Two long-period transiting exoplanets on eccentric orbits: NGTS-20 b (TOI-5152 b) and TOI-5153 b^★

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Versoix, Switzerland
e-mail: solene.ulmer-moll@unige.ch
² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Av. Diagonal las Torres 2640, Peñalolén, Santiago, Chile
⁷ Millennium Institute for Astrophysics, Chile
⁸ Department of Physics and Astronomy, University of New Mexico, 1919 Lomas Blvd NE Albuquerque, NM 87131, USA
⁹ Physikalisches Institut, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁰ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹¹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹² Institute of Planetary Research, German Aerospace Center, Ruther-fordstrasse 2, 12489 Berlin, Germany
¹³ Fisk University, Nashville, TN, USA
¹⁴ Vanderbilt University, Nashville, TN, USA
¹⁵ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
¹⁶ Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS, UK
¹⁷ Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
¹⁸ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹⁹ Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
²⁰ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
²¹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
²² Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, Santiago, Chile
²³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
²⁴ Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE Albuquerque, NM 87106, USA
²⁵ Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, 1270709, Antofagasta, Chile
²⁶ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012, Bern, Switzerland
²⁷ Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA
²⁸ ETH Zurich, Department of Physics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³⁰ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Macul, Santiago, Chile
³¹ Millennium Institute of Astrophysics, Nuncio Monsenor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³² Department of Astronomy/Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
³³ Department of Astronomy, Sofia University “St Kliment Ohridski”, 5 James Bourchier Blvd, BG-1164 Sofia, Bulgaria
³⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³⁵ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
³⁶ Centre for Astrophysics, University of Southern Queensland, West Street, Toowoomba, QLD 4350, Australia

Received: 18 March 2022
Accepted: 6 July 2022

Abstract

Context. Long-period transiting planets provide the opportunity to better understand the formation and evolution of planetary systems. Their atmospheric properties remain largely unaltered by tidal or radiative effects of the host star, and their orbital arrangement reflects a different and less extreme migrational history compared to close-in objects. The sample of long-period exoplanets with well-determined masses and radii is still limited, but a growing number of long-period objects reveal themselves in the Transiting Exoplanet Survey Satellite (TESS) data.

Aims. Our goal is to vet and confirm single-transit planet candidates detected in the TESS space-based photometric data through spectroscopic and photometric follow-up observations with ground-based instruments.

Methods. We used high-resolution spectrographs to confirm the planetary nature of the transiting candidates and measure their masses. We also used the Next Generation Transit Survey (NGTS) to photometrically monitor the candidates in order to observe additional transits. Using a joint modeling of the light curves and radial velocities, we computed the orbital parameters of the system and were able to precisely measure the mass and radius of the transiting planets.

Results. We report the discovery of two massive, warm Jupiter-size planets, one orbiting the F8-type star TOI-5153 and the other orbiting the G1-type star NGTS-20 (=TOI-5152). From our spectroscopic analysis, both stars are metal rich with a metallicity of 0.12 and 0.15, respectively. Only TOI-5153 presents a second transit in the TESS extended mission data, but NGTS observed NGTS-20 as part of its mono-transit follow-up program and detected two additional transits. Follow-up high-resolution spectroscopic observations were carried out with CORALIE, CHIRON, FEROS, and HARPS. TOI-5153 hosts a planet with a period of 20.33 days, a planetary mass of 3.26_−0.17^+0.18 Jupiter masses (M_J), a radius of 1.06_−0.04^+0.04 R_J, and an orbital eccentricity of 0.091_−0.026^+0.024. NGTS-20 b is a 2.98_−0.15^+0.16 M_J planet with a radius of 1.07_−0.04^+0.04 R_J on an eccentric 0.432_−0.023^+0.023 orbit with an orbital period of 54.19 days. Both planets are metal enriched and their heavy element content is in line with the previously reported mass-metallicity relation for gas giants.

Conclusions. Both warm Jupiters orbit moderately bright host stars, making these objects valuable targets for follow-up studies of the planetary atmosphere and measurement of the spin-orbit angle of the system.