POSEIDON III: The Aligned Orbit of the Hot Neptune Around the Hot Star WASP-195
Abstract
Stellar obliquities provide important clues as to the formation and migration histories of planetary systems, but measurements remain scarce for Neptune-mass planets, especially those orbiting hot stars (above the Kraft break). Here we present observations of the Rossiter-McLaughlin effect in the hot-star/hot-Neptune system WASP-195 (Teff=6470±100 K, vsini⋆=10.5±1.1 km s−1) obtained with the Keck Planet Finder and NEID spectrographs. A joint analysis of these observations, archival photometry, and archival radial velocities yields a sky-projected stellar obliquity of λ=−10±7∘, consistent with spin-orbit alignment. This makes WASP-195 one of the few hot-star/hot-Neptune systems with a measured obliquity. Archival radial velocities from SOPHIE exclude Jupiter-mass planets within approximately 3 au at 5σ confidence. The aligned and nearly circular orbit is naturally consistent with a history of disk-driven migration, although coplanar high-eccentricity migration or Roche-lobe overflow cannot be ruled out. We also investigate why so few Neptunes around hot stars have measured obliquities. Their scarcity likely reflects a combination of the lower intrinsic occurrence of short-period Neptunes around hot stars and the difficulty of confirming planet candidates in this regime, where rapid stellar rotation broadens spectral lines and hampers conventional radial-velocity confirmation. Rapid rotation also increases the detectability of the Rossiter-McLaughlin effect, a feature that could help to widen the planet confirmation bottleneck while expanding the obliquity census of small planets around hot stars.
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BibTeX
@misc{espinozaretamal2026poseidon,
title = {{POSEIDON III: The Aligned Orbit of the Hot Neptune Around the Hot Star WASP-195}},
author = {Juan I. Espinoza-Retamal and Joshua N. Winn and Rafael Brahm and Luke B. Handley and Elise Koo and Caleb Lammers and Cristobal Petrovich and Guðmundur Stefánsson and Andrés Jordán and Xian-Yu Wang and Songhu Wang and Nicholas Saunders and Erik A. Petigura and Lauren M. Weiss and Ashley D. Baker and Theron W. Carmichael and Fei Dai and Jerry Edelstein and Jack Foley and Benjamin J. Fulton and Steven Giacalone and Steven R. Gibson and Samuel Halverson and Andrew W. Howard and Howard Isaacson and Emma McIntyre and Pranav H. Premnath and Maleah Rhem and Kodi Rider and Ryan A. Rubenzahl and Arpita Roy and Christopher Smith and Judah Van Zandt and Josh Walawender and Elina Y. Zhang and Jingyi Zhang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.02888},
eprint = {2609.02888},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.02888},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - POSEIDON III: The Aligned Orbit of the Hot Neptune Around the Hot Star WASP-195
AU - Juan I. Espinoza-Retamal
AU - Joshua N. Winn
AU - Rafael Brahm
AU - Luke B. Handley
AU - Elise Koo
AU - Caleb Lammers
AU - Cristobal Petrovich
AU - Guðmundur Stefánsson
AU - Andrés Jordán
AU - Xian-Yu Wang
AU - Songhu Wang
AU - Nicholas Saunders
AU - Erik A. Petigura
AU - Lauren M. Weiss
AU - Ashley D. Baker
AU - Theron W. Carmichael
AU - Fei Dai
AU - Jerry Edelstein
AU - Jack Foley
AU - Benjamin J. Fulton
AU - Steven Giacalone
AU - Steven R. Gibson
AU - Samuel Halverson
AU - Andrew W. Howard
AU - Howard Isaacson
AU - Emma McIntyre
AU - Pranav H. Premnath
AU - Maleah Rhem
AU - Kodi Rider
AU - Ryan A. Rubenzahl
AU - Arpita Roy
AU - Christopher Smith
AU - Judah Van Zandt
AU - Josh Walawender
AU - Elina Y. Zhang
AU - Jingyi Zhang
PY - 2026
DA - 2026/10/07
PB - arXiv
DO - 10.48550/arxiv.2609.02888
UR - https://arxiv.org/abs/2609.02888
AB - Stellar obliquities provide important clues as to the formation and migration histories of planetary systems, but measurements remain scarce for Neptune-mass planets, especially those orbiting hot stars (above the Kraft break). Here we present observations of the Rossiter-McLaughlin effect in the hot-star/hot-Neptune system WASP-195 (T_eff=6470±100 K, vsini_⋆=10.5±1.1 km s^−1) obtained with the Keck Planet Finder and NEID spectrographs. A joint analysis of these observations, archival photometry, and archival radial velocities yields a sky-projected stellar obliquity of λ=−10±7^∘, consistent with spin-orbit alignment. This makes WASP-195 one of the few hot-star/hot-Neptune systems with a measured obliquity. Archival radial velocities from SOPHIE exclude Jupiter-mass planets within approximately 3 au at 5σ confidence. The aligned and nearly circular orbit is naturally consistent with a history of disk-driven migration, although coplanar high-eccentricity migration or Roche-lobe overflow cannot be ruled out. We also investigate why so few Neptunes around hot stars have measured obliquities. Their scarcity likely reflects a combination of the lower intrinsic occurrence of short-period Neptunes around hot stars and the difficulty of confirming planet candidates in this regime, where rapid stellar rotation broadens spectral lines and hampers conventional radial-velocity confirmation. Rapid rotation also increases the detectability of the Rossiter-McLaughlin effect, a feature that could help to widen the planet confirmation bottleneck while expanding the obliquity census of small planets around hot stars.
ER -