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1. Atmospheric Characterization of the Super-Jupiter HIP 99770 b with KPIC

   https://doi.org/10.3847/1538-3881/ad6609  

   Zhang, Yapeng; Xuan, Jerry W; Mawet, Dimitri; Wang, Jason J; Hsu, Chih-Chun; Ruffio, Jean-Bapiste; Knutson, Heather A; Inglis, Julie; Blake, Geoffrey A; Chachan, Yayaati; et al (August 2024, The Astronomical Journal)

   Abstract Young, self-luminous super-Jovian companions discovered by direct imaging provide a challenging test for planet formation and evolution theories. By spectroscopically characterizing the atmospheric compositions of these super-Jupiters, we can constrain their formation histories. Here we present studies of the recently discovered HIP 99770 b, a 16M_Juphigh-contrast companion on a 17 au orbit, using the fiber-fed high-resolution spectrograph KPIC ( $\mathcal{R}$ ∼ 35,000) on the Keck II telescope. OurK-band observations led to detections of H₂O and CO in the atmosphere of HIP 99770 b. We carried out free retrieval analyses usingpetitRADTRANSto measure its chemical abundances, including the metallicity and C/O ratio, projected rotation velocity ( $v\sin i$ ), and radial velocity (RV). We found that the companion’s atmosphere has C/O $={0.55}_{-0.04}^{+0.06}$ and [M/H] $={0.26}_{-0.23}^{+0.24}$ (1σconfidence intervals), values consistent with those of the Sun and with a companion formation via gravitational instability or core accretion. The projected rotation velocity $v\sin \left(i\right)<7.8$ km s⁻¹is small relative to other directly imaged companions with similar masses and ages. This may imply a nearly pole-on orientation or effective magnetic braking by a circumplanetary disk. In addition, we added the companion-to-primary relative RV measurement to the orbital fitting and obtained updated constraints on orbital parameters. Detailed characterization of super-Jovian companions within 20 au like HIP 99770 b is critical for understanding the formation histories of this population. 

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2. Sites of Planet Formation in Binary Systems. I. Evidence for Disk−Orbit Alignment in the Close Binary FO Tau

   https://doi.org/10.3847/1538-3881/ad354d  

   Tofflemire, Benjamin M; Prato, Lisa; Kraus, Adam L; Segura-Cox, Dominique; Schaefer, G H; Akeson, Rachel; Andrews, Sean; Jensen, Eric_L N; Johns-Krull, Christopher M; Zanazzi, J J; et al (April 2024, The Astronomical Journal)

   Abstract Close binary systems present challenges to planet formation. As binary separations decrease, so do the occurrence rates of protoplanetary disks in young systems and planets in mature systems. For systems that do retain disks, their disk masses and sizes are altered by the presence of the binary companion. Through the study of protoplanetary disks in binary systems with known orbital parameters, we seek to determine the properties that promote disk retention and therefore planet formation. In this work, we characterize the young binary−disk system FO Tau. We determine the first full orbital solution for the system, finding masses of ${0.35}_{-0.05}^{+0.06}{M}_{\odot }$ and 0.34 ± 0.05M_⊙for the stellar components, a semimajor axis of $22{(}_{-1}^{+2})$ au, and an eccentricity of $0.21{(}_{-0.03}^{+0.04})$ . With long-baseline Atacama Large Millimeter/submillimeter Array interferometry, we detect 1.3 mm continuum and¹²CO (J= 2–1) line emission toward each of the binary components; no circumbinary emission is detected. The protoplanetary disks are compact, consistent with being truncated by the binary orbit. The dust disks are unresolved in the image plane, and the more extended gas disks are only marginally resolved. Fitting the continuum and CO visibilities, we determine the inclination of each disk, finding evidence for alignment of the disk and binary orbital planes. This study is the first of its kind linking the properties of circumstellar protoplanetary disks to a precisely known binary orbit. In the case of FO Tau, we find a dynamically placid environment (coplanar, low eccentricity), which may foster its potential for planet formation. 

