Sagans List and life in the universe

[Grateful-UnDead]

There is an ongoing thread about the existence of life in the universe. An alternative approach in considering this issue is to look at “Sagan's List” .

This is a list he and some of his colleagues compiled, and which specified some forty or fifty criteria that are absolute criteria for life in the universe.

I don’t have a cite for this list, but think that it may have been a handout at a cosmology seminar I attended in graduate school. I have not been able to find any reference to it.

Therefore, I am proposing that the Dopers compile an updated Doper version of this list. With the completion of this list, it should be pretty obvious whether or not life exists anywhere else in the universe. (Contentious and inflammatory statement!!! )

As a point of reference, the list below are a few of the criteria I remember; obviously my recollection is incomplete.

I have put this in "GD" because I imagine there will be a lot of contention regarding inclusions or exclusion of criteria.

Feel free to contribute and debate the inclusions.

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 larger than “a” to ensure it can hold an atmosphere
5. must be a smaller than “b” in order to ensure that gravitational effects won’t squash life forms
6. must have a moon larger than “c” to shield the planet it from space debris
7. the moon must be smaller than “d’ in order to avoid destructive gravitational effect
8. 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.

The sun must:
1. must be of a certain age and size in order to provide the radiation spectrum conducive to life
2. must be a larger than “X” in order to have the required energy output.
3. must be smaller than “Y” in order to minimize gravitational effects

[GIGObuster]

IIRC "Sagan's list" concentrated on intelligent life. The list came from a meeting with other fellow scientists, they came to the conclusion that we humans as an intelligent species are probably alone in the universe.

While the evidence IMHO points to life being out there in the universe, I also think that intelligent life is rare.

[Grateful-UnDead]

Originally Posted by GIGObuster

IIRC "Sagan's list" concentrated on intelligent life. The list came from a meeting with other fellow scientists, they came to the conclusion that we humans as an intelligent species are probably alone in the universe.

While the evidence IMHO points to life being out there in the universe, I also think that intelligent life is rare.

Do you have a cite for the list? I haven't been able to find one.

[MrDibble]

The list is heavily weighted to life as we know it [ ], and even then, I think, ignores many examples of existing life, such as certain extemophiles with a very high tolerance for eg radiation or vacuum, only discovered recently, after the list was compiled.

My list for things necessary for life is a lot shorter:

* an energy source. Sun, black hole, chemical reaction, don't matter.
* a substrate/material source. Planet, gas cloud, ice particles, again, don't matter.

That's it.

While, like I said in another thread, one can't put any probabilities to this stuff with any scientific rigour, my personal belief is that the Universe teams with life, in many unpredicted and unfamiliar forms. I base this on the seeming Universal ubiquity of both organic precursor molecules and planets of some stripe, and the definite nature of both organic compounds and silicates to self-organize into complex forms across a wide range of conditions (not just current or past Earth-normal conditions).

Some of it may even be what we think of as intelligent (although I doubt anything near us (say, 100s of Ly) is anywhere near-human or better in intelligence)

[Wendell Wagner]

Are you really sure that Carl Sagan ever compiled any such list? I can't find any confirmation that he did. Sagan occasionally talked in his publications about the Drake equation, and other people sometimes misattributed it to Sagan. I suspect that you are misremembering an list which mentioned Sagan but didn't attribute the list to him.

[Peremensoe]

I am skeptical of the claim that Sagan and "fellow scientists...came to the conclusion that we humans as an intelligent species are probably alone in the universe." I think Sagan was well aware of how many of the Drake variables are too unknown to come to a conclusion, especially a negative conclusion.

Note, the original Drake equation was stated in terms of the likelihood of detectable civilizations in our own galaxy. If we broaden the terms to simply intelligent alien life and anywhere in the universe, then the probability becomes billions of times more likely than the Drake solution, whatever that is.

[GIGObuster]

After checking more it is likely that I got that from second hand information so, put me now in the camp that does not believe that Sagan made that list, indeed he was more into the optimistic side of the Drake equation, that come to think of it: it is a list, but a shorter one.

