Evidence for ET is mounting daily, but not proven.

[MaDeR]

Likewise, a planet found in a goldilocks zone (have we in fact discovered such a beast?) is merely the most likely location for a hypothetical lifeform.

Yes, we know planets in goldilocks zone for many years (in some cases over 15 years) already. Read up. These links are for confirmed worlds, mind you. There are further 54 unconfirmed Kepler candidates in habitable zone.
I find it surprising that you do not know about it already.

[Paul Beardsley]

Read up.

Thank you for the link.

I was aware of most of these. The planets listed are, for the most part, gas giants, highly eccentric in many cases, located in the habitability zone, but only contenders for the goldilocks zone if they happen to have large moons - and it's considered doubtful that Earth-like worlds can orbit gas giants.

[Jeff Root]

Although other suns existing is not evidence of ET life,
it certainly *is* evidence for ET life, and that is what the
title of this thread claims.

No - it just is not.

Sure it is.

We disagree, but I'm not willing to agree to disagree. Any
suggestions as to how we can resolve this disagreement?

You're categorically wrong on this. You are saying that
evidence of potential = evidence for. This is just not true.

Quite often, it is.

If I have evidence that your bathroom is usually damp,
usually dark, and not cleaned with a fungicide, then I have
evidence supporting the expectation that mildew is growing
in it. In other words, evidence for mildew growing in it.

The evidence is of more than just potential. Once we know
that the stars are distant suns, there is more than just a
potential for them to have planets. Our Solar System has
several planets and loads of smaller stuff in it. Why wouldn't
other star systems? We see vast clouds of gas and dust all
around the galaxy, containing enough material for billions of
stars and trillions of planets. The expectation should be that
some of that material actually does go into making planets,
and that other star systems actually do have planets, not
that our Solar System is uniquely the only one with planets.

Once we know that there are lots of extrasolar planets, there
is more than a potential for some of them to be "habitable".
It would be absurdly unlikely that none of them has the right
temperature, mass, and whatever to make them habitable.
That is more than potential. It is a practical certainty.

So we have a *practical* certainty that "habitable" extrasolar
planets exist. That presents more than just a potential for
extrasolar life. It says that the conditions for life are right
in places other than Earth. Even if we don't know what those
conditions are. Life exists on Earth because the conditions
are right. Why would that be different everywhere else?
It wouldn't.

If there are many places where the conditions for life are right,
then the expectation must be that life exists in many places.
Not everywhere the conditions are right, but in many places.

Let me put it another way. A lack of preclusion of something
is not evidence for it.

This is a different assertion, not putting your previous
assertion another way.

You may or may not have a nose. I have no evidence that
you have a nose, and no evidence that would preclude your
having a nose. So I cannot preclude your having a nose.
I take that lack of preclusion to be evidence supporting my
expectation that you have a nose. In other words, evidence
for your having a nose.

But in this case it isn't a question of preclusion or lack of
preclusion at all.

We have found that other suns exist in vast numbers.
We are finding that planets exist in significantly greater
numbers than do suns. It is reasonable to expect that a
sizeable fraction of those planets have whatever conditions
are required for them to be "habitable". So the rational
expectation should be that life has arisen on some planets
besides Earth.

It's an odds game. Maybe a horse is standing on the
start line with odds of 10-1.

You are stating that's evidence 'for' the horse winning.

The Wikipedia article on "Odds" says that odds are generally
expressed as odds against rather than as odds in favor of.
If that is the case here, then 10-1 odds is evidence against
the horse winning. My extremely vague impression is that
such odds are based on the amount of money bet on the horse
versus the amount of money bet against the horse. So in this
case, something like ten times as much money has been bet that
the horse will lose as has been bet that it will win. Although
I know absolutely nothing about the horse, what it ate today,
the other horses in the race, what they ate today, the jockeys,
what they ate today, the owner, the track, the weather, or the
alignment of the planets at the time of the race, I know all
the bettors quite well, and I know that they take these things
into account before placing bets. So 10-1 odds is fairly good
evidence for the horse losing.

