Q: What kind of telescope would be needed to see a person on a planet in a different solar system?
Physicist: When talking about telescopes there are two quantities to take into account; the “light gathering power” and the “resolving power” of the telescope. Coincidentally, something that’s a large distance L away, and that’s a size S across, takes up an angle of approximately . So, using ground based telescopes we can’t come even remotely close to seeing a person standing around on a planet in another solar system. Spotting dudes and ladies on one of these worlds requires, at minimum, a telescope array that’s at least 100 million km across. Of course, if you wanted to see farther, you’d need a much larger telescope array.