[acecharly (OP)]

Hi, does anybody know if we could ever detect the street lights on an exo-planet, as when it crosses between its parent star and us we see its night time side. I imagine the stars glare may be a problem. Also could it be possible to detect a rise in the light signature of sodium coming from the exo-planet assuming that E.T. were to use sodium street lights?

[greeniemax]

Problem 1: We can't even see the planet let alone the street lights.
Problem 2: We have detected 4-5 planets out of 795 that are close to Goldilocks zone.
Problem 3: We have no idea if E.T. sees light, maybe they use X-Rays or Radio Waves to see things, maybe they use something that we haven't got a clue of.

So overall its really a matter of chance and technology, but as of right now the answer is no we can't see anything we assume the planet is there because either the star dims after a certain time (the amount of dimming, size of the start gives us the idea how large the planet might be) (Kepler Mission) and the star wobbles (intensity of wobbling gives us the idea again of the size of the planet and time difference gives us the idea how far it could be from the star).

But we haven't seen any planet directly, right now its impossible.

[syhprum]

We would not be able to see street lights on the Moon with the best of telescopes availabe and that is only 2.5 light seconds away, the exoplanets at present known are 50 to 500 light years away so the possibility of seeing street lights is rather remote.

[wolfekeeper]

Oh yeah, if there were street lights on the moon, we could detect them, no problem. You can see the streetlights of Earth from orbit, and I'm pretty sure you could see them from the moon as well with a telescope.

You can't resolve individual streetlights of course, but the overall light output of a major city is no problem. And certain lines aren't likely to be naturally there, like sodium lines, they're much hotter than the surface of the moon, so you can easily spot them with a spectrogram.

I don't see any theoretical reason you couldn't do that on an exo-planet, you'd have to mask out the light of the star though.

[syhprum]

Several missions to the Moon have placed highly efficient reflectors there for the purpose of ranging, even when these are zapped with with high power lasers it is a challenge to pick out any reflections from them.

[wolfekeeper]

Yes, but they're very small, and cities are very big.

It's not a problem to detect human civilisation from space if you look at the night side.

I mean, sure a single street light, that would be very difficult indeed.

[greeniemax]

How are you going to do it, if you can't even look at the exo-planet itself?

The star that planet goes around is very bright and looking at it while its in light of the star is almost impossible and worse if you want to look at the light hiding the star light behind its impossible^2.

[wolfekeeper]

Well, direct imaging of planets has already been achieved, it is easiest if the planet is big, and far from the star.

http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets#Direct_imaging

[greeniemax]

Nice, learned something new.

But still if you want to see the dark side with lights at night the planet has to be in front of its star and that means you can't see the planet.

[wolfekeeper]

Well, if the planet is at maximum apparent distance from the star, then half the visible disk of the planet is going to be in darkness, so I don't see there's necessarily any particular problem.

[greeniemax]

Problem 2 & 3 still stand.

Problem 2: We have detected 4-5 planets out of 795 that are close to Goldilocks zone.
Problem 3: We have no idea if E.T. sees light, maybe they use X-Rays or Radio Waves to see things, maybe they use something that we haven't got a clue of.

But in case they have developed lights (which is a recent invention), we have also developed TV, Radio, Radar and many other types of radiations, so SETI has a better chance of finding out such a planet rather than looking at lights and they haven't found anything yet.