I may accept other people's assessments of odds, or
generate my own, or both. In this case my assessment is
that conditions strongly favor life being abundant in the
Universe, the evidence being everything I know about life,
the Universe, and everything.

Okay, maybe most of what I know -- or think I know -- is
irrelevant, but I'm not going to try to sort it all out for you.
As I said, I really don't consider your request for a citation
of evidence to be worth much effort at this point.

The ONLY valid stance until that race is over is that we
don't know. It might - but we simply do not know.

You don't know. I do. So at the moment, it appears that I'm
more knowledgeable than you are on this particular question.

I appreciate that you don't know and require more direct
evidence before you can know. But what doesn't work for
you does work for me.

I think there could very well be life elsewhere in the universe.
The sheer number worlds speaks volumes on the possibility
of life. Is there evidence of many worlds? Yes. Is there
evidence some are habitable? Yes.

Is there evidence that they are inhabited. No.

See now, I agree with that completely. 100%.

I'd like us to agree even more. Agreement is good.

You are making a massive massive leap in essentially
projecting evidence of habitability as evidence for life.
It isn't.

Is so.

I wish you didn't keep saying things I disagree with.

The possibility or even probability of something existing is
not evidence for it.

It can be indirect evidence. But your assertion is irrelevant.
The bit about odds of a horse race is irrelevant. Those are
assessments based on analysis of the evidence. Everyone
knows that. We don't need to argue about it. It is a trivial
diversion from the real questions here.

Even given that I think your real question isn't currently
worth much effort.

... I can say that your owning a toolbox *is* evidence that you
may own allen keys.

Correct. I MAY own allen keys. It's possible, probable even.

To use my wording, your owning a toolbox is evidence for
your owning allen keys.

Incorrect. That I own a toolbox says nothing about allen
keys actually existing. Period.

Allen keys exist whether or not you or your toolbox exist.

That you own a toolbox is evidence which supports an
expectation that you also own allen keys, even if there are
no allen keys in the toolbox, and even if you don't recall
ever having an allen key in your possession.

Just as there is other evidence for extraterrestrial life than
the existence of many suns, there is other evidence for your
ownership of allen keys than your ownership of a toolbox.
If it were just the toolbox, I wouldn't have any expectation
at all that you might own allen keys.

I really think that if you look around enough, you'll find
at least one allen key.

You have to open the toolbox and look at it before you can
state that.

Nonsense. I don't know what, if anything, is in your toolbox.
Maybe lunch. Maybe your pointe shoes. But the fact that you
own a toolbox is evidence which supports the expectation that
you own allen keys.

It would be a good place to look - that's where they tend to
be - but you have ZERO data about allen keys until you open
that box. You have NO evidence for them at all. None.

Have *you* looked in the box yet? Maybe you'll find an allen
key in there.

I have to take your word that you own a toolbox, and that
you don't own allen keys. It is more likely that you are right
about the toolbox than about the keys. Chances are good that
you saw the toolbox either shortly before you first mentioned
it or sometime since. But you could have an allen key in your
possession that you forgot about. Looking in the toolbox and
not finding one would reduce that chance some, but not much.
Chances are greater that you have an allen key elsewhere
than the tool box. If the toolbox is the only thing you own,
or if it is the only place that an allen key of yours *could*
be located (you don't own any clothes with pockets), then my
looking in it and not finding an allen key would rule out the
possibility of you owning an allen key.

You could also be lying. You might have lots of allen keys but
no toolbox. But I have evidence that that is not the case. The
evidence tells me that you definitely have a toolbox and are
pretty sure that you don't have any allen keys. But you aren't
completely sure. That's what the evidence tells me.

This may be a pedantic or semantic argument - but I feel,
very very strongly, that it is catastrophically wrong to say
'We have evidence for ET life'

We don't. We have evidence of places it might occur and
things it might be made of. These do not amount to evidence
for it.

I disagree that we don't have evidence for extraterrestrial
life, but let's suppose that we don't. Why would saying that
we do be "catastrophically wrong" rather than just "wrong"?

