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Ask around and find out what gases, vapors, and/or fumes, in the atmosphere are the result of advanced technology. Then use a spectroscopy to look for those compounds in the atmospheres of earth like exo-planets. Who is doing this?

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    $\begingroup$ Given good enough technology, why is there reason to believe there will be any gaseous emissions? $\endgroup$ – Andrew Thompson Dec 18 '16 at 1:57
  • $\begingroup$ First do such gases exist in our atmosphere, as a result of technology? $\endgroup$ – Rich Lawrence Dec 18 '16 at 2:19
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    $\begingroup$ Carbon dioxide (from combustion of fossil fuels - also could come from completely natural sources) chlorofluorocarbons (CFCs refrigerant, propellant - don't know if there are natural sources) .. $\endgroup$ – Andrew Thompson Dec 18 '16 at 2:29
  • $\begingroup$ I was thinking CFC's as wel but what other? Keep open could be certain metal fume or evaporated solvent vapors. They should have a low probability of occurring naturally. $\endgroup$ – Rich Lawrence Dec 18 '16 at 2:53
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    $\begingroup$ "Who is doing this?" Given 1) There are other processes in alien atmospheres we do not entirely (know about or) understand. 2) We don't really know what gases other 'technology' is likely to emit 3) Other technologies might not emit any gases. 4) We're really not sure what gases to look for, even for detecting life on other planets. - I suspect the answer is 'no one'. $\endgroup$ – Andrew Thompson Dec 18 '16 at 7:02
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The best method of analyzing exoplanet atmospheres is called Transit Absorption Spectroscopy. First, we analyze the spectrum of an exoplanet's star. Then, we wait for the planet to pass in front of the star. When it does so, some of the star's light is absorbed by the planet's atmosphere, changing its spectrum. Analyzing this change will tell us what chemicals are present in the planet's atmosphere.

In order to detect life, there are several compounds which researchers look for. One of these is Oxygen. Oxygen is a highly reactive element, and will disappear from a planet's atmosphere very rapidly if it is not being produced anywhere. Life uses oxygen because it is so reactive, so if oxygen is present in a planet's atmosphere, it is a strong indicator of the presence of life.

In order to look for intelligent, technologically advanced life, researchers search for compounds which are not present in nature. One of the most notable compounds is the family of chlorofluorocarbons (CFC's). CFC's have no known natural source, and have a highly noticeable infrared absorption line.

Most of the absorption lines produced by these gases are in the infrared. This means that you need a special infrared telescope to observe them. Unfortunately, since our atmosphere is opaque at some of the frequencies required to find CFC's, you need to use space-based telescopes. There are hopes that the James Webb Space Telescope and the Transit Exoplanet Survey Satellite will be able to do this work when they launch. As this paper shows, the JWST would have to focus on a planet for ~5 hours to determine the atmosphere's composition.

This work can be done from the ground, but it is more difficult and has more uncertainty.

More sources:

http://www.astrobio.net/news-exclusive/unintelligent-life-cfcs/

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  • $\begingroup$ The above journal article (astro bio news) answered my questions exactly without pause or obfuscation. Thank you Phiteros. $\endgroup$ – Rich Lawrence Dec 23 '16 at 13:30

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