Benjamin J. Fulton

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 angle between stellar spin axes and planetary orbits—stellar obliquity—probes the dynamics of planetary migration and evolution. The obliquities of giant planets have been extensively studied because they are the most easily measured. Smaller planets, while more difficult to measure, have the advantage of better reflecting the dynamics of planetary systems because they trigger negligible backreactions onto the host star. This paper introduces a new observational campaign called the Small, Low-mass Oblique Planets Experiment (SLOPE) survey with the Keck Planet Finder (KPF) spectrograph and presents four new obliquity measurements. The SLOPE survey focuses on planets smaller than Saturn across a variety of system architectures. The sky-projected obliquities of the four planets measured—TOI-1386 b, TOI-480 b, TOI-4596 b, and TOI-1823 b—are all consistent with spin–orbit alignment. We validate the planetary nature of TOI-4596 b with a significant detection of the Rossiter–McLaughlin effect. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn-mass planets in different planetary system architectures. Compared to other architectures, those in compact multiplanet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 σ confidence.

The γ Cephei system hosts one of the first exoplanets discovered and is orbited by one of the closest known stellar companions to a planet-hosting star. In this work, we derive updated orbital fits for γ Cep AB, the stellar binary, and Ab, the planet, by combining literature data with Hipparcos–Gaia astrometry, new radial velocities (RVs), and adaptive optics imaging. We acquired 328 RVs of γ Cep A with Keck/HIRES, AFP/Levy, McDonald/Tull, and Whipple/TRES, and eight adaptive optics imaging epochs with Keck/NIRC2, including the earliest spatially resolved image of γ Cep B in 2003. These observations extend the precision RV baseline of γ Cep to 45 yr and the direct imaging baseline to 23 yr, improving the inferred orbital parameter precision by a factor of 2–10 compared to previous work. For γ Cep B, we derive a semimajor axis of a _B = 20.07 ± 0.06 au, a mass of M _B = 415 ± 2 M _Jup (0.396 ± 0.002 M _⊙ ), an eccentricity of e _B = 0.422 ± 0.002, and an inclination of i _B = 119 . ° 8 ± 0 . ° 1. For γ Cep Ab, we find a semimajor axis of a _Ab = 1.978 ± 0.007 au, a minimum mass of M A b sin i = 1.6 2 − 0.04 + 0.04 M Jup , and an eccentricity of e A b = 0.0 7 − 0.03 + 0.03 . The system is dynamically compact, and its orbital architecture is consistent with von Zeipel–Lidov–Kozai (vZLK) oscillations, although the planet’s orbital inclination remains insufficiently constrained to determine whether the vZLK mechanism is active. Using the RV residuals and dynamical constraints, we rule out additional Jovian companions between 2.5 and 20 au, and companions more massive than Neptune for a < 1 au, both at >90% confidence. The absence of additional giant planets over a broad range of orbital separations is consistent with a dynamically sculpted system in which the close stellar companion has strongly limited the formation or long-term survival of other long-period giant companions.

A variety of effects can perturb the orbital properties of single planets in close orbits around their host stars. HD80606 b is a highly eccentric ( ϵ = 0.93) exoplanet orbiting its host G5V star, HD 80606. HD 80606 b became a compelling target for exoplanet research after G. Laughlin et al. discovered that the 4.16 M _Jup planet transits and is eclipsed by HD 80606, thereby enabling photometric and spectroscopic observations to yield a wealth of information about the object. The exquisite precision of recent JWST eclipse timing offers an opportunity to investigate whether the orbit of HD 80606 b is modified due to a variety of mechanisms, including General Relativity, tidal torques, or the presence of a second perturbing planet. We have used over 25 yr of radial velocity data plus eclipse and transit observations to place limits on the precession of HD 80606 b’s orbit and to assess which of these effects, if any, are observable. The new models are consistent with constant values of period, eccentricity and the argument of periastron, ω , with a limit on ω ̇ at the level of the predicted general relativistic drift. The precision radial velocity and timing data limit the mass and location of a second planet external to HD 80606 b. Timing offsets between JWST eclipses relative to the K. A. Pearson et al. predictions are attributed to the poor constraints on ϵ ( cos ω , sin ω ) with only the single eclipse measurement available in that analysis and to an over-weighting of the eclipse timing in that earlier analysis.

