Discovery of an Inflated Hot Neptune and Its Formation from Jovian Mass Loss

The production of Neptune-like planets with orbital periods of 3–6 days is challenging for conventional models of high-eccentricity migration. We present the discovery and characterization of TOI-2195~A~b, an inflated hot Neptune ($P=4.16$ days, $m_p=1.46M_{\mathrm{N}\mathrm{e}\mathrm{p}},R_p=0.79R_{\mathrm{J}}$) orbiting an early K-type star with a wide binary companion at $\sim 600$~au. Detection of the Rossiter-McLaughlin effect at $\sim 2.6\sigma$ confidence with Magellan/PFS reveals the planet is likely on a near-polar orbit with a sky-projected stellar obliquity $\lambda ={109_{-53}^{+35}}^{\circ }$. We perform coupled dynamical and structural modeling that reproduces the observed characteristics of the system. We show that the planet may have originated as a cold, Jovian planet that was excited to high eccentricities via the stellar Eccentric Kozai-Lidov (EKL) mechanism, where it lost up to $\sim 90\mathrm{\%}$ of its mass via Roche lobe overflow during close periastron passages, enabling rapid tidal migration and radius inflation due to tidal heating. TOI-2195 A b provides a test for planetary migration theories, and our simulations suggest that puffy hot Neptunes originated as more massive Jovians that underwent mass loss during high-eccentricity migration.