Photometric Orbits of Extrasolar Planets

TL;DR

This paper introduces the concept of photometric orbits (PhO) for extrasolar planets, analyzing their detectability and parameter recovery using Kepler data, with implications for determining true planetary masses.

Contribution

It defines the photometric orbit framework, studies its application to short-period giant planets, and demonstrates how Kepler data can recover orbital parameters and break mass degeneracies.

Findings

Kepler can determine orbital solutions for many short-period giant planets.

Monte Carlo experiments show successful recovery of PhO parameters from synthetic data.

Photometry can help estimate true planetary masses by breaking inclination degeneracies.

Abstract

We define and analyze the photometric orbit (PhO) of an extrasolar planet observed in reflected light. In our definition, the PhO is a Keplerian entity with six parameters: semimajor axis, eccentricity, mean anomaly at some particular time, argument of periastron, inclination angle, and effective radius, which is the square root of the geometric albedo times the planetary radius. Preliminarily, we assume a Lambertian phase function. We study in detail the case of short-period giant planets (SPGPs) and observational parameters relevant to the Kepler mission: 20 ppm photometry with normal errors, 6.5 hour cadence, and three-year duration. We define a relevant "planetary population of interest" in terms of probability distributions of the PhO parameters. We perform Monte Carlo experiments to estimate the ability to detect planets and to recover PhO parameters from light curves. We calibrate the completeness of a periodogram search technique, and find structure caused by degeneracy. We recover full orbital solutions from synthetic Kepler data sets and estimate the median errors in recovered PhO parameters. We treat in depth a case of a Jupiter body-double. For the stated assumptions, we find that Kepler should obtain orbital solutions for many of the 100-760 SPGP that Jenkins & Doyle (2003) estimate Kepler will discover. Because most or all of these discoveries will be followed up by ground-based radial-velocity observations, the estimates of inclination angle from the PhO may enable the calculation of true companion masses: Kepler photometry may break the "m sin i" degeneracy.