Bulk composition of the transiting hot Neptune around GJ 436

TL;DR

This study combines planet-structure and formation models to determine that GJ 436 b is likely a rocky planet with a small hydrogen-helium envelope, providing insights into its composition and formation history.

Contribution

It introduces a combined modeling approach to constrain the bulk composition of GJ 436 b, a unique hot Neptune, and assesses the likelihood of various formation scenarios.

Findings

GJ 436 b probably has over 45% rock content in mass.

The planet likely has a small H-He envelope of 10-20% in mass.

Ocean planet and pure gaseous models are excluded.

Abstract

The hot Neptune orbiting around GJ 436 is a unique example of an intermediate mass planet. Its close-in orbit suggests that the planet has undergone migration and its study is fundamental to understanding planet formation and evolution. As it transits its parent star, it is the only Neptune-mass extrasolar planet of known mass and radius, being slightly larger and more massive than Neptune (M=22.6 M_Earth, R=4.19R_Earth). In this regime, several bulk compositions are possible: from an Earth-like core with a thick hydrogen envelope to a water-rich planet with a thin hydrogen envelope comprising a Neptune-like structure. We combine planet-structure modeling with an advanced planet-formation model to assess the likelihood of the different possible bulk compositions of GJ 436 b. We find that both an envelope-free water planet ("Ocean planet") as well as a diminute version of a gaseous giant planet are excluded. Consisting of a rocky core with a thick hydrogen/helium envelope, a "dry" composition produces not only too small a radius but is also a very unlikely outcome of planet formation around such a low-mass star. We conclude that GJ 436 b is probably of much higher rock content than Neptune (more than 45% in mass), with a small H-He envelope (10 - 20% in mass). This is the expected outcome of the gathering of materials during the migration process in the inner disk, creating a population of which the hot Neptune is representative.