TOI-3785 b: A Low-density Neptune Orbiting an M2-dwarf Star

Using both ground-based transit photometry and high-precision radial velocity spectroscopy, we confirm the planetary nature of TOI-3785 b. This transiting Neptune orbits an M2-Dwarf star with a period of similar to 4.67 days, a planetary radius of 5.14 +/- 0.16 R-circle plus, a mass of 14.95(-3.92)(+ 4.10) M-circle plus, and a density of rho = 0.61(-0.17)(+0.18) g cm(-3). TOI-3785 b + belongs to a rare population of Neptunes (4 R-circle plus < R-p < 7 R-circle plus) orbiting cooler, smaller M-dwarf host stars, of which only similar to 10 have been confirmed. By increasing the number of confirmed planets, TOI-3785 b offers an opportunity to compare similar planets across varying planetary and stellar parameter spaces. Moreover, with a high-transmission spectroscopy metric of similar to 150 combined with a relatively cool equilibrium temperature of T-eq = 582 +/- 16 K and an inactive host star, TOI-3785 b is one of the more promising low-density M-dwarf Neptune targets for atmospheric follow up. Future investigation into atmospheric mass-loss rates of TOI-3785 b may yield new insights into the atmospheric evolution of these low-mass gas planets around M dwarfs.