Would the next generation telescope "Owl" be able to detect clearly a like-Earth planet? What kind of details would it be able to reveal?
OWL (the Overwhelmingly Large Telescope) is supposed to be a huge telescope with a diameter of about 100 m, and is supposed to operate in the visible and infrared. Whether it will be able to detect an Earth-like planet depends on the angular resolution it actually achieves.
Most of the extrasolar planets detected so far have been by detecting the slight wobble of the parent star as a result of the planet going around the star. The problem is that this wobble is too small to be detected in the case of Earth like planets.
The other side of the coin is this: to detect minute wobbles, you need a telescope that can achieve the necessary angular resolution. However, the Earth's atmosphere sets a limit on the angular resolution achievable by telescopes. Due to the turbulence of the Earth's atmosphere, telescopes cannot achieve a resolution better than about 0.5 arcseconds (and that is in the best observing sites in the world like in Mauna Kea). So, a bigger telescope will not help in improving the angular resolution even though it will enable one to see much fainter objects.
Recently, adaptive optics have been used to get around the problem a little bit and some infrared telescopes now give angular resolutions close to their theoretical limit. However, adaptive optics is extremely difficult to implement in the optical part of the spectrum. On the other hand, stars are much brighter in the optical than the infrared. So you can see that there is a whole rigmarole of issues here associated with the detection of Earth like extrasolar planets.
The best option to be able to detect Earth like planets is for going to space telescopes like Hubble (Hubble is too small to detect Earth like planets). However, there are hopes with a telescope as big as 100 m (which is what is envisioned in OWL). A telescope this big will be able to do studies of main sequence stars in the infrared (in spite of them being much fainter than in optical) due to its light collecting power. Then, one might use the adaptive optics and try to get the necessary resolution. So, it is possible that OWL will find out Earth like extrasolar planets. However, a space mission telescope designed specifically to determine extrasolar planets may settle the issue for once and for all.
What science is envisioned by OWL? According to the ESO website, OWL will be able to image extrasolar planets and determine the composition of their atmospheres and thereby reveal the possibility of biospheres. The website also claims that OWL will be able to achieve milli arcsecond resolution. If this is true (I am a bit skeptical), then it will be able to detect Earth like extrasolar planets. The other scientic projects OWL is supposed to do is to witness the birth of the first stars and galaxies, in addition to unveiling processes in star formation (and planet formation).