POSEIDON I: The Dynamical Origins of Transiting Neptunes

Abstract

We present the first results from the POSEIDON survey, aimed at constraining the dynamical origins of transiting Neptunes through stellar obliquity measurements. We report Rossiter-McLaughlin observations of two Neptunes, TOI-181 b and TOI-883 b, obtained with high-resolution spectroscopy from Magellan/PFS and WIYN/NEID. TOI-181 b is on a 4.5-day orbit with a sky-projected spin-orbit misalignment $λ= 32.0_{-6.5}^{+6.3}\,^{\circ}$ and a low eccentricity ($e<0.12$ with $2σ$ confidence). TOI-883 b has a longer orbital period of 10 days with $λ= 22_{-14}^{+15}\,^{\circ}$ and eccentricity $e = 0.16 \pm 0.03$. The significant misalignment of TOI-181 b and the significant eccentricity of TOI-883 b are suggestive of high-eccentricity migration for both systems. After adding these and other new measurements to the sample, we analyze the obliquity distribution of the host stars of transiting Neptunes. Earlier studies had suggested that the obliquity distribution is bimodal, with peaks corresponding to aligned orbits and polar orbits; the addition of more measurements has weakened the evidence for bimodality. The current sample appears to be consistent with a population of well-aligned systems and a smaller population with nearly random obliquities. This distribution resembles that observed for more massive planets, suggesting that transiting Jupiters and Neptunes originate from similar dynamical processes.

Figures (7)

• Figure 1: Orbital period versus planet radius for transiting planets. Gray points are from the TEPCat catalog Southworth2011 as of November 2025. Colored points are those for which the stellar obliquity has been measured, with five-pointed stars highlighting TOI-181 b and TOI-883 b. Dashed blue lines indicate the boundaries of the Neptune desert, ridge, and savanna as defined by Castro-Gonzalez2024. The background color conveys the density of data points.
• Figure 2: Radial-velocity and photometric observations of TOI-181. In all cases, the red curves are best-fit models and the residuals are plotted beneath the data. The error bars include a white noise jitter term added in quadrature. a) PFS velocities spanning a transit and exhibiting the RM effect. b) HARPS velocities across all orbital phases. c) MOANA photometry of the same transit observed with PFS. d) Phase-folded photometry based on TESS observations with 2-minute cadence (green). In panels c and d, the darker points are time-averaged data. Data behind the figure are available electronically.
• Figure 3: Radial-velocity and photometric observations of TOI-883. In all cases, the red curves are best-fit models and the residuals are plotted beneath the data. The RV error bars include a white noise jitter term added in quadrature. a) NEID velocities spanning a transit and exhibiting the RM effect. b) HARPS and PFS velocities across all orbital phases. c) Phase-folded photometry based on TESS observations with 30-minute cadence (blue). d) Phase-folded photometry based on TESS observations with 2-minute cadence (green). The darker points are time-averaged data. Data behind the figure are available electronically.
• Figure 4: Mass versus radius diagram. Grey points are all the transiting exoplanets from TEPCat Southworth2011 as of November 2025. Colored points convey the projected obliquity for systems with published measurements. Triangles indicate systems with only upper limits available for the mass. TOI-181 b and TOI-883 b are highlighted as stars following the same color code. The green dashed lines indicate the region where $10\leq M_p/M_\oplus \leq 50$ or $2\leq R_p/R_\oplus \leq 6$, which we considered as Neptunes. Neptune itself is highlighted as the white cross. Contours show the density of points.
• Figure 5: Stellar obliquity of Neptune ($2\leq R_p/R_\oplus\leq 6$ or $10\leq M_p/M_\oplus\leq 50$) hosts as a function of the stellar effective temperature. Upper panels show the projected obliquity while lower panels the true obliquity. Left panels show systems where no companion star has been reported and right panels show known multiple-star systems. Neptunes that have planetary companions with periods between 1/5th and 5 times the period of the Neptune are shown in blue, while those without such close companions are shown in orange. Measurements performed in this work for TOI-181 and TOI-883 are highlighted as stars with a red edge, and follow the same color code. Literature measurements come from Rossi2025 and TEPCat Southworth2011.