Searching for GEMS: TOI-5688 A b, a low-density giant orbiting a high-metallicity early M-dwarf

We present the discovery of a low-density planet transiting TOI-5688 A b, a high-metallicity M2V star. This planet was discovered as part of the search for transiting giant planets (R 8 M_⊕) through the Searching for GEMS (Giant Exoplanets around M-dwarf Stars) survey. The planet TOI-5688 A b was discovered with the Transiting Exoplanet Survey Satellite (TESS), and characterized with ground-based transits from Red Buttes Observatory (RBO), the Table Mountain Observatory of Pomona College, and radial velocity (RV) measurements with the Habitable-Zone Planet Finder (HPF) on the 10 m Hobby Eberly Telescope (HET) and NEID on the WIYN 3.5 m telescope. From the joint fit of transit and RV data, the mass of the planet is 124±24 M_⊕ and the radius is 10.4±0.7 R_⊕. This planet has a density of 0.61+0.20-0.15 g/cm³, and is on a ~2.95 day orbit around its host star. The spectroscopic and photometric analysis of the host star TOI-5688 A shows that it is a high metallicity ([Fe/H] = 0.47±0.16 dex) M2V star, favoring the core-accretion formation pathway as the likely formation scenario for this planet. In this paper, we analyze potential mechanisms of planet formation in the context of the formation of TOI-5688 A b. Additionally, observations with Gaia suggest the presence of a wide-separation binary companion, TOI-5688 B, which has a projected separation of ~5" (1110 AU) and is an M4V. This makes TOI-5688 A b part of a growing number of GEMS in wide-separation binary systems.