Transiting planets - lightcurve analysis for eccentric orbits

TL;DR

This paper introduces an analytic model for analyzing lightcurves of transiting planets in eccentric orbits, revealing effects like degeneracy, asymmetry, and transit time shifts, and applies it to reanalyze HD 209458 b data.

Contribution

The paper presents a new analytic model for transiting planet lightcurves in eccentric orbits, extending previous circular orbit models and analyzing their effects on transit observations.

Findings

Identified degeneracy in eccentricity solutions from lightcurves

Detected asymmetry and mid-transit time shifts due to eccentricity

Reanalyzed HD 209458 b data, finding a 1% larger planetary radius

Abstract

Transiting planet lightcurves have historically been used predominantly for measuring the depth and hence ratio of the planet-star radii, p. Equations have been previously presented by Seager & Mallen-Ornelas (2003) for the analysis of the total and trough transit lightcurve times to derive the ratio of semi-major axis to stellar radius, a/R*, in the case of circular orbits. Here, a new analytic model is proposed which operates for the more general case of an eccentric orbit. We aim to investigate three major effects our model predicts: i) the degeneracy in transit lightcurve solutions for eccentricity, e>0 ii) the asymmetry of the lightcurve and the resulting shift in the mid-transit time, Tmid iii) the effect of eccentricity on the ingress and egress slopes. It is shown that a system with changing eccentricity and inclination may produce a long period transit time variation (LTTV). Furthermore, we use our model in a reanalysis of HD 209458 b archived data by Richardson et al. (2006), where we include the confirmed non-zero eccentricity and derive a 24 micron planetary radius of R_P = 1.275 +- 0.082 R_J (where R_J = 1 Jovian radius), which is 1% larger than is we assume a circular orbit.