A crucial component in assessing the potential habitability of an exoplanet is an understanding of its interaction with the host star. As more terrestrial “habitable zone” exoplanets are discovered, the detailed characterization of the space environment of these planets raises new challenges, both from a physical and an observational perspective. The “space weather” environment of the planet is primarily governed by the level of magnetic activity of the star (XUV flux, stellar wind and high energy transients), the orbital distance of the planet, the nature and strength of the exoplanet’s magnetic field and the magnetic and electromagnetic interactions of this coupled system. To address this, we take advantage of the wealth of knowledge gained about the sole existing habitable system of which we are sure, namely, the Sun-Earth system. We approach this by modeling the stellar activity, which governs much of the expected star-planet interaction and so has an important role to play on potential habitability, and the planetary response, which enables us to place constraints on the expected emission signatures of the star-planet interaction.
On the stellar side, we employ a magnetic flux transport model (SFT), devised from a full 22-yr solar magnetic cycle, to characterize the asterospheric magnetic field in systems with stars of varying levels of activity, up to 10x that of the Sun. This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed solar activity cycles, as well as surface flux dynamics consistent with observed stellar relationships. We verify the viability of the SFT model for application to stars other than the Sun by reproducing the observed stellar activity-rotation relationship across a wide range of stellar types. We find that the simulations match the activity-rotation relationship in the unsaturated regime of cool stars extremely well and that the observed spread in the observations can be reasonably explained as a result of cycle variability. From our modeling of the asterospheric field at the various levels of activity consider, we are able to detail the star-exoplanet interaction through several quantitative measures such as the ratio of open to total stellar magnetic flux and its variation with stellar latitude, the location and variability of the mean stellar Alfven surface, and the strength of interplanetary magnetic field polarity inversions, all of which have the potential to influence the magnetic environment of the exoplanet.
On the planetary side, we explore the coupling of the stellar activity to the planetary magnetic environment and determine whether or not such interactions produce potentially observable signatures. In this work, we focus on the expected signatures of auroral radio emission for Earth-like planets orbiting active stars. Magnetized exoplanets are expected to produce radio emission via interaction between the host star’s stellar wind and planetary magnetosphere-ionosphere system both of which can be significantly enhanced for very active stars. Auroral radio emission is produced by field-aligned current (FAC) driven electron acceleration and this is calculated using a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model, extending the capabilities of previous work. We find that intense, sporadic FACs, driven by night-side magnetic reconnection and inner magnetosphere plasma flow, contribute significantly to the total radio power produced by wind-ionosphere interaction in terrestrial planets. During periods of strong stellar wind variability, the contribution from these secondary currents can be up to several orders of magnitude greater than the primary current systems which previous models describe. This may be even more pronounced for systems in which the primary current system is strongly limited (e.g. ionospheric saturation). The results suggest that magnetized exoplanets may temporarily produce greater radio power than previously estimated increasing their likelihood of producing a detectable signature. Additionally, due to the strong beaming of the emission, the ideal observing angle is dependent on the intensity of the interaction between the stellar wind and exoplanetary magnetosphere. Such observations could provide direct information on the strength of the planetary magnetic field and consequently knowledge about planetary dynamos, planetary evolution, atmospheric escape, and the offset of magnetic and rotation axes.