Ok, all you astronomy-types:

In theory,

What would you see of an Earth-like extra-solar system planet if the people of Earth decided to get together and build the biggest, most effective space (or earthbound) telescope - cost be darned (or maybe costs limited to the ten year budget of NASA to make this scenario seem a little bit possible.) (And assuming the glare of its sun was not an issue.)

Would you see the photons emanating from the light of an ET city at night? The blue of the oceans?

Cheers,
RBG

And now for something completely different and for your entertaiment, I offer the Arbeegee Happy Face Quartet. Take it away guys...

:-({|= =D> \/


(Hey, the whistle guy won't work for me.)

For compatibility with life, look for water and a surface temperature that allows liquid water. Spectroscopy can detect the water, I think, and perhaps some sort of infrared imaging could give you the surface temp.

For life (as we know it), we'd look for free oxygen in the atmosphere. That's pretty strong evidence that some process is producing copious quantities of oxygen, since it's very reactive and quickly combines with carbon or hydrogen or anything else it comes across. Living organisms are the only way we know to pump out that amount of good old O. Here again, spectroscopy would be our best bet.

Now, as for intelligent life similar to us... well, maybe the best evidence is that they're not coming here! Seriously, though, that would be tough, unless they're broadcasting radio waves loud enough for us to pick up.

Methane is another good indicator.

Carl Sagan and some other astronomers used the Galileo probe to see what could be detected if it were aimed at Earth. It detected oxygen, methane, significant surface color variations, significant absorption of red light (indication of chlorophyll), and radio transmissions, all signs of life. Probably all but the last could be picked up with spectroscopy done on a planet around another star.

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