Searching for GEMS: TOI-5916 b & TOI-6158 b are two Saturn-density planets orbiting M2 dwarfs

TL;DR

This paper addresses the occurrence and nature of Saturn-density giant planets around M-dwarfs (GEMS) by confirming two new systems, TOI-5916 b and TOI-6158 b, as transiting Saturn-density planets. It employs a multi-facetted approach combining TESS photometry, ground-based transit follow-up, and Habitable-zone Planet Finder radial velocities to precisely measure masses, radii, and densities, placing these planets within the broader GEMS population. The findings reinforce a trend toward Saturn-density among GEMS, with both planets in short-period orbits and TOI-6158 b exhibiting a grazing transit that challenges radius determination but not density. The study also analyzes the semi-major axis distribution of giant planets, suggesting GEMS occupy closer orbits on average than FGK giant planets, hinting at formation and migration pathways that may be largely independent of host mass across FGKM dwarfs.

Abstract

We confirm the planetary nature of (1) TOI-5916 b and (2) TOI-6158 b, two Exoplanets Transiting M-dwarf Stars (GEMS), both discovered by the Transiting Exoplanet Survey Satellite (TESS). Both systems were confirmed with ground-based photometry (Red Buttes Observatory and Swope, respectively) and radial velocity data from the Habitable-zone Planet Finder. Their radii are $R_{1}=11.8^{+0.52}_{-0.51}\text{ }R_{\oplus}$ and $R_{2}=10.4^{+2.70}_{-1.11}\text{ }R_{\oplus}$ and masses are $M_{1}=219\pm28\text{ }M_{\oplus}$ and $M_{2}=135^{+19}_{-18}\text{ }M_{\oplus}$. Both planets have Saturn-like densities ($ρ_{1} = 0.73^{+0.14}_{-0.13}\,\text{g cm}^{-3}$, $ρ_{2} = 0.66^{+0.41}_{-0.23}\,\text{g cm}^{-3}$), which appears to be a growing trend among GEMS systems and, more generally, warm Jupiters. In confirming both of these exoplanets, we add to the growing evidence for a population of Saturn-density planets among the GEMS systems. We also find evidence for a preliminary trend in which GEMS exhibit systematically closer orbits compared to FGK giants.

Figures (10)

• Figure 1: TGLC calibrated aperture light curve of TOI-5916 for TESS Sectors 55 (600s cadence) and 82 (200s cadence). Red lines correspond to the observed transits of TOI-5916 b. The blue line corresponds to an anomalous background transit that was observed by TESS during its observation of TOI-5916 (see Section \ref{['sec: joint fit']} for more on this background transit).
• Figure 2: Similiar to Figure \ref{['fig:5916_Photometry']}, but for TOI-6158.
• Figure 3: Pixel-by-pixel analysis of the anomalous transit observed in TGLC Sector 55 data. The red box indicates the $3\times3$ aperture used by TGLC, while the blue shading represents the relative baseline fluxes of individual pixels after subtracting background stars. Each panel displays the normalized and detrended light curves obtained by removing flux contributions from neighboring stars. A genuine on-target signal would produce consistent transit depths across all pixels, as observed for TOI-5916 b. Therefore, the transit anomaly detected only in a few pixels is unlikely to originate from TOI-5916 itself.
• Figure 4: The phase-folded light curve for TOI-5916. The gray data points are the raw photometry data. The black data points are the 10-minute binned data points. The red line represents the median (best joint fit model). The confidence intervals, ranging from 1$\sigma$ to 3$\sigma$, are shown in decreasing intensity of blue. For TESS Sector 55, the exposure time was 600 s, where as for TESS Sector 82 it was 200 s. The exposure time for the RBO observation was 240 s.
• Figure 5: Same as Figure \ref{['fig:5916_PFLC']}, but for TOI-6158. The exposure time for TESS Sectors 56, 82, and 83 were 600 s, 200 s, and 200 s respectively. The exposure time for the Swope observations were 40 s and 240 s respectively.