Star-Planet Interactions in Earth-like Exoplanets: Magnetospheric Dynamics and Atmospheric Evolution

Gupta, Sakshi (2026) Star-Planet Interactions in Earth-like Exoplanets: Magnetospheric Dynamics and Atmospheric Evolution. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sakshi Gupta (20RS057)) 20RS057.pdf - Submitted Version Restricted to Repository staff only Download (44MB)

Abstract

One of the crucial ingredients which shape the space environment of a planet is magnetized plasma winds emanating from the host star. The stellar wind – a continuous outflow of charged particles carrying embedded magnetic fields – interacts with the planetary magnetic field and atmosphere, sculpting the magnetospheric structure and driving atmospheric erosion. These processes fundamentally influence whether a planet can retain a substantial atmosphere over geological timescales and sustain conditions favorable for habitability. Understanding the interplay between stellar activity and planetary magnetic environments is therefore crucial for assessing the long-term evolution of (exo)planetary atmospheres across diverse star-planet systems. This thesis presents a comprehensive investigation of stellar-(exo)planetary interactions using three-dimensional resistive magnetohydrodynamic (MHD) simulations focusing on the interplay of magnetized plasma winds with exoplanetary environments. The work is motivated by three interconnected themes: How do the relative magnetic field strengths of the star and the planet determine magnetospheric configuration and atmospheric mass loss? How does varying stellar wind forcing influence Alfvén wing dynamics and reshape the planetary environment? How does a planetary magnetosphere respond to transient, extreme stellar forcing events such as interplanetary coronal mass ejections (ICMEs)? To address these questions, we carry out systematic parameter space studies using the CESSISPIM and STORMI models which are developed based upon the open source PLUTO MHD framework. By exploring a wide range of stellar wind conditions and planetary magnetic field strengths, the thesis aims to develop a comprehensive physical understanding of how magnetic interactions govern the structure, evolution, and variability of planetary magnetospheres and their atmospheres. Wherever possible, emphasis has been placed on derivation of analytic relationships and empirical scaling laws based on underlying physics and numerical experimentation so as to establish robust relationships. The thesis is organized as follows. Chapter 1 provides a broad introduction to the field of star-planet interactions. Beginning with the discovery and classification of exoplanets, it extends the well-studied Sun–Earth interaction paradigm to more general star-planet systems. The chapter discusses stellar winds, magnetic environments, and planetary magnetospheres, and places the present work in the context of ongoing efforts to understand exoplanetary habitability. Chapter 2 describes the computational framework used throughout the thesis. We outline the governing resistive MHD equations and their assumptions, followed by an overview of the PLUTO code and the numerical techniques employed. Two simulation modules, CESSI-SPIM and CESSI-STORMI, are discussed for modeling steady-state stellar wind interactions and time-dependent ICME forcing, respectively. The planetary model, atmospheric setup, boundary conditions, and diagnostics for atmospheric mass loss are also described. Chapter 3 presents a parameter space study exploring the impact of varying stellar and planetary magnetic field strengths on magnetospheric topology and atmospheric mass loss. Covering a wide range of magnetic environments, we show that the ratio of planetary to stellar wind magnetic field strength governs the global magnetospheric configuration, dayside field compression, magnetotail opening, Alfvén wing formation, and current sheet bifurcation. A semi-analytical relation connecting atmospheric mass loss to this magnetic field ratio is established putting the simulation results on a firm physical footing. Chapter 4 extends our analysis by incorporating variations in stellar wind speed alongside magnetic field strengths, spanning regimes from sub-Alfvénic to moderately super-Alfvénic flows. The study captures how the combined effects of plasma kinetic driving, magnetic tension, and planetary magnetic field strength shape the global magnetospheric topology. The morphology and scaling of Alfvén wings, magnetopause standoff distance, magnetotail structure, and plasma properties within the wings are analyzed across the parameter space to provide new insights. In Chapter 5, we investigate the time-dependent magnetospheric response to ICME impacts using the CESSI-STORMI module. The simulations explore atmospheric escape, magnetospheric topology, and current sheet evolution for both magnetized and unmagnetized planets. We show that unmagnetized planets undergo global magnetospheric polarity reversals due to reversals in the external magnetic forcing, while such reversals are less pronounced in the presence of stronger intrinsic planetary magnetic fields. Additionally, atmospheric escape and current sheet dynamics in magnetized planets is found to depend strongly on the orientation of the stellar wind field, while remaining largely insensitive to magnetic orientation of the wind for unmagnetized planets. This differential response offers a potential pathway for distinguishing magnetized and unmagnetized (exo)planets through future observations and may assist in the detection of exoplanetary magnetic fields. Chapter 6 summarizes the key findings and outlines directions for future work. Preliminary results on planetary radio emissions are presented, demonstrating that orientation-dependent radio signatures can serve as diagnostics for detecting (exo)planetary magnetic fields. The thesis establishes that the relative strengths of stellar and planetary magnetic fields play a crucial role in shaping magnetospheric dynamics and atmospheric evolution. The modeling framework and scaling relations developed in this thesis provide a foundation for advancing our understanding of how stellar magnetic activity shapes planetary environments and for guiding future observational efforts aimed at characterizing (exo)planetary habitability in the context of magnetic interactions. Chapters 3 through 5 of this thesis present the primary outcomes of the research work and are directly adapted from published and submitted research manuscripts.

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