Kepler's dark worlds: a low albedo for an ensemble of Neptunian and Terran exoplanets

Photometric phase curves provide an important window on to exoplanetary atmospheres and potentially even their surfaces. With similar amplitudes to occultations but far longer baselines, they have a higher sensitivity to planetary photons at the expense of a more challenging data reduction in terms of long-term stability. In this work, we introduce a novel non-parametric algorithm dubbed PHASMA to produce clean, robust exoplanet phase curves and apply it to 115 Neptunian and 50 Terran exoplanets observed by Kepler. We stack the signals to further improve signal to noise, and measure an average Neptunian albedo of A_g< 0.23 to 95 per cent confidence, indicating a lack of bright clouds consistent with theoretical models. Our Terran sample provides the first constraint on the ensemble albedo of exoplanets which are most likely solid, constraining A_g< 0.42 to 95 per cent confidence. In agreement with our constraint on the greenhouse effect, our work implies that Kepler's solid planets are unlikely to resemble cloudy Venusian analogues, but rather dark Mercurian rocks.