Mohamad Ali-Dib¹, Olivier Mousis¹, Jean-Marc Petit¹ and Jonathan I. Lunine²

¹Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Observatoire de Besançon, BP 1615, F-25010 Besançon Cedex, France
²Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA

The C to O ratio is a crucial determinant of the chemical properties of planets. The recent observation of WASP 12b, a giant planet with a C/O value larger than that estimated for its host star, poses a conundrum for understanding the origin of this elemental ratio in any given planetary system. In this paper, we propose a mechanism for enhancing the value of C/O in the disk through the transport and distribution of volatiles. We construct a model that computes the abundances of major C- and O-bearing volatiles under the influence of gas drag, sublimation, vapor diffusion, condensation, and coagulation in a multi-iceline 1+1D protoplanetary disk. We find a gradual depletion in water and carbon monoxide vapors inside the water’s iceline, with carbon monoxide depleting slower than water. This effect increases the gaseous C/O and decreases the C/H ratio in this region to values similar to those found in WASP 12b’s day side atmosphere. Giant planets whose envelopes were accreted inside the water’s iceline should then display C/O values larger than those of their parent stars, making them members of the class of so-called carbon-rich planets.

Reference
Ali-Dib M, Mousis O, Jean-Marc Petit J-M and Lunine JI (2014) Carbon-rich Planet Formation in a Solar Composition Disk. The Astrophysical Journal 785:125.
[doi:10.1088/0004-637X/785/2/125]

Link to Article