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The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge
Sureena Lakhanpal, Shreyas Vissapragada, Luca Naponiello, Simone Filomeno, Katia Biazzo, Vincent Bourrier, Tiziano Zingales, Erik Friden, Emily K. Pass, Aldo S. Bonomo, David W. Latham, Karen A. Collins, David R. Ciardi, David Baker, Khalid Barkaoui, Andrew Collier Cameron, Patrick Eggenberger, Akihiko Fukui, Ancy Anna John, Heather A. Knutson, Mercedes López-Morales, Jack Lubin, Luigi Mancini, Francesca Manni, Giacomo Mantovan, Annelies Mortier, Felipe Murgas, Norio Narita, Belinda A. Nicholson, Enric Palle, Matteo Pinamonti, Monica Rainer, Carolina Rodriguez, Pam Rowden, Miquel Serra-Ricart, Alessandro Sozzetti, Ivan A. Strakhov, Jhon Yana Galarza, Samuel W. Yee
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 ($R_p = 5.7 \pm 0.2\, R_\oplus$, $M_p = 26\pm 6 \,M_\oplus$, $ρ_p = 0.8 \pm 0.2 ~\textrm{g}~\textrm{cm}^{-3}$) 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.