On the Existence of Shocks in Irradiated Exoplanetary Atmospheres

TL;DR

This paper investigates the conditions under which shocks form in the supersonic flows of irradiated exoplanetary atmospheres, emphasizing the role of flow characteristics and temperature effects on shock development.

Contribution

It demonstrates that supersonic flows can be isentropic or non-isentropic, and identifies the criteria for shock formation based on flow characteristics and Mach number analysis.

Findings

Shocks are likely to form on the dayside hemisphere of hot Jupiters.

Supersonic flow can exist without shocks if flow characteristics do not intersect.

Temperature gradients influence shock development and flow behavior.

Abstract

Supersonic flows are expected to exist in the atmospheres of irradiated exoplanets, but the question of whether shocks develop lingers. Specifically, it reduces to whether continuous flow in a closed loop may become supersonic and if some portions of the supersonic flow steepen into shocks. We first demonstrate that continuous, supersonic flow may exist in two flavors: isentropic and non-isentropic, with shocks being included in the latter class of solutions. Supersonic flow is a necessary but insufficient condition for shocks to develop. The development of a shock requires the characteristics of neighboring points in a flow to intersect. We demonstrate that the intersection of characteristics may be quantified via knowledge of the Mach number. Finally, we examine 3D simulations of hot Jovian atmospheres and demonstrate that shock formation is expected to occur mostly on the dayside hemisphere, upstream of the substellar point, because the enhanced temperatures near the substellar point provide a natural pressure barrier for the returning flow. Understanding the role of shocks in irradiated exoplanetary atmospheres is relevant to correctly modeling observables such as the peak offsets of infrared phase curves.