The interpretation of (exo-)planet phase curves. Forward scattering.
In a recent paper, we investigate the phenomenon shown in the image below. Forward scattering of starlight by an extended atmosphere may produce a brightness surge in the disk-integrated brightness of a planet (or a moon, in this case Titan) when approaching back-illumination. In the image, forward scattering produces the luminous ring around the day-to-night divide (or terminator). The ring reveals a layer of strongly-forward scattering haze in Titan’s upper atmosphere.
In our study, we used about 6,000 images of Titan taken with the Cassini/ISS instrument over more than a decade and with 15 different filters. This is the most comprehensive study of its kind, and a sample of what we would like to do with exoplanets one day. Our observational-theoretical analysis shows that Titan will appear up to 200 times brighter when it is back illuminated than when it is fully illuminated. It is fair to say that the remote view of Titan’s nightside is way brighter than its dayside! This behavior is unique to Titan in our solar system. We also use this information to better constrain the reflecting properties of Titan and, in turn, its energy budget.
The brightness surge that occurs at Titan, if observed in a remote exoplanet, will allow us to put valuable constraints on the exoplanet atmosphere. In particular, it will help us understand how extended the atmosphere is and whether it contains abundant haze. This idea, in its application to exoplanets, is elaborated further in a follow-up study. The study of solar system planets (and moons) motivates new ideas to explore in the characterization of exoplanets. In turn, looking at solar system objects with an “exo-perspective” reveals new and previously unexplored aspects of them.
The paper is now published in Nature Astronomy. And is also available on astro-ph.
Characterization of giant exoplanet atmospheres with optical phase curves
The amount of starlight reflected by a planet depends critically on the optical properties of its atmosphere and the phase at which the planet is observed. In García Muñoz & Isaak (2015), we investigated the information contained in how the planet brightness varies with orbital phase for the hot Jupiter Kepler-7b. Through the exploration of a multi-million set of forward models, each of them specific to an atmospheric configuration, we concluded that Kepler-7b’s atmosphere contains an optically cloud shifted towards the dawn terminator and resting above a dark gas envelope. The cloud particles must be poorly absorbing to explain the planet’s elevated reflectance, and are likely small to explain the lack of significant back-scattering (under the assumption of Mie scattering theory). Degeneracies exist in the interpretation of the optical phase curve that could eventually be mitigated by multi-wavelength brightness phase curves or polarimetric observations (García Muñoz, in preparation).
The paper is published in the Proceedings of the National Academy of Sciences. And is also available on astro-ph.