Theron W. Carmichael

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.

Substellar companions are most often studied around solar-like or smaller stars, as the rapid rotation and variability of massive stars impede detection using traditional methods. An alternative technique, pulsation timing, uses photometry to measure light-travel-time variations—the time integral of the radial velocity signal—providing orbital constraints with strong sensitivity to intermediate-period companions. We analyze the pulsation timing variations of 16,440 δ Scuti pulsators from the Transiting Exoplanet Survey Satellite (TESS), with the goal of identifying substellar companions. We report the discovery of one candidate planet, HD 156295 b, and eight candidate brown dwarfs by fitting orbital models to the pulsation timing variations, which are consistent with the constraints from Hipparcos–Gaia astrometric acceleration, when available. We infer a mass of 6.3 ± 1.1 M _J for the planet, which is in agreement with the 2 σ upper limit of 10.5 M _J derived from Hipparcos–Gaia accelerations. If confirmed, HD 156295 would be the hottest star to host a habitable-zone planet. Given this candidate, we infer an occurrence rate of η = 0.1 0 − 0.08 + 0.21 for 5–10 Jupiter mass planets with periods between 1900 and 2600 days around A-type stars. If the system is instead a false positive, we infer the 1 σ and 2 σ upper limits on the occurrence rate to be 19% and 39%, respectively. Astrometric follow-up could confirm and further characterize the planet candidate. At least two of our candidate brown dwarf systems represent useful targets for follow-up with direct imaging.

Intense high-energy irradiation of close-in gaseous exoplanets drives the rapid escape of their atmospheres, fundamentally shaping planetary demographics. While atmospheric loss is routinely observed via transit absorption in atomic hydrogen, helium, and metal ions, the underlying physical properties, specifically the thermal structure, outflow dynamics, and mass-loss rate, remain poorly constrained due to inherent degeneracies in the transmission geometry. Here we report the first detection of atomic hydrogen emission from the escaping atmosphere of a gas giant. Using high-resolution spectroscopy of the ultrahot Jupiter KELT-9 b, we detect a hydrogen Balmer line (H α 6564.6 Å) emission signature originating from the planetary dayside. The emission line profile features a distinctive double-peaked shape with 0.1%–0.15% peak amplitudes at ±30 km s ⁻¹ and central self-absorption. This profile breaks transmission degeneracies, providing direct observational constraints on the vertical thermal structure, excited-state hydrogen populations, and wind dynamics in the upper atmosphere of KELT-9 b. Initial modeling reveals a vigorous outflow with a mass-loss rate above 10 ¹³ g s ⁻¹ , among the highest measured to date for gaseous exoplanets. Our results establish hydrogen airglow emission as a powerful diagnostic of atmospheric escape, opening a new observational window into the evolution of worlds in extreme radiation environments.

We present TOI-2155 b, a massive transiting companion, discovered using data from NASA’s Transiting Exoplanet Survey Satellite mission and confirmed with ground-based radial velocity measurements from the Tillinghast Reflector Echelle Spectrograph. We also analyze ground-based follow-up photometric data from the Wendelstein Observatory, Las Cumbres Observatory Global Telescope, and Wild Boar Remote Observatory. TOI-2155 b is a short-period companion with P = 3.7246950 ± 0.0000014 days. The radius and mass of TOI-2155 b are found to be R b = 0.97 2 − 0.008 + 0.009 R J and M b = 80 . 6 − 1.1 + 1.0 M J , respectively, corresponding to a density of ρ b = 10 9 − 3.3 + 3.1 g cm ⁻³ . The F-type subgiant host star has an effective temperature of T _eff = 6085 ± 78 K, a radius R ⋆ = 1.70 5 − 0.064 + 0.066 R _⊙ , and a mass M _⋆ = 1.33 ± 0.008 M _⊙ . With a mass close to the hydrogen-burning minimum mass, TOI-2155 b lies at the boundary between brown dwarfs and low-mass stars. Its measured mass, radius, and density place it in a transitional region, where distinguishing between a massive brown dwarf and a very low-mass star is not straightforward. TOI-2155 b therefore provides a valuable benchmark for testing evolutionary models of stellar and substellar structure near the hydrogen-burning limit.

