Steven Giacalone

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 Kepler mission has significantly advanced our understanding of exoplanet demographics. However, Kepler planet candidates showing centroid shifts are often classified as false positives and thus excluded from demographic analyses. Some of these systems may be erroneous false positives, resulting from unresolved binary stars with planets orbiting one of the stars, thereby introducing biases into past exoplanet demographic studies. Here, we present updated Kepler centroid offset values derived from Gaia Data Release 3 (DR3) and analyze KOI-1623.01, a Kepler centroid offset false positive with a clear transit signal and an orbital period of 111 days. High-contrast imaging with Keck/NIRC2 reveals that KOI-1623A has a close companion with a separation of 0 . ″ 704 ± 0 . ″ 002 , consisting of a slightly evolved G-type primary and a K dwarf secondary, while Gaia detects a tertiary companion with common proper motion at a separation of ∼ 8 . ″ 4 . By combining transit photometry, Kepler pixel-level data, direct imaging, and Gaia DR3, we find that the transit signal is consistent with a 10.0 2 − 0.31 + 0.44 R ⊕ giant planet located in the habitable zone of the secondary. We furthermore use resolved companions from Gaia to estimate that another ∼70 out of 515 of Kepler’s centroid false positives were misclassified and instead are real planets that belong to binary systems.

High-resolution imaging is widely used to constrain false positive scenarios in exoplanet validation, but it is a finite follow-up resource that reaches only a subset of candidates, and its population-level impact on validation outcomes has not been quantified through controlled removal experiments. Using an automated pipeline built on TRICERATOPS , we compute the false positive probability (FPP) of 443 TESS planet candidates. For the 264 planet candidates with high-resolution imaging observations, we compute FPP with and without the corresponding contrast curves, allowing us to quantify the impact of the additional data. We find that 72% of 68 contrast-curve-bearing validated planets would fail validation without their adopted contrast curves. The fraction requiring imaging decreases with increasing planet size, from 100% below 1.7 R _⊕ to 33% above 4 R _⊕ : within our sample and TRICERATOPS -based analysis, the availability of high-resolution imaging directly limits the yield of small-planet validation and the supply of validated targets for atmospheric characterization. Our analysis statistically validates 64 new TESS planets with sizes spanning 0.94 to 7.83 R _⊕ across hosts of spectral types: M through F. Four of these are highly amenable to JWST observations based on the transmission and emission spectroscopy metrics, and each achieves validation only with its imaging constraint.

The Transiting Exoplanet Survey Satellite (TESS) has transformed the study of nearby exoplanetary systems; however, its nominal observing strategy limits sensitivity to planets with orbital periods shorter than ∼10 days for most parts of the sky. The two TESS Continuous Viewing Zones (CVZs) provide extended temporal baselines that help overcome this limitation, enabling the detection of longer-period (>10 days) transiting planets around nearby stars. Here, we present COUNTESS , a transit-search pipeline optimized for long-baseline TESS observations that combines multisector light curves with heterogeneous cadences, and implements fast-folding box least-squares period detection, vetting, and statistical validation. As a first application of the pipeline, we conducted a search on the primary and first extended mission photometry in the TESS northern CVZ. For this analysis, we used Gaia Data Release 3 and Two Micron All Sky Survey photometry to homogeneously derive a stellar catalog of FGKM stars for the TESS northern CVZ, resulting in a sample of 391,059 stars. We used COUNTESS to search for transiting planets around 26,114 of these stars with TESS-SPOC light curves and assessed its performance, recovering 115 out of 159 known TESS Objects of Interest (TOIs; 0.85 < P < 124.72 days; 1.03 < R _p < 16.35 R _⊕ ). Additionally, we identified 10 new exoplanet candidates (1.20 < P < 34.62 days; 1.73 < R _p < 4.19 R _⊕ ) that passed vetting tests, including two new statistically validated sub-Neptunes, TIC 219893931 b and TIC 237254473 b. COUNTESS enables extended-baseline TESS analyses and identification of longer-period planets, establishing a foundation for future exoplanet demographic studies, including comparisons with the Kepler Space Telescope and K2.

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.

The characterization of Sun-like stars, including determining whether those stars are single or have stellar companions, has become increasingly important for missions seeking planets beyond the solar system. With upcoming missions like the Habitable Worlds Observatory (HWO) on the horizon, vetting stars for binarity and confirming potential targets as viable is an essential precursor science. In this paper, we present an automated algorithm that searches for double-lined spectroscopic binaries (SB2s) in high-resolution spectra. Using MOLUSC, we simulate realistic stellar populations to quantify the capability of the algorithm for detecting stellar companions around nearby stars, with potential applications to HWO target stars and Gaia DR4 planet-host candidates. In addition, we explore how the detection rate is influenced by complementary observations like high-resolution adaptive optics imaging and time-series radial velocities. We find that our SB2 detection algorithm alone can rule out 46.1% of stellar-mass companions. High-resolution adaptive optics alone rules out 79.6% of stellar companions. Radial velocity surveys alone ruled out 98.4% and 98.7% for 5 yr and 10 yr surveys, respectively. When all three methods are combined, the probability of a typical nearby star having a stellar companion is <1%. The combination of the methods allows us to better understand and resolve target lists for future surveys.

