[thedoc (OP)]

Andy Masters  asked the Naked Scientists:
   Hi Chris,

I'm no scientist, but still I'm fascinated by space and physics. One of my interests is exoplanets and the search for extraterrestrial life; i have a couple of ideas i've never heard mentioned you could give me some thoughts on if you have the time.

As an exoplanet passes between us and its parent star a small drop in the light from the star is observed from what i can gather. Being as the side of the planet we could see would be the night time side could we ever observe an intelligent life forms street lights as we can from satellite images of the Earth at night or possibly an increase in the wavelength of the sodium light signature assuming they use the same lights as us?

Thanks for your help and all your excellent bbc podcasts!!!

Andy Masters

What do you think?

[imatfaal]

It's nice idea - but I think with the distances involved we are still having to use incredibly subtle measurements of the distant stars light, let alone the light of the orbiting planet. 

Regarding actually observing the night side - Angular resolution is fixed by formula relating the apparent angle of the viewed object (basically size of object divided by distance away at that scale), the wavelength of the light, and the diameter of the light-gathering optics.  Very roughly - to discern planet sized stuff at 20 light years (I used this figure as a recent "goldilocks zone" planet was at 20 lyr) in yellow light of 580nm, you would need a light gathering optic of 20 km wide! 

On the increase of the sodium signature light - my gut instinct would be that the difference would be so intangible as to make it impossible to search for - but then the precision of the measurements used in planet searching is so amazing that I would never rule anything out.  I was looking for figures of how much light the earth gives off (which I think would be so small as  to be negligible compared to the sun) and I found this  - http://en.wikipedia.org/wiki/Planetshine#Search_for_terrestrial_planets

That is reflected light from the host star - but what is reflected can be indicative of habitable (for us) conditions