[Lord Ashtar]

Originally Posted by MrDibble

My list for things necessary for life is a lot shorter:

* an energy source. Sun, black hole, chemical reaction, don't matter.
* a substrate/material source. Planet, gas cloud, ice particles, again, don't matter.

That's it.

I must say, I never considered a black hole as a possible energy source for life. I wonder how that would work?

[Grateful-UnDead]

Originally Posted by Wendell Wagner

Are you really sure that Carl Sagan ever compiled any such list? I can't find any confirmation that he did. Sagan occasionally talked in his publications about the Drake equation, and other people sometimes misattributed it to Sagan. I suspect that you are misremembering an list which mentioned Sagan but didn't attribute the list to him.

No, given the feedback so far, and the fact that I can't find any reference to it, I am having serious doubts. While I don't doubt that I saw such a list many years ago, I now have doubts as to its origins.

But having said that, what is to prevent the Dopers from making such a list?

In this context, I have just found the following in "Popular Mechanics", July 2010, p16:

"Scientists have long believed that places without oxygen could not permanently host life more complicated than viruses or bacteria. But a team of European researchers taking core samples from the 11,000 foot deep L'Atalante Basin in the Mediterranean Sea discovered three new species of "Loricifera" that proved otherwise.
Instead of mitochondria, the power plant of most cells, which use oxygen to transfer cellurar energy, the newly found creature has organelles, that use hydrogen. They are the first multi-cellular organisms ever found that don't need oxygen to thrive"

Guess we can strike a need for oxygen from our list

[GIGObuster]

Originally Posted by Grateful-UnDead

"Scientists have long believed that places without oxygen could not permanently host life more complicated than viruses or bacteria. But a team of European researchers taking core samples from the 11,000 foot deep L'Atalante Basin in the Mediterranean Sea discovered three new species of "Loricifera" that proved otherwise.
Instead of mitochondria, the power plant of most cells, which use oxygen to transfer cellurar energy, the newly found creature has organelles, that use hydrogen. They are the first multi-cellular organisms ever found that don't need oxygen to thrive"

Guess we can strike a need for oxygen from our list

BTW, that was precisely how earth was, oxygen was not abundant at the beginning, it became abundant in the atmosphere because the early life forms produced oxygen.

[Grateful-UnDead]

Originally Posted by GIGObuster

BTW, that was precisely how earth was, oxygen was not abundant at the beginning, it became abundant in the atmosphere because the early life forms produced oxygen.

Evidently oxygen had to be present in some form.

Then a biological entity had to appear which was both "alive" in the sense that it was capable of reproduction, proliferation and evolution, and its biological processes could release oxygen in its elemental form.

So can we say one of the criteria for life is the presence of oxygen, in some form?

Leading from this, we then need to quantify the abundance of oxygen; ie: it must constitute "X" percent of the planetary mass.

So what was the form that the oxygen containing material took?

The cite above talks of metabolising hydrogen, but does not say what form the hydrogen takes prior to being metabolised.

Elemental hydrogen? Water?

Is it reasonable to assume water?

So a necessary condition for life is the presence of water?

Or some other oxygen containing material? If so, what?

[GIGObuster]

More complicated than that, Oxygen was actually a poison to early life.

http://en.wikipedia.org/wiki/Oxygen_catastrophe

The Great Oxygenation Event (GOE), also called the oxygen catastrophe or oxygen crisis or Great Oxidation, was the appearance of free oxygen (O2) in Earth's atmosphere. This major environmental change happened around 2,400 million years ago.

Photosynthesis was producing oxygen both before and after the GOE. The difference was that before the GOE, organic matter and dissolved iron chemically captured any free oxygen. The GOE was the point when these minerals became saturated and could not capture any more oxygen. The excess free oxygen started to accumulate in the atmosphere.

The rising oxygen levels may have wiped out a huge portion of the Earth's anaerobic inhabitants at the time. From their perspective it was a catastrophe (hence the name). Cyanobacteria were essentially responsible for what was likely the largest extinction event in Earth's history.

Free Oxygen was not needed for the origin of life, water (It has oxygen but in combination with Hydrogen) is a reasonable thing to assume we need as part of the mix to get life going.