The Sun is the only normal star which has been observed to
emit neutrinos. We have no evidence of other normal stars
emitting neutrinos. In your opinion, do we have evidence
for other normal stars emitting neutrinos? Do you think the
idea that other normal stars emit neutrinos is a fantasy not
based on evidence?

-- Jeff, in Minneapolis

[Jeff Root]

I totally agree with djellison over the last few posts.

If it turns out that djellison owns an allen key or two, chances
are he keeps them in his toolbox. But the existence of the toolbox
is not evidence of an allen key. Jeff's logic is not lined up in
the correct order here. (I have forgotten the Latin term here -
something like modus ponens. I'll look it up when I have time.)

If there is a problem with my logic, I would love to see it.

Modus ponens gives a deduction from definite premises:

If P, then Q.
P.
Therefore, Q.

In the case of the missing allen key, I am using premises which
are not definite, and get a conclusion which is not definite:

A person who owns something is likely to own other things.
DJ owns a toolbox.
Therefore, DJ is likely to own things other than a toolbox.

And so on. Lots of premeses, lots of conclusions.

Likewise, a planet found in a goldilocks zone ... is merely the
most likely location for a hypothetical lifeform. It does not in
any sense strengthen a case for a lifeform being located there.

Of course it does.

Let's start with two unwarranted but reasonable assumptions,
for the sake of illustration:

1) As far as we know, most stars do not have planets.
2) Life is on planets, not in space.

We see a star. We don't know whether it has any planets.
Does the system have life? Dunno.

We look closer. Hey, how about that! It has a planet in the
zone where terrestrial life could survive! Does it have life?
Still dunno, but we know it's way more likely than we thought
it was before we discovered the planet!

-- Jeff, in Minneapolis

[Jeff Root]

So we sort of have evidence that there might be life on
Titan? And that somehow equates to "we have evidence of
extraterrestrial life"?

Nope.

We have definite evidence that conditions for life on
Titan are better than previously expected -- conditions
which could support some kinds of life. So we have
evidence for extraterrestrial life.

Nobody here has said that we have evidence of
extraterrestrial life.

-- Jeff, in Minneapolis

[mutleyeng]

The Sun is the only normal star which has been observed to
emit neutrinos. We have no evidence of other normal stars
emitting neutrinos. In your opinion, do we have evidence
for other normal stars emitting neutrinos? Do you think the
idea that other normal stars emit neutrinos is a fantasy not
based on evidence?

-- Jeff, in Minneapolis

all of your examples...every one...either we know what the processes are that can allow us to make the prediction or we have seen enough examples to make the assumption.
life, we have one example and have no idea the processes that lead to it, so cannot begin to assign a probability of it occuring

for however many exo worlds science may predict are in the universe....just place a "1:" infront of it and that is what science tells you is the probability.
if you find there are double the number of worlds than you first thought, put a "1:" infront of it and see how much that changes things

[djellison]

If there is a problem with my logic, I would love to see it.

I've tried pointing it out several times, and you won't see it. You are assuming habitability is evidence for life. I've explained in as many ways as I can why that's an invalid assumption. Your reply was to make the same invalid assumption over and over again.

I'm done.

[Colin Robinson]

all of your examples...every one...either we know what the processes are that can allow us to make the prediction or we have seen enough examples to make the assumption.
life, we have one example and have no idea the processes that lead to it, so cannot begin to assign a probability of it occuring

It's true that the least understood part of the history of life on Earth is how it first emerged. Trying to work out how the first organisms came into being four billion years ago is almost as difficult as trying to work out what is happening on an exoplanet light-years away. Still, it's an exaggeration to say we have "no idea". There are a number of serious ideas, put forward by scientists on the basis of empirical data, also known as evidence (though not necessarily direct or conclusive evidence).

A few decades ago, when I.S.Shklovskii and Carl Sagan co-wrote the book Intelligent Life in the Universe, they mentioned a basic problem concerning the origin of life: enzymes are good at catalysing, but they can't function without information stored in nucleic acids like DNA; nucleic acids can store information, but they aren't catalysts; so which came first, or did they both spring into being together, or what?

