Phase Light Curves for Extrasolar Jupiters and Saturns

We predict how a remote observer would see the brightness variations of giant planets similar to those in our solar system as they orbit their central stars. Our models are the first to use measured anisotropic scattering properties of solar system giants and the first to consider the effects of eccentric orbits. We model the geometry of Jupiter, Saturn, and Saturn's rings for varying orbital and viewing parameters, using scattering properties for the (forward scattering) planets and (backward scattering) rings as measured by the Pioneer and Voyager spacecraft at 0.6-0.7 μm. Images of the planet with and without rings are simulated and used to calculate the disk-averaged luminosity varying along the orbit; that is, a light curve is generated. We find that the different scattering properties of Jupiter and Saturn (without rings) make a substantial difference in the shape of their light curves. Saturn-sized rings increase the apparent luminosity of a planet by a factor of 2-3 for a wide range of geometries, an effect that could be confused with a larger planet size. Rings produce asymmetric light curves that are distinct from the light curve that the planet would have without rings, which could resolve this confusion. If radial velocity data are available for the planet, the effect of the ring on the light curve can be distinguished from effects due to orbital eccentricity. Nonringed planets on eccentric orbits produce light curves with maxima shifted relative to the position of the maximum phase of the planet. Given radial velocity data, the amount of the shift restricts the planet's unknown orbital inclination and therefore its mass. A combination of radial velocity data and a light curve for a nonringed planet on an eccentric orbit can also be used to constrain the surface scattering properties of the planet and thus describe the clouds covering the planet. We summarize our results for the detectability of exoplanets in reflected light in a chart of light-curve amplitudes of nonringed planets for different eccentricities, inclinations, and azimuthal viewing angles of the observer.