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

Abstract

Among Neptunian mass 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 ( $\rho ={0.195}_{-0.017}^{+0.018}$ 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 and HIRES spectrographs and transit photometry from TESS, we determine a planet mass of M _p = 27.4 ± 1.7 M _⊕ and radius of R _p = 9.19 ± 0.18 R _⊕ . TOI-1173 A b is the first puffy super-Neptune planet detected in a wide binary system (projected separation ∼11,400 au). We explore several mechanisms to understand the puffy nature of TOI-1173 A b and show 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.

Keywords

• Exoplanets
• Hot Neptunes
• Wide binary stars
• Radial velocity
• Transit photometry
• Exoplanet detection methods