Since then, it has been shown that various forms of RNA (a nucleic acid, chemically related to DNA) can function not only as stores of information, but as catalysts as well. This led to the "RNA world" hypothesis – the idea that life-forms with DNA and enzymes evolved from life-forms using RNA both for information storage and as a catalyst.

There are also serious ideas about how the RNA world developed out of simple chemical building blocks and a flow of energy. One serious idea, supported by laboratory results, is that metabolic pathways such as the reductive Krebs cycle
could have begun before there were complex organic catalysts such as enzymes or catalytic RNA. The reductive Krebs cycle is a process by which quite simple carboxylic acid molecules incorporate hydrogen and carbon dioxide, obtaining energy and creating more carboxylic acid molecules...

It is still conceivable that the origin of life involved a chance event of very low probability, in which case life may be very rare in the universe. However, the tide of scientific opinion -- evidence-based scientific opinion -- seems to be flowing away from that view...

[mutleyeng]

do we have a coherent theory of Abiogenesis?
if the answer is no, then you have no means of arriving at the possibility of life for any given condition, therefore you have no means of extrapolating from a sample of one.
further, if this evidence gained from organic chemical experiments is not part of that coherant theory of Abiogenesis, then again, you have conflated evidence of one thing to be evidence of something else.

[Jeff Root]

If there is a problem with my logic, I would love to see it.

I've tried pointing it out several times, and you won't see it. You
are assuming habitability is evidence for life. I've explained in as
many ways as I can why that's an invalid assumption. Your reply
was to make the same invalid assumption over and over again.

I'm done.

Please don't give up. What I said in the quote. I want to know.

It sounds like you don't disagree with my logic, which is what
Paul suggested, but with one of my premesis. Is that correct?

Can you explain why habitability is not evidence for life in the
rest of the Universe? You do agree that habitability is evidence
for life on Earth, don't you? If a place on Earth has conditions
which are good for supporting life, you can expect to find life
there. Right? Why is that true on Earth, but nowhere else?

-- Jeff, in Minneapolis

[djellison]

If a place on Earth has conditions
which are good for supporting life, you can expect to find life
there. Right? Why is that true on Earth, but nowhere else?

-- Jeff, in Minneapolis

Sigh. Can't you see the flaw in your logic? You are assuming life blooms everywhere that is habitable. This is not a valid assumption. We have a sample of 1. ONE place. This might be the ONLY place

The range of probability is from certainty (1:1) to near impossibility (earth = the only life). We have absolutely NO idea where the likelyhood of life flourishing actually lies. None whatsoever.

You're making an assumption - an invalid one - that anywhere there is habitability there is life.

This is an untenable stance based on our sample of 1.

The evidence we have speaks to habitability

It does not speak to life. Period.

Now I really am done. I've wasted far too much time trying to explain this fundamental point to you.

[iquestor]

I think the root issue is the definition of the word 'evidence'. Evidence means 'data that supports a conclusion.' it is not proof. it is not definitive.

therefore, taking into consideration that the only real life we know requires a planet with moderate temperatures, an atmosphere, liquid water and a decent star, we can begin to try to find other places in the universe that might support the same or a similar type of life. I think jeff gave a pretty good chain of logic in an earlier post supporting that evidence for ET Life was growing, but not proven.

We now know:
1. there are many stars similar to our sun in our galaxy alone.
2. many, possibly most, stars have one or more planets (ie Kepler, etc)
3. some of theses are in the zone about a star that would allow for liquid water and moderate temperatures.

we can reasonably extrapolate the following:
1. Our Star and Solar system aren't 'special' (Anthropomorphic principal)
2. there are billions of Stars on our galaxy
3. there are even more planets than stars.

given what we know, and what we can extrapolate, makes the evidence for life somewhere else in our galaxy a near certainty just on numbers alone. In other words, what we have is evidence for ET Life, but not proof.
and it's mounting daily.

