"We have Life Signs, Jim."

Ever wondered how the Enterprise detects life signs?

Or, for that matter, what "life signs" are? The meta explanation is, of course, that it's convenient to the plot.

Except...

...we can now detect "life signs." Specifically, we now have instruments that can detect specific light refraction created by living things. It works by leveraging the fact that life is homochiral. We're not sure why it is (or whether it's true of all life), but living molecules all spiral to the right, whilst inorganic molecules can go either way.

And we can detect it from a helicopter.

It's not hard at all to go from that to detecting it from orbit. Until, of course, we find heterochiral life and get a nasty/pleasant (depending) surprise.

So "life signs" really do exist and can be remotely detected. Despite everything, we do live in an age of wonders.