Abstract
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 evolves 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 semi-analytic prescription, we investigate the entire relevant phase space of planet type, planet orbit, and stellar host mass ($1\!-\!7\, {\rm M}_{\odot }$). 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.
Highlights
IntroductionThe majority of the detected exoplanets orbit main-sequence stars, we know of over 100 planets which orbit red giant stars (e.g., Reffert et al 2015; Grunblatt et al 2019; Wittenmyer et al 2020) and four planets which orbit white dwarfs (Thorsett et al 1993; Sigurdsson et al 2003; Luhman et al 2011; Gansicke et al 2019; Vanderburg et al 2020)
To date, more than 4,000 exoplanets have been identified
We focus on the evolution of planetary magnetospheres after their host stars have evolved off the main sequence
Summary
The majority of the detected exoplanets orbit main-sequence stars, we know of over 100 planets which orbit red giant stars (e.g., Reffert et al 2015; Grunblatt et al 2019; Wittenmyer et al 2020) and four planets which orbit white dwarfs (Thorsett et al 1993; Sigurdsson et al 2003; Luhman et al 2011; Gansicke et al 2019; Vanderburg et al 2020) All these planets formed during the protoplanetary disc phase (e.g., Pinte et al 2018) and have since survived stellar evolution to reach the present time. Accompanying the non-monotonic and often sudden changes in radii and surface temperatures (or luminosities) are dramatic variations in stellar wind properties
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