HD 207897 b: A dense sub-Neptune transiting a nearby and bright K-type star

¹ Department of Physics, Shahid Beheshti University, Tehran, Iran
² 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
³ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Observatoire de Haute-Provence, CNRS, Université d’Aix- Marseille, 04870 Saint-Michel-l’Observatoire, France
⁷ Institut d’astrophysique de Paris, UMR 7095 CNRS université Pierre et Marie Curie, 98 bis, boulevard Arago, 75014 Paris, France
⁸ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Sauverny, Switzerland
⁹ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus, 28692, Villanueva de la Cañada (Madrid), Spain
¹⁰ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
¹¹ Canada France Hawaii Telescope Corporation (CFHT), 65-1238 Mamalahoa Hwy, Kamuela HI 96743 USA
¹² Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
¹³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁷ University of Maryland, Baltimore County, 1000 Hilltop Cir, Baltimore, MD 21250, USA
¹⁸ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
¹⁹ Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA 91125, USA
²⁰ Department of Physics & Astronomy, University of California Irvine, Irvine, CA 92697, USA
²¹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95060, USA
²² Department of Physics & Astronomy, University of Kansas, 1082 Malott, 1251 Wescoe Hall Dr., Lawrence, KS 66045, USA
²³ Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²⁴ International Center for Advanced Studies (ICAS) and ICIFI (CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, (1650) Buenos Aires, Argentina
²⁵ Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ Department of Physics and Astronomy, University of New Mexico, 1919 Lomas Blvd NE, Albuquerque, NM 87131, USA
²⁷ Department of Astronomy, University of California Berkeley, Berkeley, CA, 94720, USA
²⁸ Division of Geological and Planetary Sciences 1200 E California Blvd, Pasadena, CA, 91125, USA
²⁹ Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA
³⁰ NASA Exoplanet Science Institute/Caltech-IPAC, MC 314-6, 1200 E. California Blvd., Pasadena, CA 91125, USA
³¹ Millennium Institute for Astrophysics, Chile
³² Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Chile
³³ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³⁴ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA
³⁵ Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD, Australia
³⁶ Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA
³⁷ Las Campanas Observatory, Carnegie Institution of Washington, Colina el Pino, Casilla 601 La Serena, Chile
³⁸ Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles St, Baltimore, MD 21218, USA
³⁹ Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
⁴⁰ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195, Meudon, France
⁴¹ Univ. de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
⁴² Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
⁴³ SETI Institute/NASA Ames Research Center, USA
⁴⁴ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
⁴⁵ Department of Astrophysical Sciences, Princeton University, 4Ivy Lane, Princeton, NJ 08544, USA
⁴⁶ NASA Ames Research Center, Moffett Field, CA, 94035, USA
⁴⁷ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁴⁸ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
⁴⁹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁵⁰ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Received: 29 May 2021
Accepted: 8 September 2021

Abstract

We present the discovery and characterization of a transiting sub-Neptune that orbits the nearby (28 pc) and bright (V = 8.37) K0V star HD 207897 (TOI-1611) with a 16.20-day period. This discovery is based on photometric measurements from the Transiting Exoplanet Survey Satellite mission and radial velocity (RV) observations from the SOPHIE, Automated Planet Finder, and HIRES high-precision spectrographs. We used EXOFASTv2 to model the parameters of the planet and its host star simultaneously, combining photometric and RV data to determine the planetary system parameters. We show that the planet has a radius of 2.50 ± 0.08 R_E and a mass of either 14.4 ± 1.6 M_E or 15.9 ± 1.6 M_E with nearly equal probability. The two solutions correspond to two possibilities for the stellar activity period. The density accordingly is either 5.1 ± 0.7 g cm⁻³ or 5.5_−0.7^+0.8 g cm⁻³, making it one of the relatively rare dense sub-Neptunes. The existence of this dense planet at only 0.12 AU from its host star is unusual in the currently observed sub-Neptune (2 < R_E < 4) population. The most likely scenario is that this planet has migrated to its current position.