hiddenvariabl said:
I keep seeing reports about how all the planets similar to Earth in other nearby planetary systems are now being discovered. Since we won't have the technology to send probes or go there ourselves for some time, all we can do is look at them. My question is: Could we ever get better resolution images of those planets with a telescope located in our solar system? In other words can we ever make telescopes that can zoom in on the surface of another planet in another planetary system or is it just physically impossible for us to collect that much light from something so far away? Wouldn't we need a telescope with a really big collection area to do so?
Having a big collection area in order to be able to see fainter and fainter objects is always helpful, but it's not the primary reason why you'd need a REALLY big telescope. Due to diffraction, the fundamental physical limit on your angular resolution (the minimum angular separation of two points that you can resolve as being distinct from each other) is determined by your telescope aperture size. In fact, your diffraction limit scales inversely with telescope diameter (and it scales linearly with wavelength). At a wavelength of 550 nm, in order to have an angular resolution corresponding to a physical object size of 1 Earth radius at the distance to Alpha Centauri (the closest star at ~4.3 ly), you'd need a telescope aperture of >4200 m in diameter. That's more than 100 times larger in diameter than the largest optical telescope that's currently even being
proposed to be built (I believe that TMT is supposed to be 30 m and E-ELT is supposed 40 m). And that's just to get an angular resolution such that the smallest feature you can see in your image is about 6400 km at the distance of the
closest star (which is an angular resolution of about 1.5x10
-10 radians or 32 microarcseconds. To see features of 10 km in size at that (unrealistically close) distance you'd need a telescope aperture of almost 3 million metres (3000 km or almost half an Earth radius). Since the diffraction limit size scales linearly with wavelength, the situation would be even worse in the infrared (where the contrast ratio of the planet's emission to the emission from its parent star is much more favourable).
Contrast ratios are another important point. A good analogy is that directly (visually) detecting a planet is like "looking for a firefly next to a searchlight." Its tiny emission is going to get drowned out in the much much larger emission from its parent star, which is at least 10,000 times brighter, but probably a few factors of 10 more. On those few occasions in which direct imaging of planets was achieved, the light from the parent star was blocked out using an occultation disk. But even then, there was all kinds of messy spillover light that (again due to diffraction and imperfections in the optics) looked like speckles. Guess what? Any planets that might have been there would have looked very much like these speckles. Very fancy image processing techniques had to be employed in order to even see which features were real objects and which were due to optical artefacts in the image. It was also necessary to observe the same system on multiple occasions that were widely separated in time (in order to try and discern the orbital motion of the planets). So, astronomers have enough trouble as it is i
dentifying whether or not a planet is present in an image let alone resolving it into a disc, let alone resolving surface features on that disc.
