December
2026
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2026AJ....172..291E
Authors
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Espinoza-Retamal, Juan I.
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Winn, Joshua N.
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Brahm, Rafael
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Handley, Luke B.
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Koo, Elise
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Lammers, Caleb
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Petrovich, Cristobal
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Stefánsson, Gudmundur
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Jordán, Andrés
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Wang, Xian-Yu
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Wang, Songhu
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Saunders, Nicholas
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Petigura, Erik A.
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Weiss, Lauren M.
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Baker, Ashley D.
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Carmichael, Theron W.
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Dai, Fei
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Edelstein, Jerry
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Foley, Jack
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Fulton, Benjamin J.
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Giacalone, Steven
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Gibson, Steven R.
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Halverson, Samuel
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Howard, Andrew W.
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Isaacson, Howard
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McIntyre, Emma
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Premnath, Pranav H.
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Rhem, Maleah
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Rider, Kodi
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Rubenzahl, Ryan A.
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Roy, Arpita
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Smith, Christopher
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Van Zandt, Judah
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Walawender, Josh
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Zhang, Elina Y.
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Zhang, Jingyi
Abstract
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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 (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 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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