Sagans List and life in the universe

Great Debates
1

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
2

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.

3

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

4

The list is heavily weighted to life as we know it [ :dubious: ], 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)

5

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.

6

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.

7

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.

8

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

9

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

10

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.

11

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?

12

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

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/library-of-resources/annual-reports/2004/psu/projects/evolution-of-a-habitable-planet-arthur/

13

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?

14

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.

15

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”.

16

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.

17

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.

18

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.

19

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.

20

Actually, it is.

http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=408

http://archives.cnn.com/2000/TECH/space/07/26/alien.bugs/index.html

:slight_smile:

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)