A 43 d transiting Neptune and two 25 d Saturns from <i>TESS</i> , NGTS, and ASTEP

ABSTRACT Beyond orbital periods of 10 d, there is a dearth of known transiting gas giants. On longer orbits, planets are less affected by their host star, and become ideal probes of planet formation, migration, and evolution. We report the discovery of a long-period Neptune and two Saturns, each initially identified as single transits in the TESS (Transiting Exoplanet Survey Satellite) photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS (Next Generation Transit Survey) and ASTEP (Antarctic Search for Transiting ExoPlanets). High-resolution radial velocity mass measurements using CORALIE and HARPS (High Accuracy Radial Velocity Planet Searcher) confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of $43.12655_{-0.00017}^{+0.00012}~$ d, radius of $3.65\pm 0.22~\mathrm{R_{\hbox{$\oplus $}}}$, and mass of $19.1_{-4.5}^{+4.9}~\mathrm{M_{\hbox{$\oplus $}}}$ for NGTS-34 b, making it one of the longest period well-characterized transiting Neptunes. Orbiting a late F-type star, bright in the K band (Kmag$~\simeq 7.9$), it is amenable for cool atmosphere studies using James Webb Space Telescope or Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large). TOI-4940 b is a small Saturn on a $25.867811_{-0.000056}^{+0.000058}~$ d orbit with a radius of $6.61\pm 0.37~\mathrm{R_{\hbox{$\oplus $}}}$ and an upper mass limit $&amp;lt; 89~\mathrm{M_{\hbox{$\oplus $}}}$. NGTS-35 b(=TOI-6669 b) is a larger Saturn on a $25.241192\pm 0.000022~$ d, moderately eccentric orbit ($e = 0.192_{-0.033}^{+0.037}$), with a radius of $10.90\pm 0.65~\mathrm{R_{\hbox{$\oplus $}}}$ and a mass of $152_{-19}^{+22}~\mathrm{M_{\hbox{$\oplus $}}}$. With an assumed albedo $A=0.3$, each of these planets has an equilibrium temperature below 700 K, with NGTS-35 b especially cold at $450~$ K. These three giants add to the small but growing population of long-period planets that can further our understanding of planet formation mechanisms.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-12-11
DOI
https://doi.org/10.1093/mnras/staf2189
Primary Topic
Astro and Planetary Science
Type
article
Field-Weighted Citation Impact
0.00

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article

A 43 d transiting Neptune and two 25 d Saturns from TESS , NGTS, and ASTEP

Lauren Doyle, Douglas A. Caldwell, Maximiliano Moyano, F. Bouchy et al.
Monthly Notices of the Royal Astronomical Society
Astro and Planetary Science
article

A 43 d transiting Neptune and two 25 d Saturns from TESS , NGTS, and ASTEP

Lauren Doyle, Douglas A. Caldwell, Maximiliano Moyano, F. Bouchy, J. McCormac, M. R. Burleigh, Maximilian N. Günther, Andrew W. Mann, Michelle Kunimoto, Soléne Ulmer-Moll, M. Lendl, M. Steiner, J. Villaseñor, Daniel Bayliss, B. Lavie, M. R. Goad, Diana Dragomir, D. J. Armstrong, T. Guillot, Arvind F. Gupta, Alicia Kendall, Steve B. Howell, A. H. M. J. Triaud, S. L. Casewell, Samuel Gill, J. S. Jenkins, Veselin B. Kostov, Pamela Rowden, J.A. Acevedo Barroso, Colin Littlefield, P. J. Wheatley, Ares Osborn, Carl Ziegler, Faith Hawthorn, Edward M. Bryant, Louise D. Nielsen, Maddalena Bugatti, José I Vines, D. R. Anderson, Daniel J. Stevens, Suman Saha, Steven Villanueva, Matthew P. Battley, S. Udry, S. B. Fajardo‐Acosta, B. Scott Gaudi, Philippe Bendjoya, D. Mékarnia, Alastair B Claringbold, Toby Rodel, Ioannis Apergis, Tafadzwa Zivave, Guoyou Sun, Olga Suárez, D. R. Alves, George R. Harvey
article en

