Magnetohydrodynamic simulations of the space weather in Proxima b: Habitability conditions and radio emission

The habitability of exoplanets hosted by M dwarf stars dramatically depends on the space weather, where the magnetic and ram pressure of the stellar wind, and the exoplanet magnetic field are the three main players. These three parameters also likely drive the radio emission arising close to the planet. Our aim is to characterize the magneto-plasma environment and thus the habitability of the Earth-like planet Proxima b, which is inside the habitable zone of its host M dwarf star Proxima, when it is subject to average calm space weather conditions, and to more extreme space weather conditions, for example a coronal mass ejection (CME) event. We study the role of the stellar wind and planetary magnetic field, and their mutual orientation. We also determine the radio emission arising from the interaction between the stellar wind of Proxima and the magnetosphere of its planet Proxima b, which is relevant to guiding radio observations aimed at unveiling planets. We used the PLUTO code to run a set of 3D magneto-hydrodynamic simulations focused on the space weather around planet Proxima b. We considered both calm and space weather conditions for Proxima b, under three different scenarios: (a) Proxima b subject to calm space weather in a sub-Alfvénic regime, where the stellar wind magnetic pressure dominates over the wind's ram pressure; (b) Proxima b subject to calm space weather in a super-Alfvénic regime, where the ram pressure of the wind dominates, and a bow shock is formed; and (c) Proxima b subject to a coronal mass ejection event, when the dynamical and magnetic pressure of the stellar wind from its host star are increased enormously for a short period of time. We find that if Proxima b has a magnetic field similar to that of the Earth ($B_ p = B_ 0.32$ G) or larger, the magnetopause standoff distance is large enough to shield the surface from the stellar wind for essentially any planetary tilt but the most extreme values (close to degree $) under a calm space weather. Even if Proxima b is subject to more extreme space weather conditions, for example a CME event from its host star, the planet is well shielded by an Earth-like magnetosphere ( B_ $i or if it has a tilt smaller than that of the Earth. Otherwise, the planetary magnetic field must be larger to shield the planet from particle precipitation on the surface. For calm space weather conditions, the radio emission caused by the day-side reconnection regions can be as high as 7$ in the super-Alfvénic regime, and is on average almost an order of magnitude larger than the radio emission in the sub-Alfvénic cases, due to the much larger contribution of the bow shock, which is not formed in the sub-Alfvénic regime. We also find that the energy dissipation at the bow shock is essentially independent of the angle between the planet's magnetic dipole and the incident stellar wind flow. If Proxima b is subject to extreme space weather conditions, the radio emission is more than two orders of magnitude larger than when under calm space weather conditions. This result yields expectations for a direct detection (from Earth) in radio of giant planets in close-in orbits as they are expected to have magnetic fields large enough, so that their electron-cyclotron frequency exceeds the ionosphere cutoff.