From Earths to Super-Earths: Five New Small Planets Transiting M Dwarf Stars

TL;DR

This work statistically validates five Earth- to super-Earth-sized planets transiting M-dwarf hosts using a combination of TESS photometry, ground-based multi-band transits, high-resolution imaging, and spectroscopy. By joint modeling of space- and ground-based light curves and employing a Bayesian validation framework (TRICERATOPS+GomezBarrientos), the authors derive robust planet parameters and demonstrate FPPs well below acceptance thresholds, including a multi-planet TOI-5489 system. Several planets have equilibrium temperatures $ ext{T}_{eq} \\leq 400$ K, enabling atmospheric-loss tests, while their placement relative to the cosmic shoreline offers empirical constraints on atmosphere retention for rocky worlds around M dwarfs. The study discusses the metallicity and disk-population of the host stars, evaluates the likelihood of thick-disk membership, and assesses the feasibility of mass measurements and atmospheric characterization with current facilities, highlighting TOI-5728 b as a particularly informative below-shoreline case. Overall, the results expand the census of nearby, small planets around M dwarfs and provide a framework for connecting planetary atmospheres, bulk composition, and stellar environment in the context of atmospheric escape theories.

Abstract

Earth-sized planets transiting M dwarf stars present one of the best opportunities with current facilities for studying the atmospheric and bulk compositions of terrestrial worlds. Here, we statistically validate five new transiting Earth and super-Earth sized planets orbiting M dwarf stars using a combination of light curves from the Transiting Exoplanet Survey Satellite, multi-color observations from Palomar and Las Cumbres Observatory, high-resolution imaging, and stellar spectroscopy. The sample includes: TOI-5716 b, an Earth-sized planet (R_p = 0.96 $\pm$ 0.05 Rearth) with a 6.766-day orbit around a metal-poor thin-disk star ([Fe/H] = -0.54 $\pm$ 0.10); TOI-5728 b, a super-Earth (R_p = 1.31 $\pm$ 0.05 Rearth) on an 11.497-day orbit; and TOI-5736 b, a larger planet (R_p = 1.56 $\pm$ 0.07 Rearth) with an ultra-short period of just 0.649 days. We also statistically validate a multi-planet system, TOI-5489, hosting two similarly-sized super-Earths: TOI-5489 b (R_p = 1.40 $\pm$ 0.05 R_earth) and TOI-5489 c (R_p = 1.28 $\pm$ 0.07 R_earth) with orbital periods of 3.152 and 4.921 days, respectively. Due to their longer orbital periods, TOI-5716 b and TOI-5728 b both have equilibrium temperatures $\leq$ 400 K, making them useful test cases for studies of atmospheric mass loss. If TOI-5728 b is confirmed to have an Earth-like bulk composition, it would join the very small sample of rocky planets orbiting mid- to late-M dwarfs that lie below the cosmic shoreline and therefore may have retained high mean molecular weight atmospheres.

Figures (7)

• Figure 1: High-resolution images and corresponding contrast curves (black line) of our targets. There are no secondary sources detected around any of the targets.
• Figure 2: Gaia color-magnitude diagram of our targets, along with the catalog of M stars from Birky2020. The data points are color-coded by their metallicty from Birky2020. The location of TOI-5716 is consistent with the most metal-poor subset of this population, in good agreement with our spectroscopic analysis.
• Figure 3: Stacked TESS light curves for our five targets. Grey points show the 2-minute binned data, black points show the 10-minute binned data. The blue line represents the median transit model from the posterior distribution, with light blue shading indicating the 68$\%$ confidence interval.
• Figure 4: Multi-color ground-based light curves for our five targets. Data points represent 10-minute binned photometry, with light curves obtained using identical filters and exposure times phase-folded together. Black curves show the median transit model derived from the posterior distribution, with colored shading indicating the 68$\%$ confidence intervals. Blue curves represent the nominal TESS light curve model projected into the corresponding ground-based filters.
• Figure 5: The population of small exoplanets around M dwarf stars ($T_*$< 4000 K) depicting the distribution of stellar metallicities for planet hosting stars. Planets with published mass measurements $>3\sigma$ are highlighted in black whereas those without are highlighted in gray. The data points are colored based on their brightness in the $J$ band as a guide for their accessibility to radial velocity measurements.