Planetary magnetosphere evolution around post-main-sequence stars

TL;DR

This paper models how planetary magnetospheres evolve during the post-main-sequence stellar phases, revealing they are often destroyed unless planets have exceptionally strong magnetic fields, impacting habitability prospects.

Contribution

It introduces a semianalytic model to study magnetosphere evolution across various planet types and stellar masses during post-main-sequence phases, highlighting conditions for magnetosphere survival.

Findings

Magnetospheres are typically quenched during giant phases unless magnetic fields are >100 times Jupiter's.

Stellar wind variations increase ram pressure, preventing magnetosphere maintenance below 0.1 G.

Habitability around white dwarfs may be compromised due to prior magnetosphere loss.

Abstract

Accompanying the mounting detections of planets orbiting white dwarfs and giant stars are questions about their physical history and evolution, particularly regarding detectability of their atmospheres and potential for habitability. Here we determine how the size of planetary magnetospheres evolve over time from the end of the main sequence through to the white dwarf phase due to the violent winds of red giant and asymptotic giant branch stars. By using a semianalytic prescription, we investigate the entire relevant phase space of planet type, planet orbit and stellar host mass (1-7Msun). We find that a planetary magnetosphere will always be quashed at some point during the giant branch phases unless the planet's magnetic field strength is at least two orders of magnitude higher than Jupiter's current value. We also show that the time variation of the stellar wind and density generates a net increase in wind ram pressure and does not allow a magnetosphere to be maintained at any time for field strengths less than 10^{-5} T (0.1 G). This lack of protection hints that currently potentially habitable planets orbiting white dwarfs would have been previously inhospitable.