The SOPHIE search for northern extrasolar planets

XIX. A system including a cold sub-Neptune potentially transiting a V = 6.5 star HD 88986^★

¹ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
² Institut d’astrophysique de Paris, UMR 7095 CNRS université Pierre et Marie Curie, 98 bis boulevard Arago, 75014 Paris, France
e-mail: heidari@iap.fr
³ Laboratoire J.-L. Lagrange, Observatoire de la Côte d’Azur (OCA), Universite de Nice-Sophia Antipolis (UNS), CNRS, Campus Valrose, 06108 Nice Cedex 2, France
⁴ Department of Physics, Shahid Beheshti University, Tehran, Iran
⁵ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Sauverny, Switzerland
⁶ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife KY16 9SS, UK
⁷ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁸ Observatoire de Haute-Provence, CNRS, Universite d’Aix-Marseille, 04870 Saint-Michel-l’Observatoire, France
⁹ Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
¹⁰ Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Blvd., PO Box 9501, Nashville, TN 37209, USA
¹¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹³ Max-Planck-Institut für Astronomie, Heidelberg, Germany
¹⁴ Canada France Hawaii Telescope Corporation (CFHT), 65-1238 Mamalahoa Hwy, Kamuela HI 96743 USA
¹⁵ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
¹⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁷ Astrophysics Group, University of Exeter, Exeter EX4 2QL, UK
¹⁸ Univ. de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
¹⁹ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100 Providencia, Santiago, Chile
²⁰ International Center for Advanced Studies (ICAS) and ICIFI (CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, (1650) Buenos Aires, Argentina
²¹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
²² Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
²³ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁴ Instituto de Estudios Astrofísicos, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
²⁵ NASA Ames Research Center, Moffett Field, CA, 94035, USA
²⁶ Research Institute for Advanced Computer Science, Universities Space Research Association, Washington, DC 20024, USA
²⁷ Observatoire François-Xavier Bagnoud – OFXB, 3961 Saint-Luc, Switzerland
²⁸ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³⁰ Université de Paris Cité and Univ. Paris-Est Creteil, CNRS, LISA, 75013 Paris, France
³¹ Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, Santiago, Chile
³² Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, 1270709 Antofagasta, Chile

Received: 7 September 2023
Accepted: 14 November 2023

Abstract

Transiting planets with orbital periods longer than 40 d are extremely rare among the 5000+ planets discovered so far. The lack of discoveries of this population poses a challenge to research into planetary demographics, formation, and evolution. Here, we present the detection and characterization of HD 88986 b, a potentially transiting sub-Neptune, possessing the longest orbital period among known transiting small planets (<4 R_⊕) with a precise mass measurement (σ_M/M > 25%). Additionally, we identified the presence of a massive companion in a wider orbit around HD 88986. To validate this discovery, we used a combination of more than 25 yr of extensive radial velocity (RV) measurements (441 SOPHIE data points, 31 ELODIE data points, and 34 HIRES data points), Gaia DR3 data, 21 yr of photometric observations with the automatic photoelectric telescope (APT), two sectors of TESS data, and a 7-day observation of CHEOPS. Our analysis reveals that HD 88986 b, based on two potential single transits on sector 21 and sector 48 which are both consistent with the predicted transit time from the RV model, is potentially transiting. The joint analysis of RV and photometric data show that HD 88986 b has a radius of 2.49 ± 0.18 R_⊕, a mass of 17.2_−3.8^+4.0 M_⊕, and it orbits every 146.05_−0.40^+0.43 d around a subgiant HD 88986 which is one of the closest and brightest exoplanet host stars (G2Vtype, R = 1.543 ± 0.065 R_⊙, V = 6.47 ± 0.01 mag, distance = 33.37 ± 0.04 pc). The nature of the outer, massive companion is still to be confirmed; a joint analysis of RVs, HIPPARCOS, and Gaia astrometric data shows that with a 3σ confidence interval, its semi-major axis is between 16.7 and 38.8 au and its mass is between 68 and 284 M_Jup. HD 88986 b’s wide orbit suggests the planet did not undergo significant mass loss due to extreme-ultraviolet radiation from its host star. Therefore, it probably maintained its primordial composition, allowing us to probe its formation scenario. Furthermore, the cold nature of HD 88986 b (460 ± 8 K), thanks to its long orbital period, will open up exciting opportunities for future studies of cold atmosphere composition characterization. Moreover, the existence of a massive companion alongside HD 88986 b makes this system an interesting case study for understanding planetary formation and evolution.