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

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.

💡 Research Summary

This paper presents the statistical validation of five new transiting exoplanets, ranging from Earth-sized to super-Earth-sized, orbiting M dwarf stars. The discoveries were made using data from NASA’s TESS mission and confirmed through an extensive multi-wavelength, ground-based follow-up campaign.

The primary motivation of the study is to identify and characterize small, rocky planets around the most common stellar type in the Galaxy, M dwarfs. While these stars offer observational advantages for detecting and studying terrestrial planets due to their small size and mass, they also pose challenges due to their high stellar activity, which can strip away planetary atmospheres. A key goal is to find planets with low equilibrium temperatures that may have retained their atmospheres, testing the theoretical boundary known as the “cosmic shoreline” that separates rocky bodies with and without atmospheres.

The candidate signals, designated TOI-5489.01, .02, TOI-5716.01, TOI-5728.01, and TOI-5736.01, were initially identified in TESS photometry. The research team employed a robust validation strategy involving: 1) Detailed light curve analysis from TESS, including correction for systematic trends using Gaussian process convolution; 2) Multi-color ground-based transit photometry from the Palomar Observatory (WIRC, J-band), the Las Cumbres Observatory Global Telescope network (LCOGT, optical bands), and the SAINT-EX telescope (I+z band) to rule out astrophysical false positives and confirm the transit depth; 3) High-resolution adaptive optics imaging from Keck/NIRC2, Palomar/PHARO, and SAI telescopes to exclude the possibility of nearby stellar companions diluting or causing the signal; and 4) Stellar spectroscopy from APOGEE and Keck/HIRES to determine precise host star parameters (effective temperature, metallicity, radius, mass).

The validated planetary systems are:

• TOI-5489: A multi-planet system hosting two similarly-sized super-Earths: TOI-5489 b (1.40 ± 0.05 R⊕, P=3.152 days) and TOI-5489 c (1.28 ± 0.07 R⊕, P=4.921 days).
• TOI-5716 b: An Earth-sized planet (0.96 ± 0.05 R⊕) with a 6.766-day orbit around a metal-poor star.
• TOI-5728 b: A super-Earth (1.31 ± 0.05 R⊕) on a longer 11.497-day orbit.
• TOI-5736 b: A larger planet (1.56 ± 0.07 R⊕) with an ultra-short period of only 0.649 days.

Notably, TOI-5716 b and TOI-5728 b have equilibrium temperatures of ≤400 K due to their longer orbital periods. This makes them particularly valuable targets for studies of atmospheric retention and loss. The paper highlights that if TOI-5728 b is confirmed to have an Earth-like rocky composition, it would belong to a very select group of cool, rocky planets around mid-to-late M dwarfs that reside below the cosmic shoreline and are potential candidates for possessing retained high mean molecular weight atmospheres.

The statistical validation was performed using the TRICERATOPS+ tool, which calculated a very low false positive probability (FPP < 1%) for each candidate, confirming their planetary nature. The authors conclude that these newly validated planets, especially the cooler super-Earths, represent prime targets for future atmospheric characterization with facilities like JWST. They will provide crucial empirical data to constrain models of atmospheric evolution and the conditions under which terrestrial planets can maintain atmospheres around active M dwarf stars.