We report the discovery of TOI-7019b, the first transiting brown dwarf (BD) known to orbit a star that is part of the Milky Way’s ancient thick disk, as defined chemically ([Fe/H] = −0.79 ± 0.05 dex, [ α /Fe] = +0.26 ± 0.05 dex, [M/H] = −0.59 ± 0.06 dex) and kinematically ( v _⊥ ≈ 150 ± 1 km s ⁻¹ ). We estimate a system age τ = 12 ± 2 Gyr by fitting the host star’s spectrum and spectral energy distribution to alpha-enhanced isochrones and independently using the age−metallicity relation of the thick disk. This makes TOI-7019 by far the most metal-poor and ancient BD host known to date. We measure a BD mass of 61.3 M _J ± 2.1 M _J and radius of 0.82 R _J ± 0.02 R _J from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of 48.2592 ± 0.0001 days and an orbital eccentricity of 0.403 ± 0.002. The measured radius appears to be 12.3% ± 2.8% larger than predicted relative to standard evolutionary models for old, metal-poor BDs, as well as models that include stellar radiation, hinting at missing physics like the magnetic inhibition of convection. TOI-7019b lowers the probed metallicity regime for transiting BDs by over a factor of two, making it a benchmark system to test evolutionary models in the low-metallicity regime. Future measurements of TOI-7019b’s atmosphere will test whether a BD’s atmospheric composition tracks its host star’s abundances, as expected for binary-like co-formation.

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.

Studying planetary interactions in exoplanet systems informs theories of planet formation and evolution, providing essential context for understanding our own solar system. We combine spectroscopy, transit photometry, transit timing variations, and astrometry to characterize the TOI-201 system. The cotransiting system consists of a super-Earth, warm Jupiter, and massive companion at 5.8-, 53-, and 2900-day orbital periods, respectively. We perform dynamical simulations to study the past and future of the system. von-Zeipel-Kozai-Lidov oscillations emerge as the most plausible scenario to explain the outer companion’s high orbital eccentricity, with planet-planet scattering a possible but less likely contender. Because of nonzero mutual inclinations between the planets, the system is visibly evolving on very short timescales, with the current cotransiting configuration ending in 200 years.

Masses and radii of transiting brown dwarfs can be measured directly, in contrast to isolated field brown dwarfs, whose mass and radius inferences are model dependent. Therefore, transiting brown dwarfs are a test bed for the interior and evolutionary models of brown dwarfs and giant exoplanets. We have developed atmospheric and evolutionary models for this emerging population. We show that intense stellar irradiation can cause a large enhancement in the radius of transiting brown dwarfs at all masses, especially if the incident flux exceeds log ₁₀ ( F /cgs) ≥ 9 ( T _eq ≥ 1450 K). Stellar irradiation can significantly alter rates of nuclear burning in irradiated brown dwarfs, making the deuterium-burning and hydrogen-burning minimum masses strong functions of incident stellar flux. We show that the D-burning and H-burning minimum masses can decrease by 16% and 13%, respectively, between isolated and strongly irradiated brown dwarfs (log ₁₀ ( F /cgs) ≥ 10 ( T _eq ≥ 2570 K)). This shows that stellar irradiation has a larger impact on the planet-brown dwarf-star mass boundaries than metallicity or clouds. We show that metal cores or migration affect their evolution to a much lesser extent, whereas low mass highly irradiated old sources can help us test the physics of the hot Jupiter radius anomaly. We fit the observed radii of 46 transiting brown dwarfs and show that our irradiated evolutionary models fit their radii better than models that ignore the host star, especially for highly irradiated objects. However, the measured radii of 10 objects are still inconsistent at >3 σ level, indicating residual gaps in our irradiated evolutionary model.

High-resolution spectroscopy provides a unique opportunity to directly probe atmospheric dynamics by resolving Doppler shifts of planetary signals as a function of orbital phase. Using the optical spectrometer the Keck Planet Finder, we carry out a pilot study on high-resolution phase-curve spectra of the ultrahot Jupiter KELT-9 b. We spectrally and temporally resolve its dayside emission from posttransit to preeclipse (orbital phase ϕ = 0.1–0.45). The signal strength and width increase with orbital phase as the dayside rotates into view. The net Doppler shift varies progressively from −13.4 ± 0.6 to −0.4 ± 1.0 km s ⁻¹ , the extent of which exceeds its rotation velocity of 6.4 ± 0.1 km s ⁻¹ , providing unambiguous evidence of atmospheric winds. We devise a retrieval framework to fit the full time-series spectra, accounting for the variation of the line profiles due to the rotation and winds. We retrieve a supersonic day-to-night wind speed up to 11.7 ± 0.6 km s ⁻¹ on the emerging dayside, representing the most extreme atmospheric winds in hot Jupiters to date. Comparison to 3D circulation models reveals weak atmospheric drag, consistent with relatively efficient heat recirculation, as also supported by space-based phase-curve measurements. Additionally, we retrieve the dayside chemistry (including Fe i , Fe ii , Ti i , Ti ii , Ca i , Ca ii , Mg i , and Si i ) and temperature structure, and we place constraints on the nightside thermal profile. Our high-resolution phase-curve spectra and the measured supersonic winds provide excellent benchmarks for extreme physics in circulation models, demonstrating the power of this technique in understanding the climates of hot Jupiters.