/Max_the_Immortal)\ **Post date:** [November 9, 2010, 11:20pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/28 "2010-11-09T23:20:34Z")
Is it plausible that all life on Earth is the descendant of a single cell produced by a single abiogenesis event? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 9, 2010, 11:22pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/29 "2010-11-09T23:22:53Z")
> [@Blake](#): > > The Drake equation can produce any answer, including “the probability that life would arise only once is much, much greater than the probability that it would happen multiple times” and makes no comment on the likelihood of any answer. > > > [@Diogenes\_the\_Cynic](#): > > > > Not correct. The Drake equation is based on the ubiquity of earth-like conditions and the universe and the extreme improbabibilty that billions of such conditions could only lead to abiogenesis once. thereis nothing magical about the earth. Hell, rather than waiting for Dio to produce any evidence, I can prove to anyone vaguely he is wrong. Go to the Drake equation, and for the value of “the fraction of stars potentially capable of supporting life that actually go on to develop life at some point”, plug in the value 1/10^87 (the number of particles in the universe). Gee, it produces an answer that says that the probability of life arising more than once is greater than the number of particle sin the universe. Yet Dio just told us that it can never produce such an answer. Quite clearly he has no idea what he is talking about. Hell, as even the Wiki article linked to says “this evidence only looks at the Earth (a single model planet), and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). Also countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. In addition, from a classical hypothesis testing standpoint, t **here are zero degrees of freedom, permitting no valid estimates to be made**.” So I’m eagerly awaiting Dio’s calculations where he apparently managed to make valid estimates that conmstrain the Drake equation and prevent it producing any answer at all. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 9, 2010, 11:23pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/30 "2010-11-09T23:23:04Z")
> [@Stoid](#): > > Why not? Life is pretty astonishing. Life as it exists here is freakin’ magical it’s so astonishing. No wonder so many people believe in God! As mentioned, it would be more astonishing if this was the only planet in the whole universe that has life of any sort on it. Could be, we don’t know but I wouldn’t bet against life. Try this: > [@](#): > > 1. The number of galaxies. An estimated 50 billion galaxies are visible with modern telescopes and the total number in the universe must surely exceed this number by a huge factor, but we will be conservative and simply double it. That’s 100,000,000,000 galaxies in the universe. > > 2. The number of stars in an average galaxy. As many as hundreds of billions in each galaxy. > > Lets call it just 100 billion. > > That’s 100,000,000,000 stars per galaxy. > > 3)The number of stars in the universe. > > So the total number of stars in the universe is roughly 100 billion x 100 billion. > > That’s 10,000,000,000,000,000,000,000 stars, 10 thousand, billion, billion. Properly known as 10 sextillion. And that’s a very conservative estimate. > > 1. The number of stars that have planetary systems. The original extra-solar system planet hunting technology dictated that a star needed to be to close to us for a planet to be detected, usually by the stars ‘wobble’. Better technology that allows us to measure the dimming of a stars brightness when a planet crosses its disk has now revolutionised planet hunting and new planets are being discovered at an ever increasing rate. So far (August 2003) around 100 have been discovered so we have very little data to work on for this estimate. Even so, most cosmologists believe that planetary formation around a star is quite common place. For the sake of argument let us say it’s not and rate it at only one in a million and only one planet in each system, as we want a conservative estimate, not an exaggerated one. That calculation results in: > > 10,000,000,000,000,000 planets in the universe. Ten million, billion, as a conservative estimate. > > 1. The number planets capable of supporting life. Let’s assume that this is very rare among planets and rate it at only one in a million. Simple division results in: > > 10,000,000,000 planets in the universe capable of producing life. Ten billion! > > SOURCE: [Is there any other life in the Universe?](http://www.thekeyboard.org.uk/Extraterrestrial%20life.htm) Those numbers above are overly conservative but even like that you see what it comes up with. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [November 9, 2010, 11:29pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/31 "2010-11-09T23:29:13Z")
[QUOTE=Blake] Why does it seem unlikely? What do you think the likelihood of it is, and how did you arrive at that conclusion? [/QUOTE] Extrapolation. Life on this planet obviously originated. We now (approximately) the conditions necessary to have life originate. We know the (approx.) conditions that life needs to thrive in. We know that there are hundreds of billions or even trillions of planets in our own galaxy, and billions of years of windows in which some non-zero number of those planets would have the conditions and materials in which life COULD originate on them. We know that there are hundreds of billions or trillions of other galaxies out in the universe, all with the same chances. Even leaving aside the non-zero probability that like from this planet could have been spread through impacts, that’s a huge number of chances for life to be in the universe during windows of time stretching billions of years. It’s not a ‘conclusion’…it’s a belief, as was said in the subject of the OP. THAT is the basis by which ‘science’ posits that ‘extraterrestrial life is likely’. > [@](#): > > Once again, people are expounding on the probability of life arising on Earth without actually being able to tell us how they concluded what that probability is. Um, they HAVE told you how they reached the conclusions they reached. You may not agree, and since we don’t know the answer there is no way to determine who is right and who is wrong, but you can’t say no one has told you how they reached