TOI-2257 b: A highly eccentric long-period sub-Neptune transiting a nearby M dwarf

¹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: nicole.schanche@unibe.ch
² Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
³ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
⁴ Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA
⁵ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁶ Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
⁷ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁸ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
⁹ Universidad Nacional de Córdoba - Observatorio Astronómico de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
¹⁰ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
¹¹ Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹² School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmimgham B15 2TT, UK
¹³ Department of Physics & Astronomy, Vanderbilt University, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 169-224, Pasadena, CA 91109, USA
¹⁵ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
¹⁶ Sternberg Astronomical Institute, Moscow State University (SAI MSU), Universitetskii pr. 13, Moscow 119991, Russia
¹⁷ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
¹⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138, USA
¹⁹ Observatoire de l’Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
²⁰ Caltech/IPAC, 1200 E. California Boulevard, Pasadena, CA 91125, USA
²¹ Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
²² Department of Physics and Astronomy, University of New Mexico, 1919 Lomas Blvd NE, Albuquerque, NM 87131, USA
²³ NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA
²⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁵ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, Mexico
²⁶ Department of Astronomy, University of Florida, Gainesville, FL, 32611, USA
²⁷ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
²⁸ Dept. of Physics & Astronomy, Swarthmore College, Swarthmore PA 19081, USA
²⁹ Department of Astronomy, Wellesley College, Wellesley, MA 02481, USA
³⁰ Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
³¹ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
³² Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA

Received: 22 September 2021
Accepted: 28 October 2021

Abstract

Context. Thanks to the relative ease of finding and characterizing small planets around M-dwarf stars, these objects have become cornerstones in the field of exoplanet studies. The current paucity of planets in long-period orbits around M dwarfs makes such objects particularly compelling as they provide clues about the formation and evolution of these systems.

Aims. In this study we present the discovery of TOI-2257 b (TIC 198485881), a long-period (35 d) sub-Neptune orbiting an M3 star at 57.8 pc. Its transit depth is about 0.4%, large enough to be detected with medium-size, ground-based telescopes. The long transit duration suggests the planet is in a highly eccentric orbit (e ~ 0.5), which would make it the most eccentric planet known to be transiting an M-dwarf star.

Methods. We combined TESS and ground-based data obtained with the 1.0-meter SAINT-EX, 0.60-meter TRAPPIST-North, and 1.2-meter FLWO telescopes to find a planetary size of 2.2 R_⊕ and an orbital period of 35.19 days. In addition, we make use of archival data, high-resolution imaging, and vetting packages to support our planetary interpretation.

Results. With its long period and high eccentricity, TOI-2257 b falls into a novel slice of parameter space. Despite the planet’s low equilibrium temperature (~256 K), its host star’s small size (R_* = 0.311 ± 0.015) and relative infrared brightness (K_mag = 10.7) make it a suitable candidate for atmospheric exploration via transmission spectroscopy.