More information can be found here:

http://astrobiology.nasa.gov/

http://astrobiology.nasa.gov/nai/lib...planet-arthur/

[Grateful-UnDead]

Originally Posted by GIGObuster

More complicated than that, Oxygen was actually a poison to early life.

http://en.wikipedia.org/wiki/Oxygen_catastrophe

Free Oxygen was not needed for the origin of life, water (It has oxygen but in combination with Hydrogen) is a reasonable thing to assume we need as part of the mix to get life going.

More information can be found here:

http://astrobiology.nasa.gov/

http://astrobiology.nasa.gov/nai/lib...planet-arthur/

Yes, I get all that.

But in the context of building a list: oxygen in some form seems to be a pre-requisite for life; and logically, oxygen must constitute "X" percentage of planetary mass.

Correct?

[Blake]

Originally Posted by Grateful-UnDead

Yes, I get all that.

But in the context of building a list: oxygen in some form seems to be a pre-requisite for life; and logically, oxygen must constitute "X" percentage of planetary mass.

Correct?

Not really. Oxygen is 1/3 of water, so if you want to have water-based life then you need oxygen in some form. If your life form is based on liquids such as ammonia or methane or not even based on liquids at all, then you don't need oxygen.

[Grateful-UnDead]

Originally Posted by Blake

Not really. Oxygen is 1/3 of water, so if you want to have water-based life then you need oxygen in some form. If your life form is based on liquids such as ammonia or methane or not even based on liquids at all, then you don't need oxygen.

In attempting to construct the list, we are trying to establish the fundamental requirements for the development and subsequent support of "life", as opposed to the specific biological processes involved in individual life forms.

For example, it doesn't matter if the life form is based on carbon or silicone, if its structure is being torn apart by interacting stellar gravitational fields.

Similarly, if the surface radiation levels fry any molecule that comes into existence, then clearly those planetary conditions are not conductive to life.

So both of those obvious preconditions quantify where in a galaxy a life friendly planet may reside; by this process we narrow down the volume of space that is suitable, and then from this we can start to extrapolate into probabilities.

Similarly, using the case of oxygen, if we say that it is a fundamental requirement for "life", then the planet on which it develops must have a minimum of its mass constituted of oxygen. If it is silicone based, the planet must have a minimum mass of silicone.

Alternatively, life may require both a minimum mass of both oxygen and silicone.

So, of all the thousands of planets that probably exist, we have eliminated many of them. Again, this serves to provide a firm basis from which to extrapolate probabilities.

From these examples, it should be evident that defining and quantifying these issues establishes some fundamental first principles for life; and provides a pretty good basis for objectively quantifying the probability of its existence outside earth.

With reference to the NASA astrobiology studies: from what I have read, it seems that they have moved beyond these "first principles" and are looking at very specific processes and mechanisms of life on earth.

They do not appear to be considering the question of whether life on earth is a manifestation of a universal generality, or whether it is a single unique instance.

Getting an answer to this question is the the whole point of trying to reconstruct "Sagan's(???) List".

[Blake]

Originally Posted by Grateful-UnDead

Similarly, using the case of oxygen, if we say that it is a fundamental requirement for "life", then the planet on which it develops must have a minimum of its mass constituted of oxygen. If it is silicone based, the planet must have a minimum mass of silicone.

Which is perfectly circular.

If we say that it is not a fundamental requirement for "life", then the planet on which it develops needn't have any of its mass constituted of oxygen.

You can't construct a list of fundamental requirements by first constructing an elaborate on a list of fundamental requirements.

[Grateful-UnDead]

Originally Posted by Blake

Which is perfectly circular.

If we say that it is not a fundamental requirement for "life", then the planet on which it develops needn't have any of its mass constituted of oxygen.

You can't construct a list of fundamental requirements by first constructing an elaborate on a list of fundamental requirements.

What???????

What I am saying is that we specify a particular requirement.

Then taking all the planets in the universe, we delete those that don't meet that requirement.

Then we repeat the process with each of the other requirements, until such time as we have culled all those planets that don't meet all the requirements.

The planets remaining are those that meet all the specificatons, and therefore could conceivably support life.

This gives us a basis for estimating the prevalence of life in places other than earth.

