system
Given the following post, respond as if you're commenting on Reddit:
human
SETI: If a copy of our planet were 100 light years away, what are the chances we would have detected it?

Imagine a mirror copy of our planet was orbiting a Sol-like star some 100 light years away. Could current astronomers detect our own planet? What would we be able to deduce about our own planet from that range? Could we detect the actual planet using techniques being used to find exoplanets right now? What electromagnetic emissions would we see? What does SETI think about it?

So that we can ignore the timey-wimey relativistic components of this question, let's also say that this mirror-copy is 100 years further along into its timeline. That is to say, if we look through our telescopes at our theoretical mirror-planet, it appears to us to be modern-day, completely current Earth. 

Extra Credit: What's the maximum distance from which we would be able to detect life on our planet?
gpt
> Could current astronomers detect our own planet?

Almost certainly not. Same answer if it's 10 light years away.

We could only detect the planet itself with current technology if its orbit happened to be oriented just right so that it transits its sun as seen from Earth, and we happened to be watching that star with a telescope like Kepler.

It might also be possible to detect their existence if the inhabitants of that planet directed a powerful radio signal at us using their equivalent of Arecibo, and we happened to be listening in that direction with a similarly huge radio telescope at the moment it arrived. 

But the chances of either of those things happening by accident are pretty close to zero.

> Extra Credit: What's the maximum distance from which we would be able to detect life on our planet?

With current technology, much less than a light year. We'd have to be within the same solar system.