I hope I didn't miss someone answering something similar. I was doing a small thought experiment. And I was wondering whether you kind people would check up on my math.
Say we want to build a telescope to see the surface of the extrasolar planet Proxima Centauri B. Well, it is $4 \times 10^{13}\ \mathrm{km}$ away and, if we assume it as big as Earth, approximately 12 742 km large in diameter. (*)
Let us not be greedy and just say we want from our telescope a 10 x 10 resolution, i.e. we need to be able to resolve 1274 km "points" on the planet's surface. The angular size of such a point is, therefore, $3.14 \times 10^{-12}$ radians.
Using $D = \frac{\lambda}{R}$ for the telescope's needed size, we get to rather comfortable 185 km, which is a lot for practical application, but theoretically reasonable.
Maybe we can see it in classical terms, but real light is quantum. Will we see it photon wise? That is, will enough photons get into our telescope to make a useful image in a time-frame we can afford to wait on? Let us forego reality for a second and say that PC b reflects as many photons per second per square meter as Earth or $\approx 1.26 \times 10^{21}$, considering our rather large "pixel" we should get somewhere around $1.23 \times 10^{29}$ photons per 1274 $\mathrm{km^2}$ area. Using area for a half-sphere at the distance of $4 \times 10^{13}\ \mathrm{km}$ we should still get about 12 photons per km^2/s.
Which again seem fairly reasonable.
Huh, I didn't expect it to be within the realm of realistic possibility. Is my conclusion correct, that we could still collect enough photons to get an image?
(*) My (The_Sympathizer) note: Proxima b is actually somewhat larger.