Sparkling nights and very hot days on WASP-18b: the formation of clouds and the emergence of an ionosphere

¹ Centre for Exoplanet Science, University of St Andrews, St Andrews, UK
e-mail: Christiane.Helling@st-and.ac.uk
² SUPA, School of Physics & Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Département de Physique, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
⁵ Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Oxford OX1 3PU, UK

Received: 14 August 2018
Accepted: 24 January 2019

Abstract

Context. WASP-18b is an ultra-hot Jupiter with a temperature difference of up to 2500 K between day and night. Such giant planets begin to emerge as a planetary laboratory for understanding cloud formation and gas chemistry in well-tested parameter regimes in order to better understand planetary mass loss and for linking observed element ratios to planet formation and evolution.

Aims. We aim to understand where clouds form, their interaction with the gas-phase chemistry through depletion and enrichment, the ionisation of the atmospheric gas, and the possible emergence of an ionosphere on ultra-hot Jupiters.

Methods. We used 1D profiles from a 3D atmosphere simulation for WASP-18b as input for kinetic cloud formation and gas-phase chemical equilibrium calculations. We solved our kinetic cloud formation model for these 1D profiles, which sample the atmosphere of WASP-18b at 16 different locations along the equator and in the mid-latitudes. We derived the gas-phase composition consistently.

Results. The dayside of WASP-18b emerges as completely cloud-free as a result of the very high atmospheric temperatures. In contrast, the nightside is covered in geometrically extended and chemically heterogeneous clouds with dispersed particle size distributions. The atmospheric C/O ratio increases to >0.7 and the enrichment of the atmospheric gas with cloud particles is ρ_d/ρ_gas > 10⁻³. The clouds that form at the limbs appear located farther inside the atmosphere, and they are the least extended. Not all day- to nightside terminator regions form clouds. The gas phase is dominated by H₂, CO, SiO, H₂O, H₂S, CH₄, and SiS. In addition, the dayside has a substantial degree of ionisation that is due to ions such as Na⁺, K⁺, Ca⁺, and Fe⁺. Al⁺ and Ti⁺ are the most abundant of their element classes. We find that WASP-18b, as one example for ultra-hot Jupiters, develops an ionosphere on the dayside.