But first, we have to construct a list of requirements. So far we haven't been doing too well with that.

[Blake]

I no longer have any idea at all what argument you are trying to pursue.

I do however know what you said.

You said "But in the context of building a list: oxygen in some form seems to be a pre-requisite for life; and logically, oxygen must constitute "X" percentage of planetary mass."

That statement is flat out and completely and utterly without basis. There is no reason for believing that oxygen is a pre-requisite for life.

That is the sole point that I wish to make. If your list says that oxygen is a prerequisite then your list is worthless.

If you have some other point you wish to discuss pertaining to oxygen then you need to make it more clearly. But the point that oxygen is a prerequisite for life can't be supported.

[Grateful-UnDead]

Originally Posted by Blake

I no longer have any idea at all what argument you are trying to pursue.

I do however know what you said.

You said "But in the context of building a list: oxygen in some form seems to be a pre-requisite for life; and logically, oxygen must constitute "X" percentage of planetary mass."

That statement is flat out and completely and utterly without basis. There is no reason for believing that oxygen is a pre-requisite for life.

That is the sole point that I wish to make. If your list says that oxygen is a prerequisite then your list is worthless.

If you have some other point you wish to discuss pertaining to oxygen then you need to make it more clearly. But the point that oxygen is a prerequisite for life can't be supported.

Argument??? What argument?????

I have repeatedly stated that I am trying to reconstruct the list of criteria required for the support of life. Such a list may or may not have been produced by Carl Sagan.

The whole issue of oxygen was merely provided as an example as to one criterion, which may or may not be correct, and how that criterion may be applied.

So you, and anybody else, are invited to supply any such criterion which you think applies; to assist the process I supplied examples in my original post.

It's not rocket science.

[GIGObuster]

Originally Posted by Grateful-UnDead

It's not rocket science.

Actually, it is.

http://astrobiology.arc.nasa.gov/new...ews.cfm?id=408

http://archives.cnn.com/2000/TECH/sp...ugs/index.html

One of the primary goals of astrobiology is to determine how life began on Earth. A few astrobiologists argue that single-celled life forms may have originated on Mars and spread to Earth aboard meteorites. Or vice versa.

Today a NASA rocket spent twenty minutes in space putting these theories to the test.

But really, I'm agreeing more with Blake here, If you want us to help I think you need to accept (I detect some testiness here) some criticism regarding why a criterion you insist on putting on the list is not really a good one (oxygen)

[MrDibble]

Originally Posted by Lord Ashtar

I must say, I never considered a black hole as a possible energy source for life. I wonder how that would work?

"Black" holes put out a lot of energy, all across the EM spectrum, including the visible. Certainly some put out enough for photosynthesis (but the X-Rays, Gamma rays and the like make this not a nice place for Earth-autotrophs). Remember that a collapsar can be orbited by a planet/s just like any other large gravitational body.

Some sort of life could, for instance, develop around active galactic nuclei, but it would have to be so very different from "life as we know it" [] as to be very hard to describe beforehand. I don't know how such an entity would look, but I do know that it wouldn't be impossible. Some sort of broad-spectum planet-bound burrowing radiovore with silicate organ-analogues and metal-shielded casing? Or diaphanous gas-cloud entities with atom-thick gold "leaves" for trapping radiation? Who knows...

[MrDibble]

Originally Posted by Grateful-UnDead

What???????

What I am saying is that we specify a particular requirement.

Then taking all the planets in the universe, we delete those that don't meet that requirement.

I can confidently say that you're never going to find a naturally-occurring planet that doesn't have oxygen, carbon and nitrogen in some form.

[wintertime]

Speaking of silicon-based life: While we have found oxidized silicon at other places, I am not aware that we can say the same about silanes or silicones (?).. can you think of other molecules that could be an indication of a silicon-based biochemistry?

[MrDibble]

Originally Posted by wintertime

Speaking of silicon-based life: While we have found oxidized silicon at other places, I am not aware that we can say the same about silanes or silicones (?).. can you think of other molecules that could be an indication of a silicon-based biochemistry?

