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

• Published in: arXiv.org 2021-11
• Main Authors: Schanche, N, Pozuelos, F J, Günther, M N, Wells, R D, Burgasser, A J, Chinchilla, P, Delrez, L, Ducrot, E, Garcia, L J, Y Gómez Maqueo Chew, Jofré, E, Rackham, B V, Sebastian, D, Stassun, K G, Stern, D, Timmermans, M, Barkaoui, K, Belinski, A, Benkhaldoun, Z, Benz, W, Charbonneau, D, Christiansen, Jessie L, Collins, Karen A, B -O Demory, Dévora-Pajares, M, de Wit, J, Dragomir, D, Dransfield, G, Furlan, E, Ghachou, 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, Eric L N, Kunimoto, M, Latham, D W, Lester, K, McLeod, Kim 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, A H M J Triaud, Vanderspek, R, Vezie, M, Winn, J
• Format: Article
• Language: English
• Subjects: Eccentric orbits; Extrasolar planets; Image resolution; Red dwarf stars; Telescopes; Transit
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2111.01749

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 make such objects particularly compelling as they provide clues about the formation and evolution of these systems. 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 57.8pc. 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 (\(e \sim 0.5\)), which would make it the most eccentric planet that is known to be transiting an M-dwarf star. We combined TESS and ground-based data obtained with the 1.0-m SAINT-EX, 0.60-m TRAPPIST-North and 1.2-m FLWO telescopes to find a planetary size of 2.2 \(R_{\oplus}\) 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. With its long period and high eccentricity, TOI-2257 b falls in a novel slice of parameter space. Despite the planet's low equilibrium temperature (\(\sim\) 256 K), its host star's small size (\(R_* = 0.311 \pm{0.015}\)) and relative infrared brightness (K\(_{mag}\) = 10.7) make it a suitable candidate for atmospheric exploration via transmission spectroscopy.