I dont see how this is such a big deal. I think that some of you think evidence = proof. it does not. if you think it does, go watch a court proceeding sometime. evidence supports an argument, but doenst prove it.

the horse race analogy is wrong IMO - odds are for whether an event will happen, or not. Life is either out there right now, or it isn't.

[Jeff Root]

If a place on Earth has conditions which are good for
supporting life, you can expect to find life there. Right?
Why is that true on Earth, but nowhere else?

Sigh. Can't you see the flaw in your logic? You are
assuming life blooms everywhere that is habitable.

No I am not!

I said explicitly, in post #783, directly to you:

If there are many places where the conditions for life are right,
then the expectation must be that life exists in many places.
Not everywhere the conditions are right, but in many places.

Clearly, the reason you disagree with me is that you
think I said something that I did not.

This is not a valid assumption.

So you disagree with an assumption you think I made,
*not* with my logic.

We have a sample of 1. ONE place. This might be the
ONLY place

One planet. Many, many, many, many, many places on the one
planet where we observe life and learn various things about it.

The range of probability is from certainty (1:1) to near
impossibility (earth = the only life). We have absolutely NO
idea where the likelyhood of life flourishing actually lies.
None whatsoever.

Wrong.

YOU have absolutely no idea. I do have some idea. So I am
more knowledgeable on this subject than you.

You're making an assumption - an invalid one - that anywhere
there is habitability there is life.

I repeat:

I never said such a thing.

I never assumed such a thing.

I explicitly said -- directly to YOU, just a few posts back --
that life would NOT be everywhere that is habitable.

How can you get this so completely wrong? Did you not
read my reply to you? Was it so long that instead of reading
it, you just assumed I said what you want to think I said?

This is an untenable stance based on our sample of 1.

You have no idea what my views are based on. If you
were to actually read and understand what I've said here
you would at least have a clue. But as I said, your request
for citations of "evidence" for extraterrestrial life is not an
important enough problem at this point for me to bother
going into more detail, which would require a considerable
effort. Much more than I've done so far.

We know an enormous amount about life on Earth. We
know an enormous amount about chemistry. About physics.
about how things interact, and change. About how things
work. We can use our intelligence to figure out what is
going on in places that we can't get to. You are using a
bugblatterbeast argument: You can't see it, so you assume
nobody else can see it, and disagree with them when they
say that they can.

The evidence we have speaks to habitability

It does not speak to life. Period.

There is vastly more already known about the Universe
than is dreamt of in a philosophy based on the assumption
that we only have a single data point from which to work.

-- Jeff, in Minneapolis

[mutleyeng]

no, we do not think that evidence = proof.
what we do think is as the first line of your post says
"Evidence means 'data that supports a conclusion.' it is not proof"
thats fine, but the problem with what you are doing is, yet again, conflating evidence of one thing as being evidence of something else.
example: the chemistry evidence for certain lines of research dosnt become evidence for ET life until such time as it is also part of theory of abiogenesis...then, and only then can it be used as evidence for ET life.

[chrlzs]

...life may be very rare in the universe. However, the tide of scientific opinion -- evidence-based scientific opinion -- seems to be flowing away from that view...

Cites and references to support this opinion? Indeed, I would have thought that the more 'habitable' places we find and the more exploration we do without any evidence of actual life, would have to suggest it is flowing towards that view..

[Colin Robinson]

Can you explain why habitability is not evidence for life in the
rest of the Universe? You do agree that habitability is evidence
for life on Earth, don't you? If a place on Earth has conditions
which are good for supporting life, you can expect to find life
there. Right? Why is that true on Earth, but nowhere else?

The thing to consider is how life came to be in the various environments on Earth. Which breaks down into two questions:

1. How does life spread from one habitable environment to another?
2. How does life get started in the first place?

Sciences knows a lot more about question 1 than question 2.

The range of probability is from certainty (1:1) to near impossibility (earth = the only life).

The issue here (as I understand it) is the probability of life getting started on any particular habitable world. It is true that we don't have certain knowledge about this.

