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

There seems to be a widely held belief among those who study the universe that life on other planets is likely.

Since I think the fact that life exists at all is extraordinarily UNlikely…(I mean really, have we ever had a good explanation of how life arose from non-life to begin with? Not that I’ve heard.) I wonder on what basis this belief rests.

Anyone got some solid facts on this?

It sounds better on the grant request paperwork than saying “There’s nothing out there but dust, rocks, methane and hydrogen.”

Because the universe is a very large place, and it would be more extraordinary for life to only happen once than for it to happen more than once.

We have several good ideas about how life began, by the way. We have a pretty good handle on a lot of the process. we can’t confirm exactly what happened, but we have some good models that work.

Abiogenesis was not just cells springing into existence, it was a long slow slog with a lot of intermediate steps.

Life arose on Earth very quickly after the Earth cooled enough to allow liquid water.

Now, why and how did this happen? No one knows. But that argues that either Earth was seeded by life from outside Earth, in which case, there must be a lot of life out there so it could seed Earth. Or life was spontaneously generated on Earth after only a couple dozen million years when conditions were appropriate for life. In which case, it’s pretty likely that life would also spontaneously generate anywhere else in the universe where conditions are appropriate for life.

And “conditions appropriate for life” seems to mean, “liquid water is present”, and while liquid water certainly isn’t common throughout the universe, there are plenty of places where it should exist. We’ve discovered lots of extrasolar planets, and some of them are at the right distance from their star that liquid water is possible. So since there are about a hundred billion stars in our galaxy, it would be quite odd if there were only one planet with liquid water in our galaxy.

Of course, this is all guesswork. We have no direct evidence for liquid water on other planets, only indirect evidence–including the indirect evidence for liquid water under the ice of Europa.

And we don’t have a good explanation for how life arose, although there is a lot of informed speculation. Peter Ward argues that while microbial life is probably common in the universe, complex life probably isn’t: The real answer is that no one knows.

Clearly, the fact that life exists at all is as likely as anything can possibly be (i.e. probability 1.00000000…), since it is known that life does, in fact, exist.

The probability that life will originate, starting with the conditions that obtain in the formation of a solar system (twenty years ago, I would have said “the formation of a star”, but enough exosolar planetary evidence is in to bridge that step in the chain), is unknown but clearly nonzero. For Earth to be the only lifebearing planet in the universe would require that this probability falls within a very narrow range (from infinitesimally above zero to the highest level – one in quadrillions – reasonably consistent with the event happening once and only once in the Universe’s history). That in itself would seem highly unlikely.

This, and the fact that life is composed of mainly the most common possible stuff in the universe, seem to me to be enough to think it’s very plausible that there is life on some of the billions of other planets out there. Even without explaining how it came to arise. We know it’s possible, since here we are.

Because there’s so freaking much stuff out there.

“Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

To try to squeak a little bit of perspective out of a human mind, which is not designed to conceive of this kind of vastness, watch this and have that mind blown.

Because of the Drake Equation.

Recent progress on getting some real numbers for that equation:
Many Earth-like planets orbit sun-like stars:

If life arose on earth de-novo, within 300 milion years of cooling, there’s 46 billion planets in the galaxy with a nearly equal chance of having given rise to life.
If life on earth arose via a chance encounter with an interstellar bacterial spore, then life is probably thriving everywhere in the galaxy where conditions don’t actively kill it off.

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.

The OP may need to clarify the definition of “extraterrestrial life.” Microbes are life and pretty resilient. It doesn’t take a far stretch of the imagination to have them showing up in some cesspool on a planet, moon, or asteroid somewhere.

The OP might want to read A Short History of Nearly Everything. Wish I were more up to snuff on the actual facts, but science isn’t quite my bag. To paraphrase, science has a history of getting things wrong when it makes blanket statements about things not being possible (life at extreme ocean depths and pressures, age of earth, etc) [cite is the book]. Assuming extraterrestrial life is impossible feels like the less tenable stance on the issue.

OTOH, I wouldn’t bet on E.T. showing up anytime soon.

Mind if I call this “The Blake Equation”?

Perhaps a better analogy.

I surveyed all the lottery winners on my block. There was only one, and he only bought one ticket each week for a year before he won a million. So there must be a 1 in 52 chance that any lottery ticket wins a million dollars. That is totally invalid statistically and logically because, by definition, you can only sample the single winner in the planetary lottery. If our planet hadn’t evolved life then we wouldn’t be here to wonder about it. 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.

That’s got nothing to do with the anthropic principle. I’m not sure you know what that means.

What is likely (actually inevetible) is that other planets exist with conditions similar to ours which would be similiarly conducive to life. The fact that it happened on earth is only proof of concept (i.e proof that it’s possible). Given billions of trials under similar conditions, the probability that life would arise only once is smaller than the probablity that it would happen muliple times.

You obviously don’t.

Here, read this. Especially note the bit about “observations of the physical Universe must be compatible with the conscious life that observes it” if you, are still struggling after reading that article then get back to me.

Well since this is GQ, can you please show us the equations that you used to ascertain these probabilities? Because this is GQ, and not the forum for baseless opinions, so you must have calculated such probabilties before declaring what they are smaller than.

I await your equations with bated breath.

The truth, of course, is that we have absolutely no idea what the probability of life arising are, and we can have no idea at present. The probability against it may be greater than the total number of subatomic particles in the universe, for all we know. And in that case the probability that life would arise only once is much, much greater than the probability that it would happen multiple times.

This is the correct definition, but it has no application to the reasoning that if life happens once it can happen again.

Look at the Drake equation posted above. It’s not magical thinking.

That seems unlikely.

I’d say the answer to the OP is that the universe is vast, as was mentioned before. There are over a hundred billion stars in our galaxy alone. Even if we assume that the average number of planets circling those stars is 1 (which would be a highly conservative assumption), that’s a hundred billion chances for life multiplied by the time since the formation of our galaxy to when the galaxy finally goes by-by. Even if during that entire time span the Earth is the only planet to ever have life on it (10’s of billions of years, hundreds of billions of planets…:dubious:) there are over a hundred billion galaxies out there. If they even simply had one planet like ours during the 10’s of billions of years they exist, that’s still hundreds of billions of possible worlds with life on them (at one planet per galaxy).

I’d say that, probability wise and based on what we do know of life that the probability of other life in the universe outside of Earth and during the entire time span of the universe (that life COULD exist in) would pretty much be 1. YMMV, and I guess it depends on what assumptions you make, but it doesn’t seem that much of a stretch to me to assert that it’s a high probability when we are talking about such vast numbers of potential planets and such a huge expanse of time to operate in.


What are you talking about?

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.

So I ask again, since this is GQ, can you please show us the equations that you used to ascertain these probabilities? Because this is GQ, and not the forum for baseless opinions, so you must have calculated such probabilities before declaring what they are smaller than.

I’m guessing that you can’t produce any such equations, and that you have absolutely no factual basis at all for your claim.

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. 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.

I tend to have the feeling that the Earth isn’t a particularly unique spot in the universe, and therefore that the generation of life in this one spot isn’t unique. 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.

Why does it seem unlikely? What do you think the likelihood of it is, and how did you arrive at that conclusion?

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.

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.

The nub of the problem is that we have a sample size of one, constrained by the anthropic principle. Even if the probability against life is greater than the total number of subatomic particles in the universe, it still must have happened here and now for us to know about it.

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.