Abstract

ABSTRACT Beyond orbital periods of 10 d, there is a dearth of known transiting gas giants. On longer orbits, planets are less affected by their host star, and become ideal probes of planet formation, migration, and evolution. We report the discovery of a long-period Neptune and two Saturns, each initially identified as single transits in the TESS (Transiting Exoplanet Survey Satellite) photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS (Next Generation Transit Survey) and ASTEP (Antarctic Search for Transiting ExoPlanets). High-resolution radial velocity mass measurements using CORALIE and HARPS (High Accuracy Radial Velocity Planet Searcher) confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of $43.12655_{-0.00017}^{+0.00012}~$ d, radius of $3.65\pm 0.22~\mathrm{R_{\hbox{$\oplus $}}}$, and mass of $19.1_{-4.5}^{+4.9}~\mathrm{M_{\hbox{$\oplus $}}}$ for NGTS-34 b, making it one of the longest period well-characterized transiting Neptunes. Orbiting a late F-type star, bright in the K band (Kmag$~\simeq 7.9$), it is amenable for cool atmosphere studies using James Webb Space Telescope or Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large). TOI-4940 b is a small Saturn on a $25.867811_{-0.000056}^{+0.000058}~$ d orbit with a radius of $6.61\pm 0.37~\mathrm{R_{\hbox{$\oplus $}}}$ and an upper mass limit $&amp;lt; 89~\mathrm{M_{\hbox{$\oplus $}}}$. NGTS-35 b(=TOI-6669 b) is a larger Saturn on a $25.241192\pm 0.000022~$ d, moderately eccentric orbit ($e = 0.192_{-0.033}^{+0.037}$), with a radius of $10.90\pm 0.65~\mathrm{R_{\hbox{$\oplus $}}}$ and a mass of $152_{-19}^{+22}~\mathrm{M_{\hbox{$\oplus $}}}$. With an assumed albedo $A=0.3$, each of these planets has an equilibrium temperature below 700 K, with NGTS-35 b especially cold at $450~$ K. These three giants add to the small but growing population of long-period planets that can further our understanding of planet formation mechanisms.

Monthly Notices of the Royal Astronomical SocietyVol. 547(2)
Universidad de Santiago de Chile (CL), University of North Carolina at Chapel Hill (US), University of Geneva (CH), University of Bern (CH), Leiden University (NL), California Institute of Technology (US), Queen's University Belfast (GB), Ames Research Center (US), Royal Astronomical Society (GB), Goddard Space Flight Center (US), U.S. National Science Foundation (US), University of British Columbia (CA), University of Leicester (GB), Queen Mary University of London (GB), University of New Mexico (US), Stephen F. Austin State University (US), Xinjiang Astronomical Observatory (CN), Universidad Católica del Norte (CL), Department of Space (IN), University of Warwick (GB), Bay Area Environmental Research Institute (US), Xingming Observatory (CN), University of Minnesota, Duluth (US), Observatoire de la Côte d’Azur (FR), NSF NOIRLab (US), Infrared Processing and Analysis Center (US), European Space Research and Technology Centre (NL), University College London (GB), École Polytechnique Fédérale de Lausanne (CH), The Ohio State University (US), Massachusetts Institute of Technology (US), University of Chile (CL), University of Birmingham (GB), Ludwig-Maximilians-Universität München (DE), International Space Science Institute (CH), Search for Extraterrestrial Intelligence (US), Universidad Diego Portales (CL), McMaster University (CA)
National Science Foundation, National Aeronautics and Space Administration, Ohio State University, California Institute of Technology, Michigan State University, UK Research and Innovation, European Commission, Agence Nationale de la Recherche, Ministério da Ciência, Tecnologia e Inovação, Nuclear Safety and Security Commission, Korea Astronomy and Space Science Institute, Institut Polaire Français Paul Emile Victor, Université de Genève, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Ames Research Center, College of Engineering, Michigan State University, University of North Carolina at Chapel Hill, Science Mission Directorate, Natural Sciences and Engineering Research Council of Canada, Science and Technology Facilities Council
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