Hyper Illumination on Local and Global Scales

Motivation

The Sun and other stars are often approximated as a point light source in planetary sciences and remote sensing applications. The light “rays” hitting the surface are perfectly parallel. This assumption leads to a simple calculation of the illumination geometry for each surface element on the local scale and the entire planetary body on the global scale. This assumption works fine for planetary bodies orbiting far away from their host star (e.g., Earth and the Moon). However, it does not hold up for closely orbiting bodies. Mercury is much closer to the Sun, and the finite size of the Sun’s disk begins to make a difference [1]. Instead of a simple computation, more complex ray-tracing approaches are required

Exoplanets are an even more extreme case. The best-performing exoplanet detection methods (radial velocities and transit measurements) favor closely orbiting planets. The close distance leads to more than 50% of the planets surface being illuminated [2]. This affects the modeling of the reflectance and thermal emission of those exoplanets, which is a crucial task for analyzing measurements made by ground and space-based telescopes (e.g., the James Webb Space Telescope).

Plane-parallel ray model [2].

Illumination by a source with a finite angular size [2].

Literature

[1] https://meetingorganizer.copernicus.org/EPSC2024/EPSC2024-337.html

[2] https://iopscience.iop.org/article/10.3847/1538-3881/ad33c1