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

¹ Research and Scientific Support Department, European Space Agency (ESA/ESTEC), PO Box 299, 2200 AG Noordwijk, The Netherlands
e-mail: dgandolf@rssd.esa.int
² Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 La Laguna, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁵ INAF−Osservatorio Astrofisico di Catania, Via S. Sofia, 78, 95123 Catania, Italy
⁶ Physics Department “E. Fermi”, University of Pisa, largo B. Pontecorvo 3, 56127 Pisa, Italy
⁷ Istituto Nazionale di Fisica Nucleare, largo B. Pontecorvo 3, 56127 Pisa, Italy
⁸ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv Uni., 69978 Tel Aviv, Israel
⁹ Department of Physics, University of Oxford, Oxford, OX1 3RH, UK
¹⁰ Stellar Astrophysics Centre, Department of Physics and Astronomy, Århus Uni., Ny Munkegade 120, 8000 Århus C, Denmark
¹¹ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
¹² KU Leuven, Centre for mathematical Plasma Astrophysics, Celestijnenlaan 200B, 3001 Leuven, Belgium
¹³ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
¹⁴ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 16 May 2013
Accepted: 28 June 2013

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.