Not really - but that's the problem in a nutshell: you're (unconsciously) expecting a silicon-based life to use an analogue to carbon-based oxidation/solvent biochemistry, when it need do no such thing. Perhaps it uses electro-chemistry instead, or something else out of left field. Look at the models of abiogenesis that use replicating clay minerals as a substrate. Those clays are partway to living (replicating, growing) without a biochemical basis.

[wintertime]

Originally Posted by MrDibble

Not really - but that's the problem in a nutshell: you're (unconsciously) expecting a silicon-based life to use an analogue to carbon-based oxidation/solvent biochemistry, when it need do no such thing.

Ah, you might have assumed I hold a certain position when I need do no such thing to ask this question. The lack of any freely and widely available molecules that could be building blocks of silicon-based life is per se no evidence that such life cannot exist.

Their lack, however, is a reason to say that a) the existence of the analogous forms you mentioned is not supported so far by our observations and b) that we might not find the (chemical) traces we'd expect from silicon-based life, if it is indeed built upon different principles.

If someone proposed some kind of electro-chemistry, he'd still have to show how this could work to elevate the thought above wild speculation into hypothesis-territory; this way, we'd also get an idea about the characteristic traces that such a life form would leave behind, so that we would be able to identify it in the first place.

[septimus]

Two other reasons the Earth's Moon might have been helpful in the development of advanced life:

(1) Due to the feedback of conserved momentum sharing, the Moon helps stabilize the Earth's axis (obliquity of the ecliptic) or so I'm told. Otherwise climate would fluctuate much more erratically.

(2) Tides and waves, partly due to Moon's gravity, create tide pool fluctuations which assist the development of land life from sea life.

[Grateful-UnDead]

Originally Posted by MrDibble

Some sort of life could, for instance, develop around active galactic nuclei, but it would have to be so very different from "life as we know it" [] as to be very hard to describe beforehand. I don't know how such an entity would look, but I do know that it wouldn't be impossible. Some sort of broad-spectum planet-bound burrowing radiovore with silicate organ-analogues and metal-shielded casing? Or diaphanous gas-cloud entities with atom-thick gold "leaves" for trapping radiation? Who knows...

This is one of the issues that can be adressed in general terms, and it doesn't require any specific knowledge of the particular life form.

Regardless of the nature of the final life form, it would be fundamental that it would require chemical bonds be formed, producing molecules of varying complexity.

However, if the environment is such that the radiation intensity is such that those bonds are broken as soon as they are formed, then no life of any kind will develop.

Therefore, one of the requirements on the list would be that the planet be a minimum distance of "D1" from the galactic centre.

Add to this the requirement that there would need to be a certain minimum matter density for a planetary system to form in the first place, and we can see that there would be a "donut" of space within the galaxy within which the planet can form, and chemical bonding could occur.

Just these two requirements alone have culled a large volume of the galaxy.

Having said that, your idea of a "gas cloud entity" is very interesting. Would a diffuse, gaseous cloud be considered "life"? How would that work?

[Grateful-UnDead]

Originally Posted by MrDibble

I can confidently say that you're never going to find a naturally-occurring planet that doesn't have oxygen, carbon and nitrogen in some form.

Yes, and if it turns out that any or all of these elements are fundamental requirements for life, those requirements immediately put some physical parameters on the supporting planet.

Specifically, the planet would require the presence each of these elements, and they would have to be present in certain minimum abundance.

So, as a coarse screen, how many planets in the galaxy/universe have oxygen/carbon/nitrogen in abundance "A", "B", "C"?

So, now we have a "donut" within a galaxy that encloses the volume of space that can support life due to material and radiation constraints, now we add the requirement for C, N, O in specific minimum abundances, and the number of supporting planets gets smaller.

[Blake]

Originally Posted by Grateful-UnDead

Therefore, one of the requirements on the list would be that the planet be a minimum distance of "D1" from the galactic centre.

Yes, but that distance is probably incredibly short. As Mr Dibble suggests, life could develop underground even at ridiculously high radiation levels. Radiation "such that [chemical] bonds are broken as soon as they are formed" is highly ionising radiation. It can't travel far through solid matter because, well it interacts with chemical bonds, thus breaking them.

