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

Abstract

Among Neptunian mass exoplanets ($20-50$ M${}_{\oplus }$), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density ($\rho <0.3$~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 determined a planet mass of $M_{\mathrm{p}}=27.4\pm 1.7M_{\oplus }$ and radius of $R_{\mathrm{p}}=9.19\pm 0.18R_{\oplus }$. TOI-1173 A $b$ is the first puffy Super-Neptune planet detected in a wide binary system (projected separation $\sim 11,400$~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.

Accepted for publication in the Astronomical Journal on June 2, 2024