Average Albedos of Close-in Super-Earths and Neptunes from Statistical Analysis of Long Cadence Kepler Secondary Eclipse Data

TL;DR

This study estimates the average albedos of small, close-in exoplanets using long cadence Kepler data, revealing they are darker than hot Jupiters and providing insights into their atmospheric properties.

Contribution

It adapts a method for averaging light curves to long cadence data and applies it to smaller planet groups, expanding previous work and identifying false positives and orbital characteristics.

Findings

Average albedos are 0.11, 0.05, and 0.11 for different size groups.

All groups are darker than hot Jupiters, but not as dark as previously suggested.

Eclipsing binary status confirmed for some KOIs; eccentric orbit indicated for Kepler-4b.

Abstract

We present the results of our work to determine the average albedo for small, close-in planets in the {\it Kepler} candidate catalog. We have adapted our method of averaging short cadence light curves of multiple Kepler planet candidates to long cadence data, in order to detect an average albedo for the group of candidates. Long cadence data exist for many more candidates than the short cadence, and so we separate the candidates into smaller radius bins than in our previous work: 1-2 Rearth, 2-4 Rearth, and 4-6 Rearth. We find that on average, all three groups appear darker than suggested by the short cadence result, but not as dark as many hot Jupiters. The average geometric albedos for the three groups are 0.11 $\pm$ 0.06, 0.05 $\pm$ 0.04, and 0.11 $\pm$ 0.08, respectively, for the case where heat is uniformly distributed about the planet. If heat redistribution is inefficient, the albedos are even lower, since there will be a greater thermal contribution to the total light from the planet. We confirm that newly-identified false positive Kepler Object of Interest (KOI) 1662.01 is indeed an eclipsing binary at twice the period listed in the planet candidate catalog. We also newly identify planet candidate KOI 4351.01 as an eclipsing binary, and we report a secondary eclipse measurement for Kepler-4b (KOI 7.01) of $\sim$ 7.50 ppm at a phase of $\sim$ 0.7, indicating that the planet is on an eccentric orbit.