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That is assuming that the multi-terawatt Hydrogen-based emission spectrum from the Sun sun does not drown out the meager Oxygen absorption spectrum from the Earth (which it does).
Well, both the ESA and NASA have space based interfermotery telescope designs to tease out these signals, out to 40 ly or so.. And that's with near term technology.
If you built a large enough telescopic array (which is beyond our technology), you could even see surface features of extra-solar planets. Determining the atmospheric composition is the low end of what is possible. If we ever do leave the solar system, we're already going to know what the other planet's atmosphere contains. So the context of my comment was that we've been broadcasting a strong signal (powered by the sun) for a very long time. Any life like ours that is looking has had more to go on than radio waves. And for far longer.
['m not making any assumptions about reality, just entertaining possibilities. I personally don't put the prospect of alien life in the same box with elves and dragons. Your mileage may vary.
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Lack of knowledge is no reason to assume an abundance of extra-terrestrial life; or that if such life exists, that they are capable of finding out what lies beyond their sky. To do otherwise is to commit the fallacy of "Argument from Ignorance", or "Appeal to Ignorance" - where "ignorance" stands for "lack of evidence to the contrary". Such a fallacy is used to assert that a proposition is true because it has not yet been proven false. Appeals to ignorance are sometimes used to shift the burden of proof away from the person making an invalid claim, or they may be used as a rationalization by a person who realizes that he has no reason for holding the belief that he does.
Good think I didnt assume that. Don't hurt yourself trying to read.