The JADE code

II. Modeling the coupled orbital and atmospheric evolution of GJ 436 b to constrain its migration and companion^★

¹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
⁴ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁵ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

^★★ Corresponding author: mara.attia@unige.ch

Received: 21 April 2025
Accepted: 25 August 2025

Abstract

The observed architecture and modeled evolution of close-in exoplanets provide crucial insights into their formation pathways and survival mechanisms. To investigate these fundamental questions, we employed Joining Atmosphere and Dynamics for Exoplanets (JADE), a comprehensive numerical code that self-consistently models the coupled evolution of planetary atmospheres and orbital dynamics over secular timescales, rooted in present-day observations. JADE integrates atmospheric photoevaporation with high-eccentricity migration processes driven by von Zeipel-Lidov-Kozai (ZLK) cycles from an external perturber, allowing us to explore evolutionary scenarios where dynamical and atmospheric processes influence each other. Here, we specifically considered GJ 436 b, a warm Neptune with an eccentric orbit and polar spin-orbit angle that has survived within the “hot Neptune desert” despite ongoing atmospheric escape. Our extensive exploration of parameter space included over 500 000 fully coupled JADE simulations in a framework that combines precomputed grids with Bayesian inference. This allowed us to constrain GJ 436 b’s initial conditions and the properties of its putative perturbing companion within a ZLK hypothesis. Our results suggest that GJ 436 b formed at ~0.3 AU and, despite its current substantial atmospheric erosion, has experienced minimal cumulative mass loss throughout its history, thanks to a late inward migration triggered by a distant companion inducing ZLK oscillations. We find that initial mutual inclinations of 80°-100° with this companion best reproduce the observed polar orbit. By combining our explored constraints with radial velocity detection limits, we identified the viable parameter space for the hypothetical GJ 436 c. We found that it strongly disfavors stellar and brown dwarf masses, which offers a useful guide for future observational searches. This work demonstrates how coupled orbital-atmospheric modeling can shed light on the complex interplay of processes shaping close-in exoplanets and explain the survival of volatile-rich worlds near the edges of the hot Neptune desert.