The GAPS programme at TNG

XXIII. HD 164922 d: close-in super-Earth discovered with HARPS-N in a system with a long-period Saturn mass companion^★,★★

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
e-mail: serena.benatti@inaf.it
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Dipartimento di Fisica e Astronomia G. Galilei – Universtà degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁸ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
⁹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹⁰ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹³ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹⁴ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
¹⁵ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁶ Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁷ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁹ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
²⁰ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
²¹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, 50125 Firenze, Italy

Received: 12 March 2020
Accepted: 6 May 2020

Abstract

Context. Observations of exoplanetary systems demonstrate that a wide variety of planetary architectures are possible. Determining the rate of occurrence of Solar System analogues – with inner terrestrial planets and outer gas giants – remains an open question.

Aims. Within the framework of the Global Architecture of Planetary Systems (GAPS) project, we collected more than 300 spectra with HARPS-N at the Telescopio Nazionale Galileo for the bright G9V star HD 164922. This target is known to host one gas giant planet in a wide orbit (P_b ~1200 days, semi-major axis ~ 2 au) and a Neptune-mass planet with a period of P_c ~76 days. We aimed to investigate the presence of additional low-mass companions in the inner region of the system.

Methods. We compared the radial velocities (RV) and the activity indices derived from the HARPS-N time series to measure the rotation period of the star and used a Gaussian process regression to describe the behaviour of the stellar activity. We then combined a model of planetary and stellar activity signals in an RV time series composed of almost 700 high-precision RVs, both from HARPS-N and literature data. We performed a dynamical analysis to evaluate the stability of the system and the allowed regions for additional potential companions. We performed experiments on the injection and recovery of additional planetary signals to gauge the sensitivity thresholds in minimum mass and orbital separation imposed by our data.

Results. Thanks to the high sensitivity of the HARPS-N dataset, we detected an additional inner super-Earth with an RV semi-amplitude of 1.3 ± 0.2 m s⁻¹ and a minimum mass of m_d sin i = 4 ± 1 M_⊕. It orbits HD 164922 with a period of 12.458 ± 0.003 days. We disentangled the planetary signal from activity and measured a stellar rotation period of ~ 42 days. The dynamical analysis shows the long-term stability of the orbits of the three-planet system and allows us to identify the permitted regions for additional planets in the semi-major axis ranges 0.18–0.21 au and 0.6–1.4 au. The latter partially includes the habitable zone of the system. We did not detect any planet in these regions, down to minimum detectable masses of 5 and 18 M_⊕, respectively. A larger region of allowed planets is expected beyond the orbit of planet b, where our sampling rules out bodies with minimum mass >50 M_⊕. The planetary orbital parameters and the location of the snow line suggest that this system has been shaped by a gas disk migration process that halted after its dissipation.