Observational and Dynamical Constraints on an Unseen Outer Perturber in the GJ 436 Hot Neptune System

TL;DR

This study combines observational data and dynamical simulations to constrain the properties of a potential unseen outer companion influencing the orbit of GJ 436 b, a hot Neptune with a polar, eccentric orbit.

Contribution

It provides the most stringent observational constraints to date on a possible outer companion and explores its role in the planet's unusual orbital architecture through dynamical modeling.

Findings

Constraints on companion mass and orbit from radial velocity and astrometry.

Dynamical models favor a substellar perturber at >6.8 AU.

Results guide future searches for unseen companions.

Abstract

Hot Neptunes in the sub-Jovian desert offer unique insights into planetary system evolution, retaining signatures of dynamical processes that shaped their present-day architectures. Many of these planets exhibit polar orbits, yet the mechanisms responsible for these misalignments between the stellar spin axis and planet orbit normal remain under debate. GJ 436 b stands among the very few hot Neptunes with both a polar and an eccentric orbit, thereby preserving dynamical signatures that may have otherwise been erased by tidal circularization. We investigate the unusual orbital architecture of GJ 436, exploring von Zeipel-Lidov-Kozai migration induced by a distant companion as a mechanism to explain the present-day orbit of GJ 436 b. Using $\sim$20 years of archival radial velocity measurements and astrometric data from the Hipparcos-Gaia Catalog of Accelerations, we constrain a potential companion to $a_{c}<5.4$ AU for $m_{c}>0.05$ $M_{Jup}$ and $a_{c}<64$ AU for $m_{c}>24$ $M_{Jup}$ in the GJ 436 system at the $2\sigma$ confidence level, providing the most stringent constraints to date. We further perform three-body hierarchical secular simulations to determine which companion configurations could reproduce GJ 436 b's present-day orbit within the observationally constrained parameter space. Our dynamical modeling favors sub-Jovian masses on orbits with $a_\mathrm{c} \gtrsim 6.8$ AU, suggesting a substellar perturber. These observational and dynamical constraints can guide future companion searches and illuminate formation mechanisms for hot Neptune desert planets on polar orbits.