So long as the planet isn't actually made molten by the radiation it receives. molten then there is nothing at all stopping even carbon and water based life from existing on it. You really don't have to get too far from galactic centre to find solid planets, so I doubt that restriction has nay meaningful impact at all on the list.

If we posit life that isn't carbon and water based then even being molten isn't an obstacle.

Add to this the requirement that there would need to be a certain minimum matter density for a planetary system to form in the first place, and we can see that there would be a "donut" of space within the galaxy within which the planet can form, and chemical bonding could occur.

Can you provide evidence for this claim that outlying star systems don't have any planets?

Just these two requirements alone have culled a large volume of the galaxy.

No, they haven't. The first has negligible effect. The second is unproven.

[Blake]

Originally Posted by Grateful-UnDead

So, as a coarse screen, how many planets in the galaxy/universe have oxygen/carbon/nitrogen in abundance "A", "B", "C"?

100%. Or so close to as make no difference. Aside from noble gases, you have just names 3 of the 5 most abundant elements on the universe. I can't see any way that a planet could not have those elements in abundance.

[Grateful-UnDead]

Originally Posted by Blake

Yes, but that distance is probably incredibly short. As Mr Dibble suggests, life could develop underground even at ridiculously high radiation levels. Radiation "such that [chemical] bonds are broken as soon as they are formed" is highly ionising radiation. It can't travel far through solid matter because, well it interacts with chemical bonds, thus breaking them.

So long as the planet isn't actually made molten by the radiation it receives. molten then there is nothing at all stopping even carbon and water based life from existing on it. You really don't have to get too far from galactic centre to find solid planets, so I doubt that restriction has nay meaningful impact at all on the list.

If we posit life that isn't carbon and water based then even being molten isn't an obstacle.




Can you provide evidence for this claim that outlying star systems don't have any planets?



No, they haven't. The first has negligible effect. The second is unproven.

The question of radiation intensity and composition is an interesting one; we know that one of the biggest impediments for human travel beyond the moon is the lethal radiation emanating from the sun, and other stars. So until this issue is resolved, humans won't be going too far.

With regard to the issue of radiation in general: the solar wind is composed of more than just ionizing radiation. So it is an interesting to consider what a planet which is close to the galactic centre and exposed to the intense radiation there, would look like. Would it be a solid ball like the earth, or would it be reduced to a ball of plasma?

The only issue is, as you point out, is to calculate the radius of lethality.

With the regard to your other comment: I didn't say that outlying stars don't have planets.

I did say that there is an outer limit at which the density of matter would be too low for there to be sufficient mass to aggregate to make either stars or planets.

This outer radius would be a matter for both calculation and observation.

[Grateful-UnDead]

Originally Posted by Blake

100%. Or so close to as make no difference. Aside from noble gases, you have just names 3 of the 5 most abundant elements on the universe. I can't see any way that a planet could not have those elements in abundance.

This is intuitively obvious.

However, what I said was that if we assume that C,N,O are fundamental to life, then there would be a requirement that they each be a certain, yet to be defined, proportion of the planetary mass.

[MrDibble]

Originally Posted by wintertime

If someone proposed some kind of electro-chemistry, he'd still have to show how this could work to elevate the thought above wild speculation into hypothesis-territory; this way, we'd also get an idea about the characteristic traces that such a life form would leave behind, so that we would be able to identify it in the first place.

We know how such a life-form could work - we have the elements of such a "bio"-chemistry all around us, in the form of integrated circuits, batteries, electroplating, photovoltaics, motors, etc.

How such a system would actually be put together, I have no idea. Or what the traces would be.

I could speculate all day long about how nano-solenoids could make a myomere analogue, or the chance of metallic deposition on a silicate substrate in an H₂SO₄ ocean in a high magnetic-flux environment could produce primitive evolveable active circuits, but none of that speculation would be backed by any sort of numbers at all. So, not out of the realm of possibility, but in no way a scientific discussion.

[MrDibble]

Originally Posted by Grateful-UnDead

Regardless of the nature of the final life form, it would be fundamental that it would require chemical bonds be formed, producing molecules of varying complexity.

Again, this isn't necessarily so.

Having said that, your idea of a "gas cloud entity" is very interesting. Would a diffuse, gaseous cloud be considered "life"? How would that work?

