How many exoplanets with an atmospheric oxygen signature would be necessary?

[zankaon]

Currently 241 exoplanets. [http://en.wikipedia.org/wiki/Gliese_581_c/URL][/PLAIN] A few are terrestrial such as Gliese c, d at 5x and 8x Earth mass; sufficient mass to be geologically active, and hence a magnetosphere, diverting a stellar wind, and thus preventing eroding of any atmosphere and oceans. An O_2 molecular spectographic signature from a exoplanet atmosphere is a sign of photosynthesis. Earth has had such oxygen signature for 2 billion years; while just technological for say 200 yrs; that is 1 part in 10^7 (200/2 x 10^9). So if one divided 2 billion years into bins of 200 years each, one would require 10^7 such bins until one discovered an exoplanet with an oxygen signature, that had a technological civilization. This is assuming that our Earth history is typical. Re-expressed: one would need a database of 10 million exoplanets with an atmosphere oxygen signature before finding a technological civilization! How many terrestrial planets are there in a galaxy of about 10^11 stars? The pattern for exoplanets seems to be tending towards abundance. If only 1/10 of stars had 1 terrestrial, then 10^10 terrestrial exoplanets. Would and O_2 signature for 1/1000 exoplanets seem too high? If not, then aren't there approximately 1000 (10^10/10^7) technological civilizations in our galaxy?

 

[russ_watters]

It is fun to play with the Drake equation, but at the end, it is still just a pile of assumptions.

 

[dimensionless]

Once a civilization becomes technological, it might remain that way, so the number of planets you would have to look at could much lower than 10^7. On the other hand, technological civilizations could also destroy themselves.

There is also the question of what environments are hospitable. Subterranean oceans are probably more common the superterranean oceans. There is also the possibility that other liquid such as liquid methane might be suitable. Planets like Jupiter have chemistry that is somewhat exotic, and I'm not ready to rule out the possibility that they could sustain some type of life form.

There are also many comets in out solar system that, at least periodically, contain contain both liquid and gas, and are also bombarded by plenty of sunlight. Another thing about comets, is that they might be able to exchange little bits of biological material with one another, and they could potentially travel from one solar system to another. I can't imagine that a technological civilization would begin on a comet (I don't picture them as having a sufficiently large volume for life, I'm not sure though. How many cubic km of slush/water can a comet have?), but I could imagine a technological civilization colonizing a comet, and then hop scotching to other comets. How many technological civilizations are there on Earth?

 

[zankaon]

Smaller relational database

Currently 241 exoplanets. [http://en.wikipedia.org/wiki/Gliese_581_c/URL][/PLAIN] A few are terrestrial such as Gliese c, d at 5x and 8x Earth mass; sufficient mass to be geologically active, and hence a magnetosphere, diverting a stellar wind, and thus preventing eroding of any atmosphere and oceans. An O_2 molecular spectographic signature from a exoplanet atmosphere is a sign of photosynthesis. Earth has had such oxygen signature for 2 billion years; while just technological for say 200 yrs; that is 1 part in 10^7 (200/2 x 10^9). So if one divided 2 billion years into bins of 200 years each, one would require 10^7 such bins until one discovered an exoplanet with an oxygen signature, that had a technological civilization. This is assuming that our Earth history is typical. Re-expressed: one would need a database of 10 million exoplanets with an atmosphere oxygen signature before finding a technological civilization! How many terrestrial planets are there in a galaxy of about 10^11 stars? The pattern for exoplanets seems to be tending towards abundance. If only 1/10 of stars had 1 terrestrial, then 10^10 terrestrial exoplanets. Would and O_2 signature for 1/1000 exoplanets seem too high? If not, then aren't there approximately 1000 (10^10/10^7) technological civilizations in our galaxy?

If one added a relational database, cross-linking the greater than 4.6 Byrs age of a stellar system to those exoplanet terrestrials with an O_2 atmosphere signature, then one would have only those binned planets that have had a long term O_2 signature. Hence the resultant database would be much smaller; not 10^7, but rather 3 (?) orders less - that is, perhaps a manageable 10^4 planets?

 

[Gilligan08]

But wouldn't we be getting readings from these planets that are 100 - 1 milliion years old. So, I can't imagine that if we were to even have this large database of planets we would get that many civilizations because we wouldn't see the effects of the current state until much later.

Or i guess since maybe our galaxy is so much older it is reasonable to assume that many civilizations got a jump start on us. I don't know its all conjecture. though it would be awesome if we could find something.