Some eminent scientists (who have looked at all the evidence available to them, and done their best to make sense of it), have argued that the initial appearance of life on a planet is a chance evident of extremely low probability.

This view was expressed as follows by the French biologist Jacques Monod (1910 – 1976):

The universe was not pregnant with life nor the biosphere with man. Our number came up in the Monte Carlo game.

From the entry on Jacques Monod in David Darling's Encyclopedia of Science.

If the probability is low enough -- say, life has less than one chance in a billion of getting started on a habitable world -- then the number of habitable worlds could be millions, and yet Earth could be the only one with actual life.

As Paul Davies has pointed out, that would explain what he calls "the eerie silence" -- the lack of positive results in radio SETI.

However, there is another scientific view which sees the emergence of life out of complex chemistry, not as a single chance event like a lottery win, but rather as a series of incremental changes. As expressed by Christian de Duve:

many successive steps are involved. A single, freak, highly improbable event can conceivably happen. Many highly improbable events—drawing a winning lottery number or the distribution of playing cards in a hand of bridge—happen all the time. But a string of improbable events—the same lottery number being drawn twice, or the same bridge hand being dealt twice in a row—does not happen naturally.
All of which leads me to conclude that life is an obligatory manifestation of matter, bound to arise where conditions are appropriate.

Source: "The Beginnings of Life on Earth" in American Scientist

[Colin Robinson]

Cites and references to support this opinion?

Hello Chrizs. Here's what I said in the posting that you've asked for cites to support.

It is still conceivable that the origin of life involved a chance event of very low probability, in which case life may be very rare in the universe. However, the tide of scientific opinion -- evidence-based scientific opinion -- seems to be flowing away from that view...

There are a couple of cites about the probability of abiogenesis in the posting I've just done (#796). Here are some more...

2009 theoretical account of abiogenesis not a chance event, but a natural outcome of thermodynamics...

"The origin of life", by James Trefil and others, in American Scientist

Evidence favoring the hypothesis that primitive metabolism, or protometabolism, got started before complex organic catalysts appeared...

"Shallow origins", in Astrobiology Magazine

Indeed, I would have thought that the more 'habitable' places we find and the more exploration we do without any evidence of actual life, would have to suggest it is flowing towards that view..

I would agree...

IF we carefully explored habitable places without finding life, the conclusion could be drawn that life does not necessarily emerge in a habitable place.

But how many habitable places beyond Earth have yet been examined for living organisms?

[filrabat]

I know this thread is as long as a small book already, but I'll throw in my 2 cents.

Do ET civlilzations exist? To make a long story short, I ran some numbers about stars, their ages, fraction of time a planet is likely to have life, fraction of time our planet has a civlization (yes, earth-centric, but the best we can do as of now). I consistently came up with answers suggesting that ETs do exist in our galaxy, but they are few and far between. The nearest one is likely to be 10K to 20K light-years away according to my calcs. But I'm nowhere near an expert, so take it or leave it as you see fit.

However, I do think global Jurassic Parks and planetary Seringetis of 20 million yrs ago are fairly common - a few hundred light-years apart. "Pre-cambrian" type planets may be as common as a few dozen light-years apart. Again, these are just my figures, so take them for what you think they are worth.

As for more OBJECTIVE AND PROVABLE means of finding life.....

As others said, all we can do is observe, gather data, and sift through the findings. Even so, if a planet's atmospheric signature is high in ozone, water vapor, CO2 and Nitrogen, it's a pretty strong suggestion life is present there. Perhaps there is some unknown process that can generate this atmosphere via non-biological means, but it's hard to imagine what process could do so. Actual confirmation will require either enormously better optics (most likely solution) or launch a probe toward the star, then wait a few centuries to get there, then transmit data back to earth (another few decades on top of all that at least). Therefore, unless there's something about the processes or techniques for discovering "gaian" worlds that I'm unaware of, I have a difficult time believing we'll discover life outside our solar system before at least the dawn of the next century.

[Selfsim]

This thread has turned into somewhat of a dark comedy ..
I've been waiting for someone to say something which makes a difference to something real ... (no such luck, I'm afraid).

