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 (RM) effect in the hot-star/hot-Neptune system WASP-195 ( T eff = 6470 ± 100 K, v sin i ⋆ = 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 5 σ 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 RM 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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Publication Details

Journal
The Astronomical Journal
Published
2026-10-30
DOI
https://doi.org/10.3847/1538-3881/aeb1d3
Primary Topic
Stellar, planetary, and galactic studies
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article
Field-Weighted Citation Impact
0.00

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article

POSEIDON. III. The Aligned Orbit of the Hot Neptune around the Hot Star WASP-195

Samuel Halverson, Cristóbal Petrovich, Andrew W. Howard, Theron W. Carmichael et al.
2 links
The Astronomical Journal
Stellar, planetary, and galactic studies
article

POSEIDON. III. The Aligned Orbit of the Hot Neptune around the Hot Star WASP-195

Samuel Halverson, Cristóbal Petrovich, Andrew W. Howard, Theron W. Carmichael, Steven Giacalone, Rafael Brahm, Elina Y. Zhang, Benjamin J. Fulton, Erik A. Petigura, Luke B. Handley, Caleb Lammers, Guđmundur Stefánsson, Judah Van Zandt, Maleah Rhem, Ryan A. Rubenzahl, Pranav H. Premnath, Kodi Rider, Lauren M. Weiss, Josh Walawender, Nicholas Saunders, Juan I. Espinoza-Retamal, Xian-Yu Wang, Christopher Smith, Jerry Edelstein, Joshua N. Winn, Arpita Roy, Jack Foley, Elise Koo, Songhu Wang, Howard Isaacson, Fei Dai, Ashley D. Baker, Steven R. Gibson, Andrés Jordán, Emma McIntyre, Jingyi Zhang
article en

Abstract

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 (RM) effect in the hot-star/hot-Neptune system WASP-195 ( T eff = 6470 ± 100 K, v sin i ⋆ = 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 5 σ 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 RM effect, a feature that could help to widen the planet confirmation bottleneck while expanding the obliquity census of small planets around hot stars.

The Astronomical JournalVol. 172(6)
University of Notre Dame (US), University of Hawaiʻi at Mānoa (US), California Institute of Technology (US), University of Hawaii System (US), University of Kansas (US), University of California, Santa Barbara (US), University of California, Los Angeles (US), Princeton University (US), University of California, Irvine (US), Adolfo Ibáñez University (CL), W.M. Keck Observatory (US), Yale University (US), Flatiron Institute (US), Indiana University (US), University of Chile (CL), University of Amsterdam (NL), Netherlands Institute for Radio Astronomy (NL), University of California, Berkeley (US)
National Science Foundation, National Aeronautics and Space Administration, W. M. Keck Foundation, Yale University, Purdue University, Princeton University, California Institute of Technology, University of Wisconsin-Madison, Space Telescope Science Institute, NASA Exoplanet Science Institute, Jet Propulsion Laboratory
Openalex Percentile: Top 31%
Stellar, planetary, and galactic studies
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