Originally Posted by snukr:
“How will they be able to see planetary atmospheres and know what they consist of, and why do they think they will be made up from gases which exist on earth? Do they not consider the possibility, indeed likelihood there are other gases in the Universe which don't exist on Earth? Do they seriously believe that there are no other minerals, metals or gases outside of those which exist in Earth's periodic table?”
There's a whole lot of questions in that very sensible and inquiring post of yours so here goes.
The technique of astronomical spectroscopy [see
http://everything.explained.today/As..._spectroscopy/] allows us to examine the composition of stars, planetary atmospheres and galaxies without the requirement to travel to these places. That is how part of the atmospheric compositions of Venus, Mars and Titan, etc. were determined before any close up space probes ever visited those places.
Elements like hydrogen and helium were formed in the big bang and all the rest were formed in the cores or the end of life explosions of stars so the elements that we know about are pretty much the same as occur elsewhere in the universe because they were generated by the same means. It is possible by laboratory experiments to create additional new heavy elements but they tend to be unstable and short lived and don't occur in nature [see
https://en.wikipedia.org/wiki/Transuranium_element].
Since the building block elements are the same wherever you go in the universe, the compounds that result from the joining up of those elements will also be pretty much the same as the compounds that we know about or could predict. For example, the volcanic mineral olivine [see
https://en.wikipedia.org/wiki/Olivine] has been found on Earth where it's pretty widespread and it's now been found too on the Moon and Mars and the obvious extrapolation is that it will be found on worlds in other solar systems where there has also been volcanic activity.
Regarding the detection of life on other worlds, the evolution and spread of microorganisms changed the composition of Earth's atmosphere [see
https://en.wikipedia.org/wiki/Stromatolite] and the presence of life can be inferred if another world has a distinct and varied atmospheric composition similar to that of Earth and we can (or will be able to) determine that by spectroscopy which brings us back to the very first point in this thread. I hope that all helps.