[en] 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.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Schanche, N.
Pozuelos, F. J.
Günther, M. N.
Wells, R. D.
Burgasser, A. J.
Chinchilla, P.
Delrez, Laetitia ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Ducrot, Elsa ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Garcia, Lionel ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Gómez Maqueo Chew, Y.
Jofré, E.
Rackham, B. V.
Sebastian, Daniel ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Stassun, K. G.
Stern, David ; Université de Liège - ULiège > GIGA Platform Genomics
Timmermans, Mathilde ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Barkaoui, Khalid ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Belinski, A.
Benkhaldoun, Z.
Benz, W.
Charbonneau, D.
Christiansen, Jessie L.
Collins, Karen A.
Demory, B.-O.
Dévora-Pajares, M.
de Wit, J.
Dragomir, D.
Dransfield, G.
Furlan, E.
Ghachou, M.
Gillon, Michaël ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Exotic
Gnilka, C.
Gómez-Muñoz, M. A.
Guerrero, N.
Harris, M.
Heng, K.
Henze, C. E.
Hesse, K.
Howell, S. B.
Jehin, Emmanuel ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
