TOI-1728b: The Habitable-zone Planet Finder Confirms a Warm Super-Neptune Orbiting an M-dwarf Host
Cochran, William D.; Endl, Michael; Mahadevan, Suvrath; Wright, Jason T.; Ford, Eric B.; Monson, Andrew; Cañas, Caleb I.; Kanodia, Shubham; Ninan, Joe P.; Hebb, Leslie; Bender, Chad F.; Diddams, Scott A.; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Maney, Marissa; Metcalf, Andrew J.; Ramsey, Lawrence W.; Robertson, Paul; Roy, Arpita; Schwab, Christian; Terrien, Ryan C.; Lin, Andrea S. J.; Dong, Jiayin; Stefansson, Gudmundur; Baran, Helen
United States, Australia
Abstract
We confirm the planetary nature of TOI-1728b using a combination of ground-based photometry, near-infrared Doppler velocimetry and spectroscopy with the Habitable-zone Planet Finder. TOI-1728 is an old, inactive M0 star with Teff = ${3980}_{-32}^{+31}$ K, which hosts a transiting super-Neptune at an orbital period of ∼3.49 days. Joint fitting of the radial velocities and TESS and ground-based transits yields a planetary radius of ${5.05}_{-0.17}^{+0.16}$ R⊕, mass ${26.78}_{-5.13}^{+5.43}$ M⊕, and eccentricity ${0.057}_{-0.039}^{+0.054}$ . We estimate the stellar properties, and perform a search for He 10830 Šabsorption during the transit of this planet and claim a null detection with an upper limit of 1.1% with 90% confidence. A deeper level of He 10830 Šabsorption has been detected in the planet atmosphere of GJ 3470b, a comparable gaseous planet. TOI-1728b is the largest super-Neptune—the intermediate subclass of planets between Neptune and the more massive gas-giant planets—discovered around an M dwarf. With its relatively large mass and radius, TOI-1728 represents a valuable data point in the M-dwarf exoplanet mass-radius diagram, bridging the gap between the lighter Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs. With a low bulk density of ${1.14}_{-0.24}^{+0.26}$ g cm-3, and orbiting a bright host star (J ∼ 9.6, V ∼ 12.4), TOI-1728b is also a promising candidate for transmission spectroscopy both from the ground and from space, which can be used to constrain planet formation and evolutionary models.