I don't know. I could speculate - patterns or compression waveforms of varying densities could encode information, which is ultimately the fundamental unit of replication and inheritance. Think fluidics, but constrained by variant densities not hard components.

[Blake]

Originally Posted by Grateful-UnDead

The question of radiation intensity and composition is an interesting one; we know that one of the biggest impediments for human travel beyond the moon is the lethal radiation emanating from the sun, and other stars. So until this issue is resolved, humans won't be going too far.

What does that have to do with your list? We are talking about where life can arise and survive, not where life that has arisen somewhere radically different can survive. A camel can't survive in the arctic either. That doesn't tell us a damn thing about whether life can survive there

With regard to the issue of radiation in general: the solar wind is composed of more than just ionizing radiation.

So what? You were only talking about ionising radiation.

I did say that there is an outer limit at which the density of matter would be too low for there to be sufficient mass to aggregate to make either stars or planets.

Yeah, that's what we call, in technical terms, the edge of the galaxy.

This outer radius would be a matter for both calculation and observation.

So you want to calculate how many planets are in the galaxy based on how many are beyond the edge of the galaxy?

Originally Posted by Grateful-UnDead

This is intuitively obvious.

However, what I said was that if we assume that C,N,O are fundamental to life, then there would be a requirement that they each be a certain, yet to be defined, proportion of the planetary mass.

No what you asked was what proportion of planets have that abundance. And the answer is close enough to 100%.

[septimus]

Originally Posted by MrDibble

Again, this isn't necessarily so.I don't know. I could speculate - patterns or compression waveforms of varying densities could encode information, which is ultimately the fundamental unit of replication and inheritance. Think fluidics, but constrained by variant densities not hard components.

Perhaps, but I'm doubtful. You need a stable platform for some sort of entropic pump. Can large-scale storms provide that? With permanence adequate to pass significant information to "children"?

[Blake]

Originally Posted by septimus

Perhaps, but I'm doubtful. You need a stable platform for some sort of entropic pump. Can large-scale storms provide that? With permanence adequate to pass significant information to "children"?

The Red Spot on Jupiter is a large scale storm that has lasted at least 400 years and shows no signs of abating.

Since many living organisms produce daughter generations every hour or so, and some less, I would have to say that 400 years is sufficient time.

[wintertime]

Originally Posted by MrDibble

We know how such a life-form could work - we have the elements of such a "bio"-chemistry all around us, in the form of integrated circuits, batteries, electroplating, photovoltaics, motors, etc.

How such a system would actually be put together, I have no idea. (..)

Neither do I, nor if it is possible in the first place. But if we ever find something like that, I'm sure we'll have another discussion forthcoming: whether we've found first or second generation life.

[wheresmymind]

"1. lie on the outlying regions of a galaxy in order to have reduced impinging radiation levels from the galaxy center" - We have organisms here on earth that can survive radiation levels thousands or even millions of times higher than any place on earth (google deinococcus radiodurans, tardigrades). They evolved this ability basically by accident, it just turns out that some adaptations that protect and repair organisms from the effects of dessication work for radiation exposure as well.

"3. must have an iron core to provide shielding from its sun’s radiation" - see above.

"4. must be a larger than “a” to ensure it can hold an atmosphere" - I don't see why an atmosphere is necessary. Life on earth certainly first evolved in water.

"5. must be a smaller than “b” in order to ensure that gravitational effects won’t squash life forms" - Gravity would have little effect on organisms living in water (or another liquid), or very small terrestrial organisms. The force of gravity on a bacterium is so small it can be ignored - even if it was hundreds or thousands of times higher.

"6. must have a moon larger than “c” to shield the planet it from space debris" - perhaps life would have evolved a bit later without the protection of the moon, but Jupiter and the inner planets have done a good job in the past 4 billion years of clearing the inner solar system. If the moon disappeared tomorrow we wouldn't be beset by planet-sterilizing impacts.

"7. the moon must be smaller than “d’ in order to avoid destructive gravitational effect" - As long as the planet isn't literally ripped apart, I don't think tidal forces would have much of an effect on aquatic life. It could even help life along, by keeping the planet's interior molten and providing energy through volcanism.