We characterize the radius-dependent observed transiting eccentricity distribution of 219 warm ( P = 8–50 days) systems with only one transiting planetary candidate identified during Sectors 1–69 of the Transiting Exoplanet Survey Satellite (TESS) mission. Using the “photoeccentric effect” in a hierarchical Bayesian framework, we first model the population using discrete planetary size bins (sub-Neptunes, sub-Saturns, and Jovians). We then develop a continuous mixture model with weights governed by a logistic sigmoid function of radius. We find that the warm-single population is best described by two components: a dominant low-eccentricity mode (〈 e _low 〉 = 0.03 9 − 0.038 + 0.018 ) and a secondary dynamically excited mode (〈 e _high 〉 = 0.46 6 − 0.068 + 0.067 ). The fraction of planets belonging to this high-eccentricity component increases strongly with planet radius, characterized by a transition at a break radius of R _br = 9 . 2 − 1.1 + 1.9 R _⊕ . This trend places warm sub-Saturns predominantly on the same low-eccentricity track as sub-Neptunes. In contrast, warm Jovians (8–16 R _⊕ ) are frequently eccentric, with 6 5 − 12 + 13 % of the population in the high-eccentricity mode. Under the assumption of a two-component model, we see tentative evidence for a bimodal Jovian distribution at ∼2.7 σ . Finally, we identify a nonnegligible tail of highly eccentric sub-Neptunes (1–4 R _⊕ ), which comprise 16 . 2 − 6.4 + 5.2 % of the population, consistent with excitation by nontransiting external companions.

M dwarfs are the most common sites of planet formation in the Milky Way. Planet occurrence and composition are closely linked with the availability of metals in protoplanetary disks, which can be probed by measuring the metallicity of planet host stars. In this work, we measure the metallicities ([M/H] and [Fe/H]) of 59 M dwarfs hosting 76 planets and candidates using medium-resolution near-infrared spectra collected with IRTF/SpeX. We combine these results with literature metallicity measurements for planet-hosting cool dwarfs, and present a sample of 86 stars hosting 142 candidate, validated, and confirmed planets with homogeneously derived stellar parameters. Using our updated stellar radii, we calculate planet radii from TESS transit depths for both the confirmed ( N = 51, 0.6–12.5 R _⊕ , median R _p = 1.8 R _⊕ ) and candidate ( N = 25, 0.6–7.2 R _⊕ , median R _p = 2.1 R _⊕ ) planets. We compare the metallicity distributions of super-Earth and sub-Neptune host stars, finding that M dwarfs hosting sub-Neptunes are statistically more metal-rich than those hosting super-Earths. This result is robust to the radius valley prescription used and is likely not due to differences in the stellar samples considered. This result supports the hypothesized formation pathway whereby sub-Neptunes form beyond the water ice line where they can accrete volatiles before migrating inward to their observed locations. The enhanced inventories of refractory elements throughout the disk and of volatiles beyond the ice line in metal-rich disks around low-mass stars may contribute to the preference seen in the observed planet sample for sub-Neptunes to orbit metal-rich M dwarfs.

The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs; P łesssim 10 d, R_ ̊m p > 8 R_⊕) has long been a matter of debate. While most of them are found to be `lonely', there is a rare population of HJs hosting small inner-orbit companions (eight known as of May 2026). Their peculiar architecture suggests a gentle disc-migration mechanism. In this study, we present the discovery and characterisation of the multi-planet system TOI-1533, comprising an inner sub-Neptune (TOI-1533 b, P_ ̊m orb = 3.63 d, R_ ̊m p = 3.15 R_⊕) and an outer hot giant planet (TOI-1533 c, P_ ̊m orb = 8.06 d, R_ ̊m p > 7.5 R_⊕) with substantial H/He by mass (ρ_ ̊m p < 0.48 g cm -3 $), both transiting an active K-dwarf star (T_ ̊m eff ≈ 5130 K; V (mag) ≈ 11). Our joint modelling of stellar activity and planetary signals from radial velocities (HARPS-N) and transits (TESS) allowed us to detect the planetary Keplerian signals (approximately 10 σ) and isolate the stellar modulation. The inclusion of simultaneous photometry in the multi-dimensional Gaussian processes formalism was a fundamental addition to the spectroscopic activity indicators, enabling the disentanglement of stellar activity from planetary signals. The mass ratio of the two confirmed planets (M_ ̊m b / M_ ̊m c about 0.8), together with the super-Neptune mass of the large outer companion ($M_ ̊m c ≈ 40 M_⊕), makes this system unusual compared to the other known HJs with low-mass inner companions.