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With our current technology, or technology available in the near future (up to 2025), how would we detect a planet exactly like our own, and how close would it have to be to be detectable?

Which methods thus far have proven the most effective for detecting Earth-like planets and what would they be able to reveal regarding our planet's atmosphere, orbital properties, and our species?


This question is interesting food for thought on both how easily an extraterrestrial civilization with comparable technology might find us, and for how easily we might find them.

For the purpose of the question we will assume that there is a planet with a civilisation with equivalent technological advancements that could exist anywhere.

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What I'm trying to get at is that if there was a planet/civilisation exactly like us, would we be able to detect that. –  Jordan Brown Nov 30 '13 at 6:30
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3 Answers

"probably not". Unless it were so close that television or radio signals like we've been inadvertently sending out can be detected above background radiation levels we can't know about the civilisation.
The signals are extremely weak, and not on any band scientifically interesting (deliberately of course, because we don't want our televisions to pick up natural signals that would interfere with our artificial signals) unless your scientific interest is specifically to snoop on communications sent out by potential aliens (SETI comes to mind, a nice hobby and something to do with radio telescopes for the times there's nothing more interesting to listen to).

As to planets the size of earth, we've never yet been able to find one, let alone a rocky one in the habitable zone of a star. The mechanisms used to detect exoplanets simply aren't sensitive enough.

Leaves one option: their equivalent of Voyager 1 enters our solar system and we actually spot and recognise it for what it is, and find a way to recover it. But If they're at our level of technology it will be thousands of years yet before that happens.

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The universe is pretty old, they might have sent it a long time a go in a galaxy far away... –  Jordan Brown Dec 6 '13 at 9:13
    
you want to detect something that's identical to us, assumption is they're identical to us right now... –  jwenting Dec 6 '13 at 9:35
    
That's not the point of the question, assuming there is a planet identical to us, would we be able to detect it and realise it has intelligent life. –  Jordan Brown Dec 7 '13 at 6:09
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@JordanBrown detect it has intelligent life NOW? No. Speed of light makes that impossible. Detect it might at some point have had intelligent life? Only if they sent out incredibly strong signals (way beyond what we're doing, and as our technology improves what we send out gets weaker as we're moving away from omnidirectional EM transmissions) and/or is very, very close. –  jwenting Dec 9 '13 at 6:03
    
-1 Radio signals are not the only way to detect a civilization. –  called2voyage Dec 9 '13 at 15:33
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Avi Loeb and Edwin Turner have written a paper about the possibility of detecting a civilization similar to our own on another world by looking for city lights.

Their proposed method suggests observing the dark sides of planets when they transit in front of their parent star. Though they said that this method will require future generations of telescopes.

Source:

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If there were a planet with same civilazion "exactly like us", probability that it were near enough to be detectable are extremely remote. Not zero, but as low as those of trespassing a wall by means of quantum mechanics.

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In recent years we have been discovering hundreds of exoplanets and your saying that if one of those was as advanced as us, we would have no way of telling? –  Jordan Brown Dec 6 '13 at 3:32
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@JordanBrown any planet the size of earth would be extremely difficult to detect, yes. Any civilisation on the level of our own on such a planet would be impossible to detect with any sensors we possess. –  jwenting Dec 6 '13 at 8:25
    
-1 Does not answer the question. Even if the probability were remote, there are definite techniques that could be used to detect such a planet should it happen to be close enough to detect. The question asks "how would we" not "could we" so this does not answer the question. –  called2voyage Dec 9 '13 at 15:35
    
@called2voyage IF there were such a planet close enough to detect, it would be one of our Solar System, so it would have been detected long ago. We can not detect them nowadays (nor in any near future), as explained by jwenting –  Envite Dec 9 '13 at 23:34
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