I explicitly said -- directly to YOU, just a few posts back --
that life would NOT be everywhere that is habitable.

Why is life NOT everywhere that is habitable, now Jeff ? Can you please tell us how, and why, you know this ?
I'm fascinated.

Actual confirmation will require either enormously better optics (most likely solution) or launch a probe toward the star, then wait a few centuries to get there, then transmit data back to earth (another few decades on top of all that at least). Therefore, unless there's something about the processes or techniques for discovering "gaian" worlds that I'm unaware of, I have a difficult time believing we'll discover life outside our solar system before at least the dawn of the next century.

Hi filrabat;
If we're talking of the order of hundreds, (or even scores, tens), of light-years distant, none of what you mention is feasible (probes, getting data back to Earth, ...). Its good that you have a difficult time believing we'll discover exo-life, too. Why would this be dependent on time .?. Its more dependent on making a chance discovery.

IF we carefully explored habitable places without finding life, the conclusion could be drawn that life does not necessarily emerge in a habitable place

What if the life there was unrecognisable to us ? What makes it possible to rule out the existence of life in a habitable zone which has been searched ? What are the criteria (confidence levels) for determining such a conclusive null finding ? Who is it who would be drawing this conclusion ? (Ie: Is it you ?... Or are you speaking on behalf of science ?)

Also, do you favour the views of the folk whose links you published in post #796 ? Or, did you make this post for purely for educative reasons ?

Regards

[mutleyeng]

@selfism
just for my own curiosity,
is it your view that the best way forward on answers to the problem of life would be sample return missions?
If so, where would you prioritise. I appreciate that requires speculation, but hey, its a fairly speculative thread.

(if i am reading you right, you are interested in looking for where there may be complex chemistry rather than little green men)

[KABOOM]

My $0.02

In order of decreased likelihood.

#1 Life exists elsewhere in the universe
#2 Given that life exists elsewhere, such life is "complex life" (multicellular animals)
#3 Given that complex life exists elsewhere such complex life actually encounters complex life somewhere else.

While we have not yet cracked the code and distilled abiogenesis down to a recipe, a lot of good work has been down and I think that it is not a "one in a billion" chance that given the right organic ingredients in a habitable zoned planet that life will eventually arise.

The author Nick Lane (Rare Earth, Oxygen - the molecule that made the world) opined that he thought that it is much more difficult for multicellular life to evolve from bacterial life than it would be for bacterial life itself to arise from the right (but not yet biological) organic ingredients.

Given the vastness of space, the technological developments, expense and timelines involved I frankly see #3 as the event that would happen with less frequency (on a % basis) than 1 or 2. That is, given that "complex" life has already occurred somewhere what is the likelihood that such life directly finds other complex life elsewhere.

[Jeff Root]

Why is life NOT everywhere that is habitable, now Jeff ?
Can you please tell us how, and why, you know this ?

Conditions change. A place that is habitable now may not
have been habitable ten million years ago. A place that was
habitable ten million years ago may not be habitable now.

-- Jeff, in Minneapolis

[John Mendenhall]

Jeff, I think, after reading the Wiki article on 'evidence', that you've got a good case for your case. Being in the habitability zone greatly increases the probability that a planet can support life. It does not prove it; but since the only evidence we have is indirect, we'll take it as supporting evidence. If we stop and think about it, much of astronomy is inferential (black holes, neutron stars, Oort cloud, etc.)

Regards, John M.

[Colin Robinson]

do you favour the views of the folk whose links you published in post #796 ? Or, did you make this post for purely for educative reasons ?

In post #796 I cited 2 contrasting theories:

1.Origin of life a chance event of very low probability (Jacques Monod, Paul Davies)
2.Origin of life not an event but a process, governed by natural laws (Christian de Duve)

In post #797 I cited another article by James Trefil and others, which is a quite detailed argument for a form of theory 2.

I mentioned these contrasting theories in response to the debate between Jeff Root and djellison, about whether evidence for habitable worlds is evidence for life beyond Earth. The answer depends very much on whether theory 1 or theory 2 is right.

