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There are several ways we are coming closer to answering the question "is there life elsewhere in the universe?". One is by first understanding very well the origin life on our own planet. Another is by trying to look for radio signals from extraterrestrial civilizations. Another is to try to search for habitable exoplanets, and eventually examine the habitable ones in detail via direct imaging. Another is too look for evidence of life or life that existed in the past right in our own solar system (for exmaple on Mars, Titan, Europa, etc).

1) Have I missed any other astronomy-related approaches to answering this question?

2) Short of radio contact from an ET civilization, or a visit by aliens, or the discovery of life/past-life in our solar system, what would give conclusive evidence of alien life in the galaxy?

For example:

In 20 or so years when we develop the technology to directly image exoplanets, we will be able to determine their chemical composition and characterize their structure. If astronmers who study exoplanets found what they are looking for -evidence of oxygen and other biomarker gases - this might be highly suggestive of life, but not conclusive evidence. What kinds of observations of an exoplanets could enable us to say with ceartainty "there is life on this planet"? Short of telescopic observations of truly unbelievable detail, I cannot think of a way that we can look at a planet and say "yes there is life on it, or no there is not" other than perhaps if seeing lights on the dark side of the planet.

Edit: Actually, I forgot the obvious scenario where there are trees/grass on the exoplanet. But I don't think the plant life is required to be a different color from the rest of the terrain, and a planet with aliens does not have to have plant life. So we might not be able to detect life visually.

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    $\begingroup$ Altough I am not a fan of ET discussions, I still can not see why downvote the question: it is well posed and legitimate. To the OP: imaging of extrasolar planets (not exoplanets) it is still MUCH far way, if ever possible. $\endgroup$ – Py-ser Feb 11 '15 at 9:33
  • $\begingroup$ It seems some telescopes will be able to image extrasolar planets in the near future (20s), but what I meant in the last comment, is the ability to image small structures (trees, plants, buildings), like the ones asked in the OP question. $\endgroup$ – Py-ser Feb 11 '15 at 9:59
  • $\begingroup$ @Py-ser: As far as I know, "extrasolar planets" and "exoplanets" are synonymous (and a quick Google search for "exoplanet" tends to confirm that). $\endgroup$ – Keith Thompson Feb 12 '15 at 2:36
  • $\begingroup$ @KeithThompson, Wikipedia also confirms that, but I would not trust it. Extrasolar planets are planets outside the solar system, exoplanet are exotic planets, i.e., planets that do not follow our models, like supergiants as close to the star as Mercury to the Sun. Rarely in astronomy two different words have the same meaning, but I am not an expert of the field (planets). $\endgroup$ – Py-ser Feb 12 '15 at 10:00
  • $\begingroup$ @Py-ser: I've never seen the word "exoplanets" used to refer to "exotic planets". As far as I know, exotic planets are just called "exotic planets". There's an "exoplanet" tag for this site; its description says "Planets that lie outside our solar system." The "exo-" prefix is quite common, and the word "exoplanet" has a well established definition. $\endgroup$ – Keith Thompson Feb 12 '15 at 15:41
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Lots of frontier science lectures to watch on nearby topics here: https://astrobiology.nasa.gov/seminars/

Your very question seems to be the hottest topic in astronomy today.

My impression is that if oxygen gas and methane were on the same exoplanet, then it certainly must be living biological processing creating that there. Nothing else could explain the coexistence of the two. And even if one of those two gasses could occur abiologically, and a range of exoplanets with one of them would be found, statistically one could deduce a frequency of planets which must have life, because the abiotic processes could only explain a certain maximum part of them.

It is a pitty that most exoplanets are in size between Venus and Neptune, which are at the same time the least explored planets in our Solar system. When a sub-Neptune or a super-Venus has its atmosphere examined for the first time, we won't have as good references as we could've had.

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  • $\begingroup$ Hot topic perhaps, but certainly one of the least scientificly conducted ones. $\endgroup$ – AtmosphericPrisonEscape Feb 11 '15 at 17:20
  • $\begingroup$ +1 It is a subject where astronomers can help exo-biologists (no sillier than many other university departments) but it would be nice if the time and resources were spent on the current problems in astronomy. $\endgroup$ – C. Towne Springer Feb 12 '15 at 6:09
  • $\begingroup$ I don't understand. Isn't searching for life an important scientific pursuit and basically the reason for all the nasa missions to mars? $\endgroup$ – Joshua Benabou Feb 12 '15 at 20:43
  • $\begingroup$ @JoshuaBenabou Of course it is! It is what motivates most young astronomers today. It is the base of the design of the big spaceflight and telescope missions. It is what made the president about 20 years ago hold a press conference about what might've been evidence of ancient life on Mars, caught in a meteorite. Some who research extragalactic plasma might not care, but the search for extraterrestrial life is de facto THE mission of astronomy in our day. Foolish or not, that's how it is. 50 years ago a test pilot on the Moon was cool. Today no one cares. Just find life! $\endgroup$ – LocalFluff Feb 12 '15 at 21:33
  • $\begingroup$ Right, so what in the world is C. Towne Springer saying? How is exo-biology a silly university department in the same way that ethics or philosophy is a silly department? $\endgroup$ – Joshua Benabou Feb 12 '15 at 21:36
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I don't see why this question would be down voted. I think that the most we could determine is that the conditions for life are present on a Planet. The only way to know for certain would be to actually go there . However we could perhaps detect certain gases that could only be emitted from living organisms and this would be be some proof.

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