We present results of numerical simulations of flux F and degree of polarization P of light that is reflected by Earth-like extrasolar planets orbiting solar type stars. Our results are presented as functions of the wavelength (from 0.3 to 1.0 µm, with 0.001 µm spectral resolution) and as functions of the planetary phase angle. We use different surface coverages for our model planets, including vegetation and a Fresnel reflecting ocean, and clear and cloudy atmospheres. Our adding-doubling radiative transfer algorithm, which fully includes multiple scattering and polarization, only handles horizontally homogeneous planets, so we simulate fluxes and polarization of horizontally inhomogeneous planets by weighting results obtained for homogeneous planets. Like F, P of the reflected starlight is shown to depend strongly on the phase angle, on the composition and structure of the planetary atmosphere, on the reflective properties of the underlying surface, and on the wavelength, in particular in wavelength regions with gaseous absorption bands. The sensitivity of P to a planet's physical properties appears to be different than that of F. Combining flux with polarization observations thus makes for a strong tool for characterizing extrasolar planets. The calculated total and polarized fluxes are made available through the CDS.

Abstract. Using numerical simulations of flux and polarization spectra of visible to near-infrared starlight reflected by Jupiter-like extrasolar planets, we show that polarimetry can be used both for the detection and for the characterization of extrasolar planets. Polarimetry is valuable for detection because direct, unscattered starlight is generally unpolarized, while starlight that has been reflected by a planet will generally be polarized. Polarimetry is valuable for planet characterization because the degree of polarization of starlight that has been reflected by a planet depends strongly on the composition and structure of the planetary atmosphere.

To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low-cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud-free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground-based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts.

We present an efficient numerical method for integrating planetary radiation over a planetary disk, which is especially interesting for simulating signals of extrasolar planets. Our integration method is applicable to calculating the full flux vector of the disk-integrated planetary radiation, i.e. not only its observed flux (irradiance), but also its state of polarization (linear and circular). Including polarization is important for simulations of the light reflected by a planet, in particular, because this will generally be polarized. Our integration method is based on the expansion of the radiation field of a spherical, horizontally homogeneous planet into generalized spherical functions. With the expansion coefficients, the flux vector of the disk-integrated, reflected starlight can be obtained rapidly for arbitrary planetary phase angles. We describe the theory behind the disk-integration algorithm and results of accuracy tests. In addition, we give some illustrative examples of the application of the algorithm to extrasolar planets.Key words. methods: numerical -polarization -radiative transfer -stars: planetary systems IntroductionDuring the past decades, the spatial resolution of the observations of planets in our solar system has increased significantly. This increase stems from planetary missions such as Voyager, Galileo, and Cassini-Huygens, from space-bound telescopes like the Hubble Space Telescope, and from the development of ground-based adaptive optics systems. Together with this increasing spatial resolution, the spatial resolution of numerical simulations for the interpretation of observations of solar system planets has increased, too. Consequently, efficient numerical methods to integrate reflected starlight across a planetary disk have received little attention lately. Recent discoveries of extrasolar planets, however, have renewed interest in such numerical integration methods.Because extrasolar planets are very faint compared to their parent star, and because the angular distance between a star and an orbiting planet is very small, observing the planet itself by detecting the stellar light it reflects or the thermal radiation it emits is extremely difficult. Consequently, almost all of the known extrasolar planets have been found by indirect methods, in which not the planet itself but rather its influence on its parent star is detected. Although very useful for detecting an extrasolar planet, indirect detection methods give, however, little information on the planet itself, apart from its mass and some orbital elements. Information on the physical structure and chemical composition of a planet, for example, can be derived from direct observations of the planetary radiation. To succeed in detecting the very faint planetary radiation, dedicated instruments and space missions are being designed, such as the Planetfinder instrument that has been designed for use on one of ESO's VLTs and ESA's Darwin mission (Fridlund 2004) with space telescopes flying in formation and performing infra...