Letter to the Editor

A correlation between the heavy element content of transiting extrasolar planets and the metallicity of their parent stars

¹ Observatoire de la Côte d'Azur, CNRS UMR 6202, 06304 Nice Cedex 4, France e-mail: guillot@obs-nice.fr
² Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
³ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁴ Centro de Geofisica de Evora, Rua Romeo Ramalho 59, 7002-554 Evora, Portugal
⁵ Department of Physics, University of Florida, PO Box 118440, Gainesville 32611-8440, USA
⁶ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁷ Institut d'Estudis Espacials de Catalunya – CSIC, Campus UAB, Facultat de Ciències, Torre C5-parell-2a, 08193 Bellaterra, Spain

Received: 22 April 2006
Accepted: 30 April 2006

Abstract

Context.Nine extrasolar planets with masses between 110 and 430  are known to transit their star. The knowledge of their masses and radii allows an estimate of their composition, but uncertainties on equations of state, opacities and possible missing energy sources imply that only inaccurate constraints can be derived when considering each planet separately.

Aims.We seek to better understand the composition of transiting extrasolar planets by considering them as an ensemble, and by comparing the obtained planetary properties to that of the parent stars.

Methods.We use evolution models and constraints on the stellar ages to derive the mass of heavy elements present in the planets. Possible additional energy sources like tidal dissipation due to an inclined orbit or to downward kinetic energy transport are considered.

Results.We show that the nine transiting planets discovered so far belong to a quite homogeneous ensemble that is characterized by a mass of heavy elements that is a relatively steep function of the stellar metallicity, from less than 20 earth masses of heavy elements around solar composition stars, to up to ~ for three times the solar metallicity (the precise values being model-dependant). The correlation is still to be ascertained however. Statistical tests imply a worst-case 1/3 probability of a false positive.

Conclusions.Together with the observed lack of giant planets in close orbits around metal-poor stars, these results appear to imply that heavy elements play a key role in the formation of close-in giant planets. The large masses of heavy elements inferred for planets orbiting metal rich stars was not anticipated by planet formation models and shows the need for alternative theories including migration and subsequent collection of planetesimals.