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.

We present spectroscopic follow-up observations of binary systems from the Gaia Data Release 3 (DR3) binary catalog that were selected to have large enough mass functions for their companions to be black holes or neutron stars. The selection includes 20 stars that are astrometric and/or spectroscopic binaries, as well as 11 stars with large accelerations both in the plane of the sky and along the line of sight but no DR3 orbital solution. We provide classifications for this entire sample, including radial velocity orbital solutions for 12 binaries. Apart from the previously published binaries Gaia BH1, Gaia BH , and Gaia NS1, we show that the Gaia orbits are incorrect for all of the stars with candidate dark companions above 2 M _⊙ . We suggest more conservative cuts on the significance and goodness-of-fit parameters that may be useful for identifying reliable orbital solutions in the tail of the binary star distribution. Although we find no new confirmed black hole or neutron star companions, one accelerating system has a minimum companion mass of 1.16 ± 0.01 M _⊙ that is likely to be a neutron star or an ultramassive white dwarf. The acceleration catalogs may therefore provide a largely unexplored source of additional wide binaries containing compact objects.

The upcoming release of Gaia DR4 will yield thousands of giant planet candidates, eventually enabling studies of giant planet eccentricities, masses, and occurrence rates across a broad range of stellar host masses, metallicities, and ages. However, some of these planet candidates are expected to be false positives and even genuine detections will require additional observations to precisely determine their orbits and masses. We present here the first results of the Gaia Exoplanet Orbits, Demographics, and Evolution Survey, an observational campaign to identify the most promising planet candidate hosts for pre-DR4 vetting and post-DR4 validation and characterization. In this paper, we showcase three systems from our broader sample exhibiting both tangential and radial accelerations, each representing a distinct outcome of our survey strategy. We combine Hipparcos, Hipparcos-Gaia, Gaia DR2, and Gaia DR3 absolute astrometry with adaptive optics (AO) imaging and precision radial velocities (RVs) to constrain companion masses and orbits. HIP 18512, a nearby (15.3 pc) K4V dwarf, hosts a low-mass stellar companion at 10 . ″ 87 ± 0 . ″ 07 (166 au) which produces significant RV and astrometric accelerations on its host star. The RV trend and astrometric acceleration of the nearby (24.2 pc) K4V star HIP 45839, together with an AO imaging nondetection, constrain the companion to a = 17 . 9 − 2.7 + 4.8 au ( P = 70–127 yr) and M = 45 . 2 − 12.7 + 10.5 M _Jup . In the case of HIP 81991 (43.8 pc, G5V), the astrometric and RV data indicate that the companion has a separation of 6 . 4 − 0.3 + 0.6 au ( P = 14.4–17.7 yr) and a mass of 9 . 5 − 2.2 + 5.4 M _Jup , and is more likely a planet (65%) than a brown dwarf (35%).

