In-depth characterization of the Kepler-10 three-planet system with HARPS-N radial velocities and Kepler transit timing variations

¹ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Academy for the Mathematical Sciences, c/o Isaac Newton Institute for Mathematical Sciences, 20 Clarkson Road, Cambridge CB3 0EH, UK
⁴ Astrophysics Group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
⁶ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁷ Université Aix Marseille, CNRS, CNES, LAM, Marseille, France
⁸ Department of Physics, University of Oxford, Oxford OX1 3RH, UK
⁹ Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 2, 8093 Zurich, Switzerland
¹⁰ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹³ SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁴ Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3FD, UK
¹⁵ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁶ DTU Space, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
¹⁷ Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK
¹⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Tenerife, Spain
¹⁹ Department of Astrophysics, University of Exeter, Stocker Rd, Exeter EX4 4QL, UK
²⁰ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
²¹ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
²² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy

^★ Corresponding author; aldo.bonomo@inaf.it

Received: 15 November 2024
Accepted: 10 February 2025

Abstract

The old G3V star Kepler-10 is known to host two transiting planets, the ultra-short-period super-Earth Kepler-10 b (P_b = 0.837 d; R_b = 1.47 R_⊕) and the long-period sub-Neptune Kepler-10 c (P_c = 45.294 d; R_c = 2.35 R_⊕), and a non-transiting planet that causes variations in the Kepler-10 c transit times. Measurements of the mass of Kepler-10 c in the literature have shown disagreement, depending on the radial-velocity dataset and/or the modeling technique used. Here we report on the analysis of almost 300 high-precision radial velocities gathered with the HARPS-N spectrograph at the Telescopio Nazionale Galileo over ∼11 years, and extracted with the YARARA-v2 tool, which corrects for possible systematics and/or low-level activity variations at the spectrum level. To model these radial velocities, we used three different noise models and various numerical techniques, which all converged to the solution: M_b = 3.24 ± 0.32 M_⊕ (10σ) and ρ_b = 5.54 ± 0.64 g cm⁻³ for planet b; M_c = 11.29 ± 1.24 M_⊕ (9σ) and ρ_c = 4.75 ± 0.53 g cm⁻³ for planet c; and M_d sin i = 12.00 ± 2.15 M_⊕ (6 σ) and P_d = 151.06 ± 0.48 d for the non-transiting planet Kepler-10 d. This solution is further supported by the analysis of the Kepler-10 c transit timing variations and their simultaneous modeling with the HARPS-N radial velocities. While Kepler-10 b is consistent with a rocky composition and a small or no iron core, Kepler-10 c may be a water world that formed beyond the water snowline and subsequently migrated inward.