The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge

Abstract

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 ($R_p=5.7\pm 0.2R_{\oplus }$, $M_p=26\pm 6M_{\oplus }$, ${\rho }_p=0.8\pm 0.2\text{g}{\text{cm}}^{-3}$) 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 $1.20_{-0.30}^{+0.16}$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.

arXiv comment22 pages, 11 figures, 2 tables, resubmitted to AJ