51 Pegasi harbors the first confirmed extrasolar planet orbiting a Sun-like star. Decades of continued radial velocity (RV) observations have since uncovered signatures of an additional distant companion in the system from a shallow radial acceleration. We present new constraints on the mass and separation of a potential outer companion based on a synthesis of RVs, absolute astrometry, and new high-contrast imaging. Our analysis combines 31 yr of new and previously published RV measurements from the OHP/ELODIE, Lick/Hamilton, Keck/HIRES, and Automated Planet Finder (APF)/Levy spectrographs; a ∼25 yr baseline of absolute astrometry from Hipparcos and Gaia; and deep imaging from Keck/NIRC2 and HST/WFPC2. We find evidence for curvature in the RVs, which, when combined with nondetections from imaging and astrometry, point to a super-Jupiter at ≃15–100 au or a brown dwarf companion at ≈20–170 au. However, the inferred radial acceleration of the host star is driven primarily by the Lick/Hamilton dataset, and its slope is consistent with long-term instrument drift, calling into question the nature of the long-period signal. If an outer companion is present, it could explain the origin of the inner hot Jupiter if 51 Peg b arrived at its current location through high-eccentricity migration. On the other hand, if the signal is spurious, the exceptional baseline rules out Jovian planets within ∼10 au and most brown dwarfs within several tens of astronomical units, implying that the system is devoid of massive companions. Continued RV and astrometric monitoring together with high-contrast imaging can be used to distinguish these scenarios.

We introduce the Keck Planet Finder (KPF) Spectroscopy of the Upper Atmospheres and Refractory Species in Ultrahot Planets (SURFS-UP) Survey, a high-resolution survey to investigate the atmospheric composition and dynamics of a sample of ultrahot Jupiters with the KPF. Due to the unique design of KPF, we developed a publicly available pipeline for KPF that performs blaze removal, continuum normalization, order stitching, science spectra combination, telluric correction, and atmospheric detection via cross correlation. As a first demonstration, we applied this pipeline to a transit of WASP-76 b and achieved some of the highest signal-to-noise ratio (SNR) detections of refractory species in WASP-76 b to date (e.g., Fe I is detected at an SNR of 14.5). We confirm previous observations of an asymmetry in Fe I absorption, but find no measurable ingress–egress asymmetry in Na I and Ca II . Together, these results suggest variations within different layers of the atmosphere of WASP-76 b: neutral metals such as Fe I trace deeper regions with stronger asymmetries, while Na I and Ca II probe regions higher in the atmosphere where the ingress–egress asymmetries are weaker. Unlike some other ultrahot Jupiters, our results are qualitatively consistent with general circulation model predictions of decreasing velocity asymmetry with altitude and do not require a high-altitude superrotating jet that has been invoked for other planets (e.g., WASP-121 b). These results suggest that atmospheric circulation patterns in ultrahot Jupiters may be more diverse than previously thought, highlighting the need for broader surveys to study how atmospheric dynamics depend on planetary and stellar properties.

We present high-precision measurements for HR 7672AB, composed of a Sun-like (G0V) star and an L dwarf companion. Three nights of precise (70 cm s ⁻¹ ) radial velocity (RV) asteroseismology with the Keck Planet Finder clearly detect 5 minute oscillations from the primary, HR 7672A, and modeling of the frequency spectrum yields an asteroseismic age of 1.87 ± 0.65 Gyr. We also determine a gyrochronological age of 2.58 ± 0.47 Gyr, and we combine these two results for a final age of 2.26 ± 0.40 Gyr. In addition, we obtained new RVs for HR 7672A and new astrometry for the companion HR 7672B. From a joint orbit fit, we measured a dynamical mass of 1.111 ± 0.017 M _⊙ for HR 7672A and 75.39 ± 0.67 M _Jup for HR 7672B. This places the companion near the stellar/substellar boundary and is thus particularly sensitive to differences in model predictions. The joint precision in host star age (18% uncertainty) and companion mass (0.9% uncertainty) makes HR 7672AB an exceptional substellar benchmark. Combined with the companion’s luminosity, we use these measurements to test predictions from six brown dwarf cooling models. The best agreement occurs with the G. Chabrier et al. models, which incorporate a new equation of state, resulting in predictions that agree within σ with all the observations. The other five sets of models agree at the 1 σ –3 σ level, depending on the particular test, and some models struggle to predict a sufficiently low luminosity for HR 7672B at any age given its dynamical mass. We also detected a weak seismic signal in near-simultaneous TESS photometry of HR 7672A, with the resulting RV-to-photometry oscillation amplitude ratio consistent with solar values.