The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge
Recent exoplanet demographics studies have uncovered the Neptune ridge: an overdensity of intermediate-size planets at orbital periods between 3--6~d. The origin of this pile-up remains unclear, but it could be a consequence of high-eccentricity migration (HEM). Here, we confirm TOI-4129\,b, a planet that may help to reveal the origin of the Neptune ridge. Using radial velocities from HARPS-N and transit photometry from TESS and four ground-based telescopes, we found that TOI-4129\,b has a relatively low density ($R_p = 5.7 \pm 0.2\, R_\oplus$, $M_p = 26\pm 6 \,M_\oplus$, $ρ_p = 0.8 \pm 0.2 ~\textrm{g}~\textrm{cm}^{-3}$) and orbits a $V = 12.9$ K dwarf star every 3.27~d, placing it at the inner edge of the Neptune ridge. We detect the stellar rotation period at 12.7~d in the photometry, implying a fairly young gyrochronal age of $1.20^{+0.16}_{-0.30}~$Gyr for the system. With a Transmission Spectroscopy Metric of $112\pm 30$, this adolescent Neptune is a good target for atmospheric observations with \textit{JWST} and \textit{Ariel}, and we also find that it is amenable to future constraints on the (three-dimensional) stellar obliquity. TOI-4129\,b will therefore be an important testing ground for the idea that planets in the Neptune ridge underwent HEM to arrive at their current locations.