Basis of science's belief that extraterrestrial life is likely?

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.

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.

Where do you live? My local lottery draws a winner every single week. My analogy even specified that the prize was drawn every week.

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.

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.

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?

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.

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.

You understand wrong. Completely wrong. Dead wrong. 100% wrong.

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 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.

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.

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!

Is it plausible that all life on Earth is the descendant of a single cell produced by a single abiogenesis event?

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, there 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.

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:

Those numbers above are overly conservative but even like that you see what it comes up with.

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’.

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).

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.

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.

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?’…


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.

I am not good with math and statistics, but I am good at logic. This seems very clear to me.

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?

I wouldn’t either,because we simply have no idea what the odds are.

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?

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.

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.

What does this mean? I don’t follow. (I told you I’m bad at this stuff…)

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

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?

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.