Effects of Orbital Eccentricity on Extrasolar Planet Transit Detection and Lightcurves

TL;DR

This paper explores how orbital eccentricity affects transit detection and lightcurve analysis of extrasolar planets, revealing biases, measurement challenges, and potential methods to infer eccentricity from photometry.

Contribution

It introduces a quantitative analysis of eccentricity effects on transit probability, duration, and lightcurve asymmetry, and discusses implications for habitability assessments.

Findings

Eccentric planets are more likely to transit than circular ones at the same semimajor axis.

Transit duration varies systematically with eccentricity and periastron longitude.

Current technology cannot measure transit asymmetry caused by eccentricity.

Abstract

It is shown herein that planets with eccentric orbits are more likely to transit than circularly orbiting planets with the same semimajor axis by a factor of (1-e^2)^{-1}. If the orbital parameters of discovered transiting planets are known, as from follow-up radial velocity observations, then the transit-detected planet population is easily debiased of this effect. The duration of a planet's transit depends upon of its eccentricity and longitude of periastron; transits near periastron are shorter, and those near apoastron last longer, for a given impact parameter. If fitting for the stellar radius with the other transit parameters, this effect causes a systematic error in the resulting measurements. If the stellar radius is instead held fixed at a value measured independently, then it is possible to place a lower limit on the planet's eccentricity using photometry alone. Orbital accelerations cause a difference in the planet's ingress and egress durations that lead to an asymmetry in the transit lightcurve that could be used along with the transit velocity measurement to uniquely measure the planet's eccentricity and longitude of periapsis. However, the effect is too small to be measured with current technology. The habitability of transiting terrestrial planets found by Kepler depends on those planets' orbital eccentricities. While Kepler will be able to place lower limits on those planets' orbital eccentricity, the actual value for any given planet will likely remain unknown.