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.; L.J. GARCIA; GOMEZ MAQUEO, Y.; JOFRE E.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2022

0004-6361

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 make 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 that is known to be transiting an M-dwarf star. Methods. 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⊕ 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 in 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.