Panel a: diagram showing the essential planes and angles needed to characterise the reflected light intensity: the orbital phase of the planet ϕ, the inclination of the orbital plane with respect to the sky plane i and the scattering angle α. The scattering angle α is measured along the scattering plane and our definition of the direction of a positive polarisation p(α) is perpendicular to this plane. In special cases the polarisation of the reflected light could be negative. This would correspond to a direction of p(α) perpendicular to the red arrows. Panel b: normalised intensity and polarised intensity of the reflected light as a function of the orbital phase ϕ, calculated with the reference planet atmosphere model used in this paper: a Rayleigh scattering atmosphere from Buenzli & Schmid (2009) with an optical depth of τ_sc = 2, a single scattering albedo of ω = 0.90, and a ground surface (= cloud) albedo of A_S = 1. The different colours show the phase functions of planets on circular orbits seen at four different inclinations: 0° (black), 30° (red), 60° (green), and 90° (blue).