Reflected Light from a Giant Planet’s Periastron Passage
HD 20782b is a giant planet with at least twice the mass of Jupiter. What makes HD 20782b bizarre is that its orbit around its host star, a Sun-like star, is the most eccentric orbit known for any exoplanet. HD 20782b takes 597 days to go around its host star once in an extremely elongated orbit with eccentricity 0.956. Once per orbit, HD 20782b swings in for a brief fiery encounter with its parent star. At closest approach (periastron), HD 20782b is only 0.061 AU from its host star, and the planet is 2.73 AU from its host star at its furthest (apastron).
The highly elongated orbit of HD 20782b means that the intensity of flux it receives from its host star varies from 3.6 to over 7000 times the intensity of flux Jupiter receives from the Sun. Stephen R. Kane et al. (2015) present evidence for reflected light from HD 20782b during the planet’s periastron passage. The signature of reflected light is indicated as a tiny increase in the brightness of the planet’s host star as reflected light from the planet adds to the overall observed brightness of the host star.
Short-period giant planets tends to have relatively low albedos (i.e. low reflectivity) while long-period giant planets tend to have relatively high albedos (i.e. high reflectivity). Because HD 20782b spends the vast majority of its time far from its host star, it will have a high albedo similar to a long-period giant planet. HD 20782b spends only a very small amount of time around periastron and the planet’s high albedo atmosphere does not have enough time to react to the intense flux from its host star. As a consequence, HD 20782b, with its high albedo atmosphere, is able to reflect a larger amount of flux from its host star than a typical short-period giant planet. This makes HD 20782b easier to detect in reflected light when it is around periastron. Planets with highly eccentric orbits like HD 20782b can provide a good opportunity for the study of planetary atmosphere in reflected light.
Stephen R. Kane et al. (2015), “Evidence for Reflected Light from the Most Eccentric Exoplanet Known”, arXiv:1511.08679 [astro-ph.EP]