Transit of Exomoon Plasma Tori: New Diagnosis

Abstract

In the solar system, moons largely exceed planets in number. The Kepler database has been shown to be sensitive to exomoon detection down to the mass of Mars, but the first search has been unsuccessful. Here, we use a particles-in-cell code to predict the transit of the plasma torus produced by a satellite. Despite the small size of a moon, the spatial extent of its plasma torus can be large enough to produce substantial transit absorptions. The model is used for the interpretation of Hubble Space Telescope early ingress absorptions apparently observed during WASP-12b and HD 189733b UV transits for which no consistent explanation exists. For HD 189733b an exomoon transiting $\sim 16$ $R_p$ ahead of the planet and loading $\sim 10^{29}$ C II ions/s into space is required to explain the tentative early ingress absorption observed for C II. For WASP-12b, a moon transiting $\sim 6$ $R_p$ ahead from the planet and ejecting $\sim 10^{28}$ Mg II ions per second is required to explain the NUV early ingress absorption feature. Interestingly, both HD 189733b and WASP-12b predicted satellites are outside the Hill sphere of their planets, an indication that the moons, if present, were not formed in situ but probably captured later. Finally, our simulations show a strong electromagnetic coupling between the polar regions of planets and the orbital position of the moons, an expected outcome of the unipolar induction DC circuit model. Future observations should test our predictions with a potential opportunity to unambiguously detect the first exomoon plasma torus.