THE construction of several large ground-based telescopes1,2 and the anticipated launches of new space-based ones3-5, have prompted renewed interest in the means by which extra-solar planets might be discovered1,6,7-11. The direct detection of light from such a planet would be the most compelling means of discovery, and it may soon be technically feasible1,6. Jupiter has traditionally been used as a benchmark for observability, but extra-solar giant planets could have a wide range of masses and ages12, and could be significantly brighter than Jupiter. Here we present calculations estimating the optical and infrared fluxes of extra-solar giant planets with a range of ages, and demonstrate the conditions under which they will be observable with several new telescopes. Giant planets with masses greater than that of Jupiter, and younger than about 1 billion years, are the best targets, and they should be visible using the generation of telescopes now under construction.
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