Bulk composition of the transiting hot Neptune around GJ 436

¹ Observatoire de l'Université de Genève, 1290 Sauverny, Switzerland e-mail: pedro.figueira@obs.unige.ch
² University of Exeter, The Queens Drive, Exeter, Devon, UK
³ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁴ Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France

Received: 28 October 2007
Accepted: 11 October 2008

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 understand 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 (, ). 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, and comprising a Neptune-like structure. We combine planet-structure modelling 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.