Planet-induced Periodic Modulation of Stellar Activity in GJ~436: Insights into a Warm Neptune's Magnetic Field

TL;DR

This study analyzes 17 years of spectroscopic data of GJ 436, revealing star-planet magnetic interactions that suggest the planet's magnetic field is between 6 and 110G, providing insights into planetary magnetic properties.

Contribution

It introduces a new geometrical model for interpreting star-planet magnetic interactions and estimates the magnetic field of GJ 436 b from observational data.

Findings

Repeated stellar activity enhancements at specific orbital phases.

Estimated planetary magnetic field between 6 and 110G.

Proposed a new model for star-planet magnetic interaction signals.

Abstract

Interactions between stellar and planetary magnetic fields are expected to produce observable radio and optical signals modulated by their orbital periods, but direct detections remain elusive. We analyze 17 years of spectroscopic data of the GJ 436 system. This M2.5 V star hosts a transiting Neptune-sized planet in a close-in, inclined orbit. The data shows repeated enhancements of the stellar chromospheric activity at approximately the same phase of its 8-year activity cycle modulated by a combination of the planet's orbital period and the stellar rotation. We interpret this modulation as star-planet interaction. We propose a new geometrical model to interpret these signals, then, estimate the power of the interaction and, from models, estimate the magnetic field of GJ 436 b to be between 6 and 110G. This finding opens new pathways to detect star-planet interactions and to investigate planetary magnetic fields and their implications on atmospheric retention and detectability.