From how far away can it be deduced that there is sentient life on earth?

The Aargh are a humanoid alien race a couple solar systems away. Their home planet is quite earth-like (oxygen atmosphere, a lot of water, traditional organic chemistry even if a few proteins are different to earth), and in the habitable zone of their solar system. They have colonized several planets and moons in their own system, and have started to branch out to uninhabited solar systems.

They haven't found another sentient species yet, however, nor discovered traces of one. This, they want to change. They deliberately send out scientific discovery missions to solar systems with at least one planet in the habitable zone. Since they are very careful and do not want to provoke a hostile first contact (especially when they don't know the technological level of their potential adversary) they stop several times along the way to check if they can't find a hint of sentient life on the other planet.

One of those planets is earth, in the year of 2020 (with current technology levels).

Question: How far away do the Aargh have to be to get unmistakable proof of sentient / intelligent life on earth?

Can they distinguish that from their home planet some 10 lightyears away by running a spectral analysis on our atmosphere and discover that there are X gases that should not be that prevalent in a non-technological but habitable planet? If so - which gases or changes in gas concentration?

Or do they need to be close enough to receive the radio / electromagnetic emissions that our technology radiates into space? If so, how far away is that if they want to make sure that it is deliberate and not some sort of strange natural radiation?

Or can they detect the light from earth's night side from further away?

posted over 4 years ago

subrunner‭

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