 

[keinve]

i see no reason for it to require oxygen at all. there are many others gases. what's to say a technological species would require oxygen? they may be able to breath nitrogen only. or carbon dioxide. your assumptions are based on the idea that all life requires oxygen. also, you said that the Earth has only been technological for two hundred years. don't you think that a sword is a piece of technology? because last time i checked, you can't get a claymore out of a piece of iron without doing something to it first. the same goes for spears, and bows. and armor. technology extends back to the stone age, when something as simple as a sharpened stone attached to a stick was common. that was technology. brewing alcohol requires technology. ancient egyptions brewed beer. i believe the term you meant was technologically advanced. i know i went off topic, but when people mess up the definition of technology, i tend to get a bit carried away. so, when you count all all the technologically advanced civilations dependent on oxygen, it would probably be a bit less than one thousand. when you count all technologically advanced civilations, it's eaisly several thousand. even a sea based race could be technologically advanced. that could increase your numbers even more.

 

[russ_watters]

There are certain things about carbon and water that make them uniquely suited for complex chemical reactions to take place in. That's why it is expected that they are required for life. Also, even if there is something we haven't thought of, the fact that we haven't thought of it also means we wouldn't know how to look for it! So we may as well just stick with what we know.

And I think when people say "technologically advanced" in this context, they generally mean advanced enough to be able to detect from a great distance. This requires things like artificial lights, radio, certain artificial chemicals in the atmosphere, etc.

 

[keinve]

There are certain things about carbon and water that make them uniquely suited for complex chemical reactions to take place in. That's why it is expected that they are required for life. Also, even if there is something we haven't thought of, the fact that we haven't thought of it also means we wouldn't know how to look for it! So we may as well just stick with what we know.

And I think when people say "technologically advanced" in this context, they generally mean advanced enough to be able to detect from a great distance. This requires things like artificial lights, radio, certain artificial chemicals in the atmosphere, etc.

i never said that it would be non carbon based life forms or that they would be able to exist without water. i just said that he assumed that they were dependent on exygen. i know that water need oxygen to exist, but he just said that the signature needed to be oxygen, indicating that the society is directly dependent on it. humans are. there might be a species that is dependent on a natural gas that is poisonous to us. they would be suprised to learn that we breath oxygen.

 

[russ_watters]

there might be a species that is dependent on a natural gas that is poisonous to us.

Via what chemical reaction would methane provide energy to these species?

 

[keinve]

who knows? they may have a biology that is dependent on methane. besides, it might not be just methane. carbon dioxide, over a long period of time with no oxygen, can be deadly.

 

[DaveC426913]

who knows? they may have a biology that is dependent on methane. besides, it might not be just methane. carbon dioxide, over a long period of time with no oxygen, can be deadly.

Right, but then we're just fantasizing. At the risk of being facetious, one might as well look for unicorns.

The point is that, there are good reasons why we think water is likely essential for life. It's not just an Earth-centric bias. The same very good reasons for water's role in life apply on any other planet too. It has a host very interesting properties that, when you add them up, make it a virtually unique substance in the universe.

Looking for non-water-based life is the equivalent of looking in a water fountain only for pennies that have landed balanced on their edge. Oh sure, you can't rule it out, but what would you bank on?

 

[russ_watters]

who knows?

Chemists know exactly what chemical reactions with methane produce energy. I'll give you a hint: the simplest ones use some form of oxygen. That's kinda the point here. We already know what the likely chemical bases of life are. There are only so many elements and so many natural compounds. Certain ones have specific properties that make them ideally suited for harboring life.

Liquid methane has actually been considered as a possible primary liquid for supporting life (as opposed to water), but it has certain disadvantages over water that make it unlikely.

carbon dioxide, over a long period of time with no oxygen, can be deadly.

Early in the Earth's evolution, carbon dioxide was predominant and oxygen was poisonous. Plants, of course, breathe in carbon dioxide and expunge oxygen. But there is good reason why animals evolved to breathe oxygen (carbon dioxide producing reactions don't produce enough energy to support animals).

The point, again, is that it isn't just about "life as we know it" and being able to recognize vastly different life if we saw it. The proportions of the elements in the universe, and the chemical properties of the elements make it extremely unlikely that life would be able to form with a chemical makeup fundamentally different from ours. The unique properties of carbon, water, and oxygen make them uniquely suited for being the chemical makeup of life.

 

[russ_watters]

Right, but then we're just fantasizing. At the risk of being facetious, one might as well look for unicorns.

It is also worth pointing out that just because we aren't looking for unicorns, that doesn't mean we wouldn't recognize them if we saw them. So there really isn't any downside to a focused approach to our search.

 

[888eddy]

keinve. you're right, in that its possible because we just don't know.

however we are much more likely to find a planet with life similar to Earth's since we know what we are looking for. if we found a planet with an atmosphere completely different from anything we have ever seen we might guess it has some other form of life but it could be something else. whereas if we found a planet with the same signatures as life on Earth it would have a much higher probability of harboring life and therefore is the better option for further study.

i see your point that we don't know how life might form and that there could be all sorts of organisms around the galaxy. as far as putting a value on the number of technologically advanced organisms in the galaxy it is possible that there is a completely different form of life and that would multiply the value. but our best chances of finding extra terrestrial life is to search for traits of the only form of life we know of.