Solid-surface exoplanet detector

Exoplanet surfaces imprint unambiguous signatures on phase curves via coherent backscattering, which can allow us to identify planets with visible surfaces from planets with atmospheres. As the community looks to smaller, rocky worlds, surface/atmospheric signals can be hard to uncover, which often leads to non-detections. Furthermore, none of these current techniques can be used for planets which don’t transit at all. In this talk we will show how a feature, studied for over half a century in Solar System body phase curves, could be a tool to help solve this problem. When we look at the reflected light from e.g. the Moon, Mars, Enceladus and Jupiter as a function of phase, the brightness observed near full-illumination peaks very sharply. This is due to a process called ‘coherent backscattering’, whereby, due to a non-homogeneous scattering surface or atmosphere, the incoming light coherently interferes with the backscattered light, dramatically increasing the intensity as viewed by the observer at zero phase. The shape of this ‘peak’ is much sharper for atmosphere-less solid bodies, such as the Moon and Enceladus, as for those with an atmosphere, such as Jupiter and Titan. In this talk, we will show that this method could be used for exoplanet phase curves to detect whether the planet has an atmosphere, even if they don’t transit. We have developed a phase curve model with coherent backscattering and demonstrate that it can be used to correctly distinguish Solar System planets with and without atmospheres. Additionally, we model this effect on some well-known rocky exoplanets, simulating observations from both JWST and the future Habitable Worlds Observatory and reveal the results.