the conclusions they reached. There is no way to know a lot of things in science, so extrapolation and theory are used. We have theories on how life might have originated, and we have experimentation that shows how it might have happened. We have theories and some data on what the conditions may have been like on the early Earth when life first originated (and even theories that life might have come here from somewhere else). > [@](#): > > Why is “the probability against life is greater than the total number of subatomic particles in the universe” less likely than any other probability that could be conceived of. Because based on experimentation and theory into the conditions and possible methods that caused life to originate it seems ridiculously pessimistic. Also, I’ve never seen any scientists posit such a low probability for anything outside of something like magic or leprechauns spontaneously originating somewhere in the universe. Considering that humans are taking the first steps to creating life ourselves, your assertion seems extreme to me. > [@](#): > > The nub of the problem is that we have a sample size of one, constrained by the anthropic principle. We have a sample size of one, no doubt. So what? I’ve seen theories that spring up about a single bone from a single species that lived 10’s of millions or 100’s of millions of years ago. That’s what science is all about…making theories and testing them. And this theory (that there is life in the universe) is certainly testable…heck, we are testing for it right here in our own solar system. > [@](#): > > So long as that remains true, I fail to see how you can make declarations about some probability being less than or greater than some other. I’m not making a declaration…I’m saying that this is why ‘science’ (by which I presume the OP means a large number of interested or involved scientists) believe ‘that extraterrestrial life is likely’. They are making assumptions and extrapolating from data gathered about the Earth, life and the universe in general and they are theorizing that the probability of life in such a vast area with such a vast number of potential bodies and in such vast time frames is ‘likely’. Seems reasonable to me…certainly more reasonable than your ‘probability against it may be greater than the total number of subatomic particles in the universe’ based on what we know. It’s a testable theory, and one that will probably be tested for as long as there are humans around with the ability and desire to test it. We may never know, considering the distances involved…or, we may find former life on Mars, or life on one of the various moons, which would pretty much move us to ‘what is the probability of INTELLIGENT life in the universe?’… -XT ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [November 9, 2010, 11:29pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/32 "2010-11-09T23:29:16Z")
> [@Lemur866](#): > > But the problem with the lottery analogy is that we know in advance that there will be only one winner of a lottery, because that’s the way the game is structured. No it’s not. There could be two or three or twelve. But it’s so vanishingly rare for a set of numbers to be picked a bunch of times in a given week that we usually only get one winner. But there’s nothing specifically preventing more…except the statistical unlikeliness of it. Which makes it a pretty usable analogy, actually. From what I’ve read in this thread and elsewhere, even our best guesses at what it takes to create a fertile ground for life to emerge assume a great deal more than 5 numbers plus a mega, taken from 50 or so possibilities. More like picking the right 500 numbers in an exact sequence, from a billion possibilities! The fact that it actually happened once in no way whatsoever means it would happen over and over again, and the assumption that it would is exactly what I am questioning. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [November 9, 2010, 11:33pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/33 "2010-11-09T23:33:44Z")
> [@Blake](#): > > I surveyed all the lottery winners in my town. There were only _two_, and one only bought one ticket each week for a year before he won a million and the other bought a ticket every week for 10 years before he won a million. Therefore the average time that a person has to play the lottery before winning a million is just 5 years. > > Do you believe that reasoning is correct? Or do you think that selecting just the winners prevents us from drawing any conclusions at all about the probability of winning? I mean, there could be billion of people who play every week for 70 years and never win, right? > > As far as I can see, no matter how improbable the odds of an event are, if you only sample the winners you are going to conclude that it is likely. When you only have a single, self-selected sample that is a winner, you can conclude absolutely nothing. I am not good with math and statistics, but I am good at logic. This seems very clear to me. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 9, 2010, 11:35pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/34 "2010-11-09T23:35:55Z")
> [@Whack-a-Mole](#): > > As mentioned, it would be more astonishing if this was the only planet in the whole universe that has life of any sort on it. As has been asserted before, without any actual basis. I ask again, why would it be astonishing? If the odds against life arising on Earth are greater than the number of particles in the universe, why would you be astonished if this was the only planet with life? At those odds, wouldn’t it be more astonishing if we _weren’t_ the only planet with life? > [@](#): > > Could be, we don’t know but I wouldn’t bet against life. I wouldn’t either,because we simply have no idea what the odds are. > [@](#): > > Those numbers above are overly conservative … No, they aren’t. Number 5 is just an wild arsed guess. How do we know it is conservative? How we you know it isn’t overblown by a billion orders of magnitude? How do we know the real answer isn’t 10^87? ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [November 9, 2010, 11:37pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/35 "2010-11-09T23:37:55Z")
