Radio-telescopes (e.g. the Very Large Array (VLA)) can simulate one gigantic dish by using separate smaller dishes.
Q: Could such an array of optical telescopes potentially see an exoplanet at say 20 LY away?
Assumptions:
- The array is space-based, at an ideal location such as the Earth-Sun L2 Lagrangian Point
- Assume the exoplanet of interest is defined as a rockey planet up to 5 times the diameter of earth
Answer
Sure...why not? The answer is really in the diffraction limit.
Two NASA missions planned to address this based upon sparse apertures: SIM and TPF. SIM (http://en.wikipedia.org/wiki/Space_Interferometry_Mission) was a variable distance pair of telescopes. TPF (http://exep.jpl.nasa.gov/TPF-I/tpf-I_index.cfm) was a synchronized flying mission of several telescopes phased together.
Concepts today are based upon Fourier Telescopy (several apertures phased together sort of like an interferometer or a star nulling telescope). The Navy does something similar on the ground (see: NPOI http://www.lowell.edu/research_telescopes_npoi.php). JPL / Caltech have done something similar on the ground at Palomar Observatory (http://www.astro.caltech.edu/palomar/pti.html)
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