Spin–Orbit Alignment of Two Neptune-size Planets Younger Than 500 Myr: TOI-560 b and TOI-5082 b

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Abstract Stellar obliquity measurements provide a direct probe of planetary system dynamics, but remain sparse for Neptune-size planets, particularly at young ages. We present Rossiter–McLaughlin measurements for two young Neptune-size planets, TOI-560 b and TOI-5082 b, using time-resolved Keck Planet Finder (KPF) spectroscopy and joint modeling with TESS transit photometry. We measure sky-projected obliquities of λ b = −25° ± 16° for TOI-560 b and λ b = 1 9 ° − 13 ° + 17 ° for TOI-5082 b. Combining these constraints with stellar rotation periods and spectroscopic estimates of v sin i ⋆ , we obtain 95% upper limits of ψ < 69 . ° 7 and ψ < 50 . ° 5, respectively. Both systems are therefore consistent with low to moderate true obliquities and show no evidence of strong spin-orbit misalignment. With ages of 480 ± 190 Myr for TOI-560 b and 180 ± 9 Myr for TOI-5082 b (a likely member of the CRIUS197 stellar association), the systems are in a key evolutionary phase when postdisk dynamical processes such as secular interactions may begin to manifest. Nevertheless, both systems remain consistent with low obliquity. In the broader context of young systems with existing measurements, these results support an emerging picture in which Neptune-size planets at ≲1 Gyr are commonly found in low-obliquity configurations.

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