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

January 2022 • 2022A&A...657A..45S

Authors • 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.

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


IPAC Authors


Jessie Christiansen

Associate Scientist