Kepler-77b: a very low albedo, Saturn-mass transiting planet around a metal-rich solar-like star

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Abstract
We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry (quarters 1−13) with high-resolution spectroscopy from the Sandiford at McDonald and FIES at NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of M p = 0.430 ± 0.032 M Jup, a radius of R p = 0.960 ± 0.016 R Jup, and a bulk density of ρ p = 0.603 ± 0.055 g cm-3. It orbits a slowly rotating (P rot = 36 ± 6 days) G5 V star with M ⋆ = 0.95 ± 0.04 M ⊙, R ⋆ = 0.99 ± 0.02 R ⊙, T eff = 5520 ± 60 K, [M/H] = 0.20 ± 0.05 dex, that has an age of 7.5 ± 2.0 Gyr. The lack of detectable planetary occultation with a depth higher than ~10 ppm implies a planet geometric and Bond albedo of A g ≤ 0.087 ± 0.008 and A B ≤ 0.058 ± 0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of ~0.5 min, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transitsobserved in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet’s passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.

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