Which telescope to see exoplanets?

[jumpjack]

Considering that this is the better image Hubble can offer for a "planet" just 1/16 lightyear apart (our Pluto),

how big should a telescope mirror be to see something on Gliese 581d exoplanet, around 20 lightyears apart???

[Kullat Nunu]

Pluto isn't 1/16 light year apart. Near the perihelion it is about 4 light hours distant!

The problem detecting Gliese 581 d is not that it is dim, but it is very close to its star which needs extraordinary resolution and therefore a huge mirror. However the light probably would swamp the planet anyway so a device called nulling interferometer (a group of telescopes which are placed so that the light from the star is canceled out) is the simplest possibility. In order to see any details you would need a mirror perhaps hundreds of kilometers wide. The only remotely practical way to achieve that is to place an interferometer consisting of several distant telescopes in space.

[yedprior]

This is the answer from NASA to your question:

http://origins.jpl.nasa.gov/library/...enhouse30.html