Best wayto image rocky planets is in the mid-IR where they are brighter and their parent star is less bright.
Mid-IR detectors weren't very good 30years ago when todays large telescopes were planed and the mid IR telescopes that have been built were based around poor detectors from 30years earlier and so are small.
Of course what you claim a telescope is for and what is the fashionable topic once it's running can be different. That's the nice thing about telescopes compared to say an LHC - decades after they open they are finding new unusual things with new instruments.
Wow. How many photons that bounce of the planet, travel a few lightyears and get caught in this 40m mirror are we talking about here? 1 million per night? Are we close to the limits of detection or what else is possible?
A million photons would be overkill for a solid detection. A standard CCD saturates at about 65,000 counts (or about that many photons, assuming the efficiency is high). A source that is detected well enough to do interesting measurements usually has a ~1000 - 20,000 photons per pixel. Spread out over ~15 pixels, a good detection could require as few as ~15,000 photons.
In this particular application it would be more difficult since you not only have to detect the source on its own, you have to block out the glare from the parent star. Usually the main problem in this sort of work is making sure that the source you are detecting is actually a real thing and is not just an artifact from the way you block out the light from the parent star.
The proposed adaptive optics imager for ELT has a (1 sigma/1hour) limit in the visible of a 32mag object - with better than HST resolution.
A 0mag star gives you about 1000photons/s/cm^2/Angstrom so the 40m telescope in visible light would capture about 2E13 photons/s for the brightest star in the sky.
But 32mag is 2.51^32 = 6E12 times fainter so only a few 1000 photons in an hour long exposure.
I wish the Terrestrial Planet Mapper and Terrestrial Planet Imager had been funded. It was proposing to fly an armada of nulling interferometers in space using optical interferometry capable of imaging planets around other stars at 25x25 pixel resolution.
This is unprecedented, right? I have been wondering for a while if such a thing is or will be possible. I assumed the distances were simply too great.
I see now that it is possible to directly image gas giants, but not to my knowledge small rocky planets.
http://en.wikipedia.org/wiki/Direct_imaging#Direct_imaging