Characterization of extra-solar planets with direct-imaging techniques

In order to characterize the physical properties of an extra-solar planet one needs to detect planetary radiation, either visible (VIS) to near-infrared (NIR) reflected starlight or infrared (IR) thermal radiation. Both the reflected and thermal flux depend on the size of the planet, the distance between the planet and the star, the distance between the observer and the planet, and the planet’s phase angle (i.e. the angle between the star and the observer as seen from the planet). Moreover, the planetary radiation also depends on the composition and structure of the planet’s atmosphere and/ or surface, the wavelength of the observation, and other effects such as the presence and physical characteristics of planetary rings or moons. In this white paper, we describe the techniques needed to learn about the properties of the planet from various observations of its radiation. In a companion white paper, we discuss how planets might be found using direct-detection methods (Turnbull et al. 2009); here we focus on the more detailed photometry, spectroscopy, polarimetry, and time-variability that can reveal the true nature of these planets and the systems in which they were born.