TOI-179: A young system with a transiting compact Neptune-mass planet and a low-mass companion in outer orbit^★,★★

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: silvano.desidera@inaf.it
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
⁴ Aix-Marseille Univ., CNRS, CNES, LAM, 13000 Marseille, France
⁵ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Clayton 3800, Melbourne, Australia
⁶ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁷ Dipartimento di Fisica e Astronomia – Universtà di Padova, Via Marzolo 8, 35121 Padova, Italy
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Dipartimento di Fisica e Astronomia – Universtà di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁰ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, Netherlands
¹³ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁴ SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Laboratory for Atmospheric and Space Physics 1234 Innovation Dr. Boulder, CO 80303, USA
¹⁶ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
¹⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁸ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
¹⁹ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
²⁰ University of Exeter, Astrophysics Group, Physics Building, Stocker Road, Exeter EX4 4QL, UK

Received: 27 July 2022
Accepted: 25 September 2022

Abstract

Context. Transiting planets around young stars are key benchmarks for our understanding of planetary systems. One such candidate, TOI-179, was identified around the K dwarf HD 18599 by TESS.

Aims. We present the confirmation of the transiting planet and the characterization of the host star and of the TOI-179 system over a broad range of angular separations.

Methods. We exploited the TESS photometric time series, intensive radial velocity monitoring performed with HARPS, and deep high-contrast imaging observations obtained with SPHERE and NACO at VLT. The inclusion of Gaussian process regression analysis was effective to properly model the magnetic activity of the star and identify the Keplerian signature of the transiting planet.

Results. The star, with an age of 400±100 Myr, is orbited by a transiting planet with period 4.137436 days, mass 24±7 M_⊕, radius 2.62_−0.12^+0.15 R_⊕, and significant eccentricity (0.34_−0.09^+0.07). Adaptive optics observations identified a low-mass companion at the boundary between brown dwarfs and very low-mass stars (mass derived from luminosity 83₋₆⁺⁴ M_J) at a very small projected separation (84.5 mas, 3.3 au at the distance of the star). Coupling the imaging detection with the long-term radial velocity trend and the astrometric signature, we constrained the orbit of the low-mass companion, identifying two families of possible orbital solutions.

Conclusions. The TOI-179 system represents a high-merit laboratory for our understanding of the physical evolution of planets and other low-mass objects and of how the planet properties are influenced by dynamical effects and interactions with the parent star.