TOI-1173 A b: The First Inflated Super-Neptune in a Wide Binary System

Among Neptunian mass planets exoplanets ($20-50$ M_⊕), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density (ρ < 0.3 g cm⁻³). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107 b, TOI-1420 b, and WASP-193 b. In this paper, we report the discovery of TOI-1173 A b, a low-density (ρ = 0.269-0.024+0.028 g cm⁻³) super-Neptune with $P = 7.06$ days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X spectrograph and transit photometry from TESS, we determined a planet mass of Mₚ = 26.1±1.9\ M⊕ and radius of Rₚ = 8.10±0.17\ R⊕. TOI-1173 A b is the first puffy Super-Neptune planet detected in a wide binary system (separation ~ 11,400 AU). We explored several mechanisms to understand the puffy nature of TOI-1173 A b, and showed that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 A b likely has maintained its orbital stability over time and may have undergone von-Zeipel-Lidov-Kozai migration followed by tidal circularization given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 A b will shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin-orbit analyses may elucidate the dynamical evolution of the system.