Detection of visible light from the darkest world

We present the detection of visible light from the planet TrES-2b, the darkest exoplanet currently known. By analysis of the orbital photometry from publicly available Kepler data (0.4-0.9 mu m), we determine a day-night contrast amplitude of 6.5 +/- 1.9 ppm (parts per million), constituting the lowest amplitude orbital phase variation discovered. The signal is detected to 3.7 sigma confidence and persists in six different methods of modelling the data and thus appears robust. In contrast, we are unable to detect ellipsoidal variations or beaming effects, but we do provide confidence intervals for these terms. If the day-night contrast is interpreted as being due to scattering, it corresponds to a geometric albedo of A(g) = 0.0253 +/- 0.0072. However, our models indicate that there is a significant emission component to dayside brightness, and the true albedo is even lower (<1 per cent). By combining our measurement with Spitzer and ground-based data, we show that a model with moderate redistribution (P-n similar or equal to 0.3) and moderate extra optical opacity (kappa' similar or equal to 0.3-0.4) provide a compatible explanation to the data.