I favor theory 2, especially as argued by Trefil and his friends. Nonetheless, theory 1 could conceivably be true.

[Colin Robinson]

What if the life there was unrecognisable to us ? What makes it possible to rule out the existence of life in a habitable zone which has been searched ? What are the criteria (confidence levels) for determining such a conclusive null finding ? Who is it who would be drawing this conclusion ?

How did zoologists on Earth come to a position where they can rule out deer-like unicorns, fire-breathing dragons, and mermaids, while affirming the existence of the black swan and the llama? What do you think a zoologist would say if you said: Hold on, what if unicorns have some property which makes them impossible to detect?

[Eclogite]

Jeff, I think, after reading the Wiki article on 'evidence', that you've got a good case for your case. Being in the habitability zone greatly increases the probability that a planet can support life. It does not prove it; but since the only evidence we have is indirect, we'll take it as supporting evidence.

John, I believe you are continuing the mistake made by Jeff. Let's suppose the probability that a planet can support life if it is in the Goldilocks zone is 1. It's a certainty. Being in the zone means the plane can support life.

But - and what a huge but - it says absolutely nothing about whether or not the planet does support life. That is contingent upon the emergence of life on that planet, or its transfer from another source. And we have insufficient data, as yet, on which to make an informed decision about the odds for either of these mechanisms. As such habitable zones are not evidence for life.

[KABOOM]

John, I believe you are continuing the mistake made by Jeff. Let's suppose the probability that a planet can support life if it is in the Goldilocks zone is 1. It's a certainty. Being in the zone means the plane can support life.

But - and what a huge but - it says absolutely nothing about whether or not the planet does support life. That is contingent upon the emergence of life on that planet, or its transfer from another source. And we have insufficient data, as yet, on which to make an informed decision about the odds for either of these mechanisms. As such habitable zones are not evidence for life.

But it is still evidence. Indirect evidence. I was a guest at the hotel on the night of the hotel murder. That is evidence against me. Not enough evidence to prove beyond a doubt that I committed the murder.

[djellison]

That's a very poor analogy. If 100 people were in that metaphorical hotel...one of them MUST have committed the murder. Without any further information, there's a 1% chance that any individual was the murderer.

That's nothing like the situation regarding life elsewhere. To stretch your poor analogy - you have one convicted murderer in custody, and a hotel. You don't know if there's even been a murder in that hotel. There's people in it. You're saying that because you have one murderer in custody (the earth, in this dreadful analogy) then it's evidence that others in that hotel are murderers as well. Without even a dead body to look at, how can you infer that?

We know that this one habitable place, Earth, has life. We know of many other potential habitable places. We have, to date, no idea if those places have life or not.

We have ever mounting evidence for habitability. We do not have ever mounting evidence for life.

Habitability is not life. It's habitability. To call it evidence for life is willfully misleading.

[John Mendenhall]

(snip)
But - and what a huge but - it says absolutely nothing about whether or not the planet does support life. That is contingent upon the emergence of life on that planet, or its transfer from another source. And we have insufficient data, as yet, on which to make an informed decision about the odds for either of these mechanisms. As such habitable zones are not evidence for life.

Agreed. That is what I said. It is evidence that a planet in the habitable zone is a good place to look for life, provided that other parameters are met. Goldilocks conditions, like not too big, not too small, not too hot, not too cold, etc. What is 'evidence' provokes arguments in many fields (see the Wiki article), and certainly is not confined to this thread. Some philosophers regard all evidence as indirect. I was surprised.

Regards, John M.

[MaDeR]

and it's considered doubtful that Earth-like worlds can orbit gas giants.

Yeah, ratio of parent body/orbiting body is AFAIK about 1:10000 (barring exceptional cases as Pluto/Charo or Earth/Moon). Fortunately, it is enough for heavy Jupiter (say 10 Mj, it is still in planethood per 13 Mj cutoff) with Mars-sized and larger world as it's moon as non-exceptional case.
Smaller gas giants can be probably written off, at least for worlds with habitable environment on surface.