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

Schanche, N.; Pozuelos, F. J.; Günther, M. N.; Wells, R. D.; Burgasser, A. J.; Chinchilla, P.; Delrez, L.; Ducrot, E.; Garcia, L. J.; Gómez Maqueo Chew, Y.; Jofré, E.; Rackham, B. V.; Sebastian, D.; Stassun, K. G.; Stern, D.; Timmermans, M.; Barkaoui, K.; Belinski, A.; Benkhaldoun, Z.; Benz, W.; Bieryla, A.; Bouchy, F.; Burdanov, A.; Charbonneau, D.; Christiansen, J. L.; Collins, K. A.; Demory, B. -O.; Dévora-Pajares, M.; de Wit, J.; Dragomir, D.; Dransfield, G.; Furlan, E.; Ghachoui, M.; Gillon, M.; Gnilka, C.; Gómez-Muñoz, M. A.; Guerrero, N.; Harris, M.; Heng, K.; Henze, C. E.; Hesse, K.; Howell, S. B.; Jehin, E.; Jenkins, J.; Jensen, E. L. N.; Kunimoto, M.; Latham, D. W.; Lester, K.; McLeod, K. K.; Mireles, I.; Murray, C. A.; Niraula, P.; Pedersen, P. P.; Queloz, D.; Quintana, E. V.; Ricker, G.; Rudat, A.; Sabin, L.; Safonov, B.; Schroffenegger, U.; Scott, N.; Seager, S.; Strakhov, I.; Triaud, A. H. M. J.; Vanderspek, R.; Vezie, M.; Winn, J.
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Astronomy and Astrophysics

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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 (Kmag = 10.7) make it a suitable candidate for atmospheric exploration via transmission spectroscopy.
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