Exomoon Phase Curves: Toroidal Exosphere Simulations of Exo‐Ios Orbiting 8 Exoplanets in Alkali Spectroscopy

Abstract

Toroidal atmospheres and exospheres characterized at exoplanets may be fueled by volcanically active exomoons, often referred to as exo‐Ios. We study the neutral outgassing and volatile evolution of a close‐orbiting, evaporating satellite at eight candidate exoplanet‐exomoon systems WASP‐49,‐96,‐69,‐17 b, XO‐2N b, HAT‐P‐1 b, HD‐189733 b, and HD‐209458 b by developing a 3‐D test‐particle Monte Carlo simulation, Simulating the Evolution of Ring Particles Emergent from Natural Satellites. The module is coupled to dishoom, approximating the minimum mass‐flux needed to reproduce observations of alkali line profiles identified in dozens of transmission spectra. We focus on sputtered neutral sodium, limited by photoionization and radiative effects. By considering Earth‐, Io‐, and Enceladus‐like masses, we systematically simulate the imprint of a non‐hydrostatic medium (characteristic of volcanic exospheres) in density and velocity space using a novel Delaunay tesselation field estimator algorithm. Our results demonstrate how exomoons can considerably modulate gas density observations probed near exoplanet transit, depending on the orbital phase of the putative satellite at the time of observation. The density evolution, therefore, manifests on orbital timescales as “exomoon phase curves” from shadow to occultation. We find two regimes of density evolution, characteristic of a: (a) localized cloud and (b) an azimuthally symmetric exoring/torus, degenerate with an exoplanet atmosphere, ranging from ∼109.5±0.5 cm−2 to ∼1015±0.25 cm−2 at our leading candidate WASP‐69b I. In certain orbital architectures, the smallest evaporating satellite mass surprisingly generates the brightest sodium signal, fueling optimism for discovering photometrically indiscernible rocky exomoons. We suggest long baseline monitoring of alkali and SO2 systems in spectroscopy to search for the temporal and spatial variability predicted here.