@misc{10481/72660,
year = {2022},
month = {1},
url = {http://hdl.handle.net/10481/72660},
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 similar to 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-circle plus 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 (similar to 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.},
organization = {Swiss National Science Foundation (SNSF)},
organization = {European Commission	PP00P2-163967	
PP00P2-190080},
organization = {MIT's Kavli Institute},
organization = {European Space Agency
European Commission},
organization = {Ministry of Science and Higher Education of the Russian Federation	075-15-2020-780 (N13.1902.21.0039)},
organization = {Heising-Simons Foundation},
organization = {French Community of Belgium},
organization = {DGAPA},
organization = {Programa de Apoyo a Proyectos de Investigacion e Innovacion Tecnologica (PAPIIT)
Universidad Nacional Autonoma de Mexico	BG-101321},
organization = {TESS Guest Investigator Program	80NSSC19K1727},
organization = {NASA Exoplanet Research Program	18-2XRP18_2-0136},
organization = {Centre for Space and Habitability (CSH) of the University of Bern},
organization = {Swiss National Science Foundation (SNSF)},
organization = {European Research Council (ERC)	336480},
organization = {Actions de Recherche Concertee (ARC) grant - Wallonia-Brussels Federation},
organization = {UK Research & Innovation (UKRI)},
organization = {Science & Technology Facilities Council (STFC)},
organization = {Science and Technology Development Fund (STDF)	ST/S00193X/1},
organization = {Fonds de la Recherche Scientifique - FNRS	T010920F},
organization = {Simons Foundation	327127},
organization = {MERAC foundation},
organization = {Fonds de la Recherche Scientifique - FNRS	PDR T.0120.21},
organization = {Swiss National Science Foundation (SNSF)},
organization = {National Science Foundation (NSF)},
organization = {NASA Exoplanet Exploration Program		
NASA's Science Mission Directorate},
organization = {European Research Council (ERC)	679030	
803193/BEBOP},
organization = {Balzan Prize Foundation		
BEL-SPO/BRAIN2.0 research program (PORTAL project)},
publisher = {EDP},
keywords = {Planets and satellites: detection},
keywords = {Stars: individual: TOI-2257},
keywords = {Stars: individual: TIC 198485881},
keywords = {Techniques: photometric},
title = {TOI-2257 b: A highly eccentric long-period sub-Neptune transiting a nearby M dwarf},
doi = {10.1051/0004-6361/202142280},
author = {Schanche, N. and Dévora Pajares, Martín},
}