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

Abstract

<jats:p><jats:italic>Context.</jats:italic> 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.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> 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 (<jats:italic>e</jats:italic> ~ 0.5), which would make it the most eccentric planet known to be transiting an M-dwarf star.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> 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 <jats:italic>R</jats:italic><jats:sub>⊕</jats:sub> 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.</jats:p> <jats:p><jats:italic>Results.</jats:italic> 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 (<jats:italic>R</jats:italic><jats:sub>*</jats:sub> = 0.311 ± 0.015) and relative infrared brightness (<jats:italic>K</jats:italic><jats:sub>mag</jats:sub> = 10.7) make it a suitable candidate for atmospheric exploration via transmission spectroscopy.</jats:p>