High C/O Ratio and Weak Thermal Inversion in the Very Hot Atmosphere of Exoplanet WASP-12b

TL;DR

This study reveals that the hot Jupiter WASP-12b has a super-solar C/O ratio, lacks a thermal inversion, and exhibits efficient energy circulation, challenging existing models of highly irradiated exoplanet atmospheres.

Contribution

It provides the first evidence of a super-solar C/O ratio and weak thermal inversion in WASP-12b's atmosphere, contrasting with theoretical expectations.

Findings

C/O ratio >= 1 in WASP-12b's atmosphere

Depletion of water vapor and enrichment of methane

Absence of a thermal inversion despite high irradiation

Abstract

The carbon-to-oxygen ratio (C/O) in a planet provides critical information about its primordial origins and subsequent evolution. A primordial C/O greater than 0.8 causes a carbide-dominated interior as opposed to a silicate-dominated composition as found on Earth, and the atmospheres can also differ from those in the Solar System. The solar C/O is 0.54. Here we report an analysis of dayside multi-wavelength photometry of the transiting hot-Jupiter WASP-12b that reveals C/O >= 1 in its atmosphere. The atmosphere is abundant in CO. It is depleted in water vapor and enhanced in methane by over two orders of magnitude each compared to a solar-abundance chemical-equilibrium model at the expected temperatures. We also find that the extremely irradiated atmosphere (T > 2,500 K) of WASP-12b lacks a prominent thermal inversion, or a stratosphere, and has very efficient day- night energy circulation. The absence of a strong thermal inversion is in stark contrast to theoretical predictions for the most highly irradiated hot-Jupiter atmospheres.