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

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

Uncovering the true periods of the young sub-Neptunes orbiting TOI-2076

“…Two of the planets found by K2 to orbit HIP 41378 are duotransiters (Becker et al 2019), and a combination of radial velocities and groundbased transit photometry were able to recover the true period of HIP-41378 f (Santerne et al 2019;Bryant et al 2021). In TESS, the true period of TOI-2257 b, which produced two 0.4% transits in TESS Year-2 photometry consistent with four possible period aliases, was recovered through ground-based photometry (Schanche et al 2021). However, the majority of the planets so far followed up on in this way typically either show few period aliases, or they produce deep eclipses easily observable from the ground (depth > 0.4%).…”