(October 4, 2010 at 12:57 pm)downbeatplumb Wrote: . 20 light years showed be no problem for a next gen space based telescope.
BTW, I am not sure what is meant by "no problem"? Let's say the planet is 15,000 Kilometers in diameter and the largest features on it, say a giant impact crater, an ocean or a continent, is 7,000 Kilometers. Using small angle approximation, from 20 light years a theoretically perfect optical telescope will need an aperture of about 800 Kilometers to resolve it.
Hubble's aperture is about 5 meters.
Failing to resolve surface details, or having any intelligent alliens the most we can do it to try to detect through spectral analysis whether the atmosphere contains any gases which should not remain in the atmosphere for long without being continuously resupplied by biological or technological activity. The chance of finding a smoking gun with this technique that will definitely put aside any doubt seems small. Even having set foot on Mars and surveyed its atmopshere with multiple low orbit satellites we can't be sure if what is Martian atmosphere is indicative on ongoing biological processes or not.