"8. 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." - Volcanism, radioactive decay, and tidal forces could also warm a planet (or moon) just fine. Examples in our own solar system include Io, Europa, Ganymede and Titan.

The sun must:
1. must be of a certain age and size in order to provide the radiation spectrum conducive to life - Solar radiation isn't necessary for life (see 8 above). On our own planet we have many chemoautotrophic organisms that get their energy from inorganic compounds. Hydrothermal vent communities are one obvious example. They've even found photopigments that can harvest photons of blackbody radiation coming from the vents.

2. must be a larger than “X” in order to have the required energy output. - See above.

3. must be smaller than “Y” in order to minimize gravitational effects - See 7 above. I think the only thing in this regard that is necessary is that the orbit of the planet is stable and the planet is far enough away for water or some other liquid solvent to exist.

I'm with Mr. Dribble in that all I think is really necessary is some sort of energy source, and some sort of "stuff" that can react.

[Blake]

Originally Posted by wheresmymind

"4. must be a larger than “a” to ensure it can hold an atmosphere" - I don't see why an atmosphere is necessary. Life on earth certainly first evolved in water.

Unless you are talking about molten metal, any liquid is going to have an appreciable vapor phase. So surface liquid=atmosphere. This is most definitely rue of water. If a planet can't hold an atmosphere it sure can't hold liquid water.

[wheresmymind]

Originally Posted by Blake

Unless you are talking about molten metal, any liquid is going to have an appreciable vapor phase. So surface liquid=atmosphere. This is most definitely rue of water. If a planet can't hold an atmosphere it sure can't hold liquid water.

True, but I'll counter that surface liquid isn't necessary. Galilean moons Europa and Ganymede and Saturn's moon Enceladus are all believed to have liquid water (warmed by tidal heating) beneath their surface.

[Blake]

Originally Posted by wheresmymind

True, but I'll counter that surface liquid isn't necessary. Galilean moons Europa and Ganymede and Saturn's moon Enceladus are all believed to have liquid water (warmed by tidal heating) beneath their surface.

Correct me if I'm wrong, but aren't all those bodies large enough to retain an atmosphere?

[wheresmymind]

Originally Posted by Blake

Correct me if I'm wrong, but aren't all those bodies large enough to retain an atmosphere?

Actually, I guess those moons ARE large enough to have an atmosphere, they just don't for whatever reason. They have trace atmospheres, much like our own moon, but their surface pressures are measured in fractions of a micro Pascal, whereas 1 atm is about 100,000 pascals. So their "atmosphere" is on the order of one trillionth to one hundred billionth as dense as our own - similar to the very hardest laboratory vacuums. Titan is the only moon that has a dense atmosphere. Titan is similar in size to the moons I mentioned, and its atmosphere is more dense and more massive than Earth's despite Titan's small size/mass (compared to Earth). So there are factors besides size that play a role in the formation and retention of an atmosphere.

[wintertime]

Originally Posted by wheresmymind

So there are factors besides size that play a role in the formation and retention of an atmosphere.

A pretty good online source concerning these factors is Origins of Atmospheres by Kevin Zahnle. A couple of interesting articles by Catling, Kasting and Zahnle, published in Science, deal with the possible influence of life on the depletion of certain gases.

In case someone is interested in the new field of "astrobiology" in general, I've enjoyed the book Planets and Life - The Emerging Science of Astrobiology, edited by Woodruff T. Sullivan III and John Baross.

[wheresmymind]

Originally Posted by wintertime

A pretty good online source concerning these factors is Origins of Atmospheres by Kevin Zahnle. A couple of interesting articles by Catling, Kasting and Zahnle, published in Science, deal with the possible influence of life on the depletion of certain gases.

In case someone is interested in the new field of "astrobiology" in general, I've enjoyed the book Planets and Life - The Emerging Science of Astrobiology, edited by Woodruff T. Sullivan III and John Baross.

Thanks a lot! It's a small world; John Baross is actually my grand-advisor (he was my MS advisor's advisor). I didn't realize he was so involved in astrobiology, I know him as a marine microbial ecologist.