The detection of reflected light from a planet can allow for the study of the planet's atmosphere. However, the challenge is that the planet-to-star flux ratio is very small. Even for giant planets in close-in orbits, the flux ratio is still below ~1/10,000. This ratio decreases as the planet's orbital distance increases. Nevertheless, the reflected light from giant planets in the habitable zone of their host stars may be detectable with next generation telescopes such as ESO’s European Extremely Large Telescope (E-ELT). Even so, the planet-to-star flux ratio for giant planets in the habitable zone is less than ~1/10,000,000. The E-ELT is predicted to be able to detect the reflected light from several known giant planets in the habitable zone with less than 100 hours of observations for each planet.
Martins et al. (2016), "Reflected light from giant planets in habitable zones: Tapping into the power of the Cross-Correlation Function", arXiv:1604.01086 [astro-ph.EP]