A Model for Thermal Phase Variations of Circular and Eccentric Exoplanets

TL;DR

This paper introduces a semi-analytic model for exoplanet atmospheres that explains phase curve variations, accounting for orbital geometry, advection, and re-radiation, and applies it to eccentric planets to infer wind and radiative properties.

Contribution

The study develops a new semi-analytic model calibrated with HD 189733b, enabling analysis of phase variations for eccentric exoplanets and linking observable features to atmospheric dynamics.

Findings

Phase curve morphology depends on eccentricity and argument of pericenter.

Timing of phase maximum indicates wind direction but not wind speed or radiative time.

Eccentric planets can exhibit 'ringing' allowing measurement of wind speed and radiative time.

Abstract

We present a semi-analytic model atmosphere for close-in exoplanets that captures the essential physics of phase curves: orbital and viewing geometry, advection, and re-radiation. We calibrate the model with the well-characterized transiting planet, HD 189733b, then compute light curves for seven of the most eccentric transiting planets. We present phase variations for a variety of different radiative times and wind speeds. In the limit of instant re-radiation, the light curve morphology is entirely dictated by the planet's eccentricity and argument of pericenter: the light curve maximum leads or trails the eclipse depending on whether the planet is receding from or approaching the star at superior conjunction, respectively. For a planet with non-zero radiative timescales, the phase peak occurs early for super- rotating winds, and late for sub-rotating winds. We find that for a circular orbit, the timing of the phase variation maximum with respect to superior conjunction indicates the direction of the dominant winds, but cannot break the degeneracy between wind speed and radiative time. For circular planets the phase minimum occurs half an orbit away from the phase maximum -despite the fact that the coolest longitudes are always near the dawn terminator- and therefore does not convey any additional information. In general, increasing the advective frequency or the radiative time has the effect of reducing the peak-to-trough amplitude of phase variations, but there are interesting exceptions to these trends. Lastly, eccentric planets with orbital periods significantly longer than their radiative time exhibit "ringing" whereby the hot spot generated at periastron rotates in and out of view. The existence of ringing makes it possible to directly measure the wind speed (the frequency of the ringing) and the radiative time constant (the damping of the ringing).