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

@misc{lakhanpal2026neptune,
  title = {{The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge}},
  author = {Sureena Lakhanpal and Shreyas Vissapragada and Luca Naponiello and Simone Filomeno and Katia Biazzo and Vincent Bourrier and Tiziano Zingales and Erik Friden and Emily K. Pass and Aldo S. Bonomo and David W. Latham and Karen A. Collins and David R. Ciardi and David Baker and Khalid Barkaoui and Andrew Collier Cameron and Patrick Eggenberger and Akihiko Fukui and Ancy Anna John and Heather A. Knutson and Mercedes López-Morales and Jack Lubin and Luigi Mancini and Francesca Manni and Giacomo Mantovan and Annelies Mortier and Felipe Murgas and Norio Narita and Belinda A. Nicholson and Enric Palle and Matteo Pinamonti and Monica Rainer and Carolina Rodriguez and Pam Rowden and Miquel Serra-Ricart and Alessandro Sozzetti and Ivan A. Strakhov and Jhon Yana Galarza and Samuel W. Yee},
  howpublished = {arXiv},
  year = {2026},
  month = nov,
  doi = {10.48550/arxiv.2611.03810},
  eprint = {2611.03810},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2611.03810},
}

TY  - UNPB
TI  - The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge
AU  - Sureena Lakhanpal
AU  - Shreyas Vissapragada
AU  - Luca Naponiello
AU  - Simone Filomeno
AU  - Katia Biazzo
AU  - Vincent Bourrier
AU  - Tiziano Zingales
AU  - Erik Friden
AU  - Emily K. Pass
AU  - Aldo S. Bonomo
AU  - David W. Latham
AU  - Karen A. Collins
AU  - David R. Ciardi
AU  - David Baker
AU  - Khalid Barkaoui
AU  - Andrew Collier Cameron
AU  - Patrick Eggenberger
AU  - Akihiko Fukui
AU  - Ancy Anna John
AU  - Heather A. Knutson
AU  - Mercedes López-Morales
AU  - Jack Lubin
AU  - Luigi Mancini
AU  - Francesca Manni
AU  - Giacomo Mantovan
AU  - Annelies Mortier
AU  - Felipe Murgas
AU  - Norio Narita
AU  - Belinda A. Nicholson
AU  - Enric Palle
AU  - Matteo Pinamonti
AU  - Monica Rainer
AU  - Carolina Rodriguez
AU  - Pam Rowden
AU  - Miquel Serra-Ricart
AU  - Alessandro Sozzetti
AU  - Ivan A. Strakhov
AU  - Jhon Yana Galarza
AU  - Samuel W. Yee
PY  - 2026
DA  - 2026/11/02
PB  - arXiv
DO  - 10.48550/arxiv.2611.03810
UR  - https://arxiv.org/abs/2611.03810
AB  - 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 ± 0.2 R_⊕, M_p = 26± 6 M_⊕, ρ_p = 0.8 ± 0.2 g 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_−0.30Gyr for the system. With a Transmission Spectroscopy Metric of 112± 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.
ER  -