Blake is wrong about the necessity of a larger sample size: A sample size of one can establish conclusions with virtual certainty, given the right epistemic circumstances. (For example, a sample size of one single observed member of a new species, say, that it has wings, is enough to establish with a virtual certainty that every other member of that gender of that species, at that age, also has wings.) But he’s right, I think, that we don’t have a basis for predicting the frequency of life until we know more about the mechanisms that give rise to life. The mechanisms could be very common, or could be incredibly rare. We just don’t know, and until we do, we won’t know how common life is in the universe. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [November 9, 2010, 11:47pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/36 "2010-11-09T23:47:59Z")
> [@Whack-a-Mole](#): > > Remember life does not need the narrow conditions that humans live in. > > Right here on earth life has filled just about every ecological niche imaginable including some places distinctly hostile to human life. > > From bacteria in an [incredibly isolated underground lake in Antarctica](http://www.i-fink.com/articles/articles/lonely_old_bacteria/) to an ecosystem living on the bottom of the Atlantic Ocean near volcanic vents. Pitch black, acid “water”, very high temperatures and incredible pressure and [all sorts of life is there](http://www.onr.navy.mil/focus/ocean/habitats/vents2.htm). > > Presumably if there is liquid water available life is possible. > > Certainly is so here on earth so that suggests even a planet we would deem hostile could have life thus increasing the chances it could happen elsewhere in the universe. Yes, this is another issue I have… why in the world would smarty-pants scientists make the silly assumption that life only comes in the “needs water & oxygen in a narrow range of temperatures” variety? That’s extremely limiting and lowers the likelihood considerably. If you are willing to allow for elements existing elsewhere in the universe that we’ve never heard of because they don’t exist here, and the possibility that life could exist that is thoroughly unlike anything we’ve ever imagined, surviving in ways that leave us drooling with cluelessness…well, that expands the possibilities a lot. ---
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [November 9, 2010, 11:51pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/37 "2010-11-09T23:51:15Z")
> [@Blake](#): > > In addition, from a classical hypothesis testing standpoint, t **here are zero degrees of freedom, permitting no valid estimates to be made**." What does this mean? I don’t follow. (I told you I’m bad at this stuff…) ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 9, 2010, 11:54pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/38 "2010-11-09T23:54:53Z")
All of the above is very interesting, but we can actually get a somewhat better quantitative, and objective, feel for the probability of life by reconstructing Sagan’s (?) list. Let me provide some of the criteria that I vaguely remember, and others can fill in the remaining forty or fifty blanks. Note that in the construction of this list, there is no necessity for the construction of fanciful statistics. However, as the list grows, the improbability of the criteria being met becomes obvious. The planet has to: 1. lie on the outlying regions of a galaxy in order to have reduced impinging radiation levels from the galaxy center 2. be a long way away from other stars so as to avoid interacting gravitational tides which would tear the planet, and any life forms, apart. 3. must have an iron core to provide shielding from its sun’s radiation 4. must be a certain minimum size to ensure it can hold an atmosphere 5. must be a certain maximum size in order to ensure that gravitational effects won’t squash life forms 6. must have a moon of a certain size to shield it from space debris 7. must be a certain distance from its sun in order to provide sufficient energy to warm the planet, but not too close, or not too far. 8. The sun must be of a certain age and size in order to provide the radiation spectrum conducive to life 9. Etc etc ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 9, 2010, 11:55pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/39 "2010-11-09T23:55:12Z")
> [@Blake](#): > > No, they aren’t. > > Number 5 is just an wild arsed guess. How do we know it is conservative? How we you know it isn’t overblown by a billion orders of magnitude? How do we know the real answer isn’t 10^87? You can nitpick all you like. You are correct that we just do not know. There could be only us or there could be billions of planets with life on them out there. Currently there is no way to say and we can only guess. The point is what would you do if you were a betting man? Pretend you are forced to put all your money and worldly possession up on a single bet (you have no choice in my hypothetical). This is an all or nothing bet…double your net worth or lose it all. You have to bet if the Earth is the only planet in the whole universe with life on it or if at least one other planet, besides the earth, has life on it. Once you place your bet the answer will be revealed and with the magic of hypotheticals we will pretend that answer is indisputably correct. So, which way would you place your bet and why? ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [November 9, 2010, 11:56pm UTC](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078/40 "2010-11-09T23:56:49Z")
> [@Stoid](#): > > Yes, this is another issue I have… why in the world would smarty-pants scientists make the silly assumption that life only comes in the “needs water & oxygen in a narrow range of temperatures” variety? That’s extremely limiting and lowers the likelihood considerably. If you are willing to allow for elements existing elsewhere in the universe that we’ve never heard of because they don’t exist here, and the possibility that life could exist that is thoroughly unlike anything we’ve ever imagined, surviving in ways that leave us drooling with cluelessness…well, that expands the possibilities a lot. At the same time, reducing the basis for speculation. I think good science tries to speculate only when there is a basis–and also tries to widen that basis. [Previous page](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078.md?page=1) [Next page](https://boards.straightdope.com/t/basis-of-sciences-belief-that-extraterrestrial-life-is-likely/560078.md?page=3)