Star–planet interaction

Abstract

Context. Electromagnetic star-planet interaction (SPI) describes the phenomenon of a planet coupling to its host star via electromagnetic forces. Alfvén waves can establish such a coupling by forming Alfvén wings. Star-planet interaction allows for phenomena that we do not otherwise know from the Solar System. Wing-wing interaction is such an example, whereby the Alfvén wings of two planets merge and interact in a nonlinear way. Aims. In this paper, we focus on the effects that SPI has on other planets and the stellar wind. First, we analyze the different wave structures connected to SPI and then we investigate the wing-wing interaction. Methods. Our study applies a magnetohydrodynamic model to describe a stellar system with multiple possible planets. As an example, we chose TRAPPIST-1 and its two innermost planets. We extended the PLUTO code to simulate collisions between atmospheric neutral particles and plasma ions. Neutral gas clouds imitate the planets and move through the simulation domain. That allows for the simulation of fully time-dependent stellar systems. Results. We analyzed the wave structures that result from the interaction between stellar wind and TRAPPIST-1 b. The resultant wave structure propagating inward is an Alfvén wing. The outwardly directed part of the interaction consists of an Alfvén wing, slow mode waves, the planetary wake, and a slow shock. We quantified the strength of the respective wave perturbations at the outer planets to be on the order of 10% to 40% of the local background values of thermal, magnetic, and dynamic pressure. Wing-wing interaction occurs due to the relative position of two planets during their conjunction and shows three phases. First there is an initial, nonlinear intensification of the Poynting flux by 20%, an intermediate phase with reduced Poynting flux, followed by a third phase when the Alfvén wing of planet c goes through planet b’s wave structures with another intensification phase of the Poynting flux.