Hidden Gems in TESS: sherlock finds two new rocky planets around nearby M dwarfs

TL;DR

The study demonstrates the Hidden Gems project's capability to uncover sub-threshold transiting planets around nearby M-dwarfs using sherlock, resulting in the discovery and validation of two rocky planets, TOI-237 c and TOI-4336 A c, via extensive ground-based follow-up. Comprehensive stellar characterization and a global photometric analysis yield precise planetary radii and orbital parameters, revealing near-resonant, dynamically stable configurations with negligible TTVs. The planets reside in the warm, rocky regime near the radius valley, offering valuable test cases for atmospheric retention and evolution around low-mass stars, with clear prospects and challenges for JWST observations and mass measurements. Overall, the work expands the census of small planets around M dwarfs and illustrates the effectiveness of combining space-based surveys with targeted ground-based campaigns and robust statistical vetting.

Abstract

The Hidden Gems project searches the TESS data for additional planets transiting low-mass stars in confirmed systems. Our goal is to identify planet candidates that are below the detection threshold set by the SPOC and QLP pipelines using sherlock, a specialized pipeline for robust detection and vetting of transit signals in TESS data. We present the discovery of two inner rocky planets in the TOI-237 and TOI-4336 A systems, confirmed with ground-based photometry from the TRAPPIST, SPECULOOS, ExTrA, and LCO facilities. TOI-237 c has a radius of 1.21 +/- 0.04Re, orbits its mid-M host star every 1.74 days, and is close to a 3:1 mean-motion resonance with TOI-237 b. TOI-4336 A c has a radius of 1.17 +/- 0.06Re, and orbits with a period of 7.58 days an M3.5 host star which is part of a hierarchical triple system. We performed model comparison to search for non-zero eccentricities, and found that the circular transit models are statistically favored. Dynamical simulations show that both systems are in stable configurations, and the TTVs expected for the TOI-237 system are of the order of seconds. TOI-237 c and TOI-4336 A c join the high-interest population of warm likely super-Earths below the so-called "radius valley". In particular, TOI-237 c is a good candidate for phase curve observations with JWST/MIRI thanks to the small radius of the host star and its short period.

Figures (13)

• Figure 1: TESS 2-min cadence photometry of TOI-237 obtained with the TESS automatic aperture. Each of the panels shows the Sectors 2, 29, and 69, respectively. TOI-237 b is highlighted in blue, and TOI-237 c (TIC 305048087.02) in orange.
• Figure 2: TESS 2-min cadence photometry of TOI-4336 A obtained with the same custom apertures as in 2024_Timmermans. Each of the panels shows the Sectors 11, 38, and 64, respectively. TOI-4336 Ai̱s highlighted in blue, and TOI-4336 A c (TIC 166184428.02) in orange.
• Figure 3: Optical spectrum of TOI-237 (black line) normalized at 7100 Å, compared to its best fit spectral template, a combination M4 and M5 dwarf types 2017ApJS..230...16K. Key molecular and atomic spectral features are labeled, as are uncorrected telluric features ($\oplus$). The inset box shows a close-up of the 6500--6750 Å region encompassing H$\alpha$ and Li I lines.
• Figure 4: Phase coverage from the observations of TOI-237 obtained with TRAPPIST-South described in Sect. \ref{['sec:TS']}. The light gray line represents the percentage of phase covered for each orbital period from 0.001 days to 8 days with steps of 0.001 days. The black line shows the phase coverage for periods binned by 2.4 hours. The dashed orange line corresponds to P=1.74 days, the period of TOI-237 c, at which we find a coverage of $\sim$98%. The blue line corresponds to P=5.15 days at which a coverage of $\sim$80% is reached.
• Figure 5: Bar plot representing the Bayes factor of the tested models for the TOI-237 system. The null hypothesis is taken as a one-planet circular fit, the other models are two-planet fits: two circular orbits, planet b circular and planet c eccentric, the inverse, and two eccentric orbits.