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3. Gaia22dkvLb: A Microlensing Planet Potentially Accessible to Radial-velocity Characterization

   https://doi.org/10.3847/1538-3881/ad5203  

   Wu, Zexuan; Dong, Subo; Yi, Tuan; Liu, Zhuokai; El-Badry, Kareem; Gould, Andrew; Wyrzykowski, L; Rybicki, K A; Bachelet, Etienne; Christie, Grant W; et al (July 2024, The Astronomical Journal)

   Abstract We report discovering an exoplanet from following up a microlensing event alerted by Gaia. The event Gaia22dkv is toward a disk source rather than the traditional bulge microlensing fields. Our primary analysis yields a Jovian planet with $M_{\mathrm{p}}={0.59}_{-0.05}^{+0.15}{M}_{\mathrm{J}}$ at a projected orbital separation $r_{\perp }={1.4}_{-0.3}^{+0.8}$ au, and the host is a ∼1.1M_⊙turnoff star at ∼1.3 kpc. At $r\prime \approx 14$ , the host is far brighter than any previously discovered microlensing planet host, opening up the opportunity to test the microlensing model with radial velocity (RV) observations. RV data can be used to measure the planet’s orbital period and eccentricity, and they also enable searching for inner planets of the microlensing cold Jupiter, as expected from the “inner–outer correlation” inferred from Kepler and RV discoveries. Furthermore, we show that Gaia astrometric microlensing will not only allow precise measurements of its angular Einstein radiusθ_Ebut also directly measure the microlens parallax vector and unambiguously break a geometric light-curve degeneracy, leading to the definitive characterization of the lens system. 

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4. New Orbital Constraints for YSES 1 b and HR 2562 B from High-Precision Astrometry and Planetary Radial Velocities ^∗

   https://doi.org/10.3847/1538-3881/ae07cb  

   Roberts, Jonathan; Thompson, William; Wang, Jason J; Blunt, Sarah; Balmer, William O; Bourdarot, Guillaume; Bowler, Brendan P; Chauvin, Gael; Eisenhauer, Frank; Henning, Thomas K; et al (October 2025, The Astronomical Journal)

   Abstract We present new VLTI/GRAVITY astrometry and updated orbit fits for the directly imaged companions YSES 1 b and HR 2562 B, substellar objects straddling the planet–brown-dwarf boundary. Using high-precision astrometry, radial velocity data, and proper motions, we derive revised orbital parameters withorbitize!. For YSES 1 b, the inclusion of GRAVITY astrometry and a relative radial velocity measurement from Y. Zhang et al. (2024) overcomes the traditional challenge of constraining eccentricities for distant companions, enabling the first orbit fit and yielding a constrained eccentricity of 0.44  ±  0.20. This represents the first full orbit fit for the system. Additionally, we calculate a median line-of-sight stellar obliquity of $12_{-8}^{+11}$ deg, providing further insight into the system’s dynamical architecture. For HR 2562 B, our analysis agrees with S. Y. Zhang et al., confirming a low-eccentricity orbit (0.34  ±  0.20) and an inclination of 87  ±  1 deg. We find HR 2562 B’s orbit to be nearly coplanar with the debris disk, with a mutual inclination of 3.7  ±  0.3 deg. For both YSES 1 b and HR 2562 B the lower eccentricities favor an in situ formation scenario over extreme scattering or cloud fragmentation. 

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5. Searching for GEMS: Discovery and Characterization of Two Brown Dwarfs Around M Dwarfs*

   https://doi.org/10.3847/1538-3881/adbb54  

   Larsen, Alexander; Swaby, Tera N; Kobulnicky, Henry A; Cañas, Caleb I; Kanodia, Shubham; Libby-Roberts, Jessica; Monson, Andrew; Gupta, Arvind F; Cochran, William; Mahadevan, Suvrath; et al (April 2025, The Astronomical Journal)

   Abstract Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( $68.0_{-2.2}^{+2.2}{M}_{\mathrm{J}}$ ) and TOI-5610b ( $40.4_{-1.0}^{+1.0}{M}_{\mathrm{J}}$ ), two moderately massive brown dwarfs orbiting early M-dwarf hosts (T_eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are periodP = 10.40046 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.824_{-0.031}^{+0.033}$ R_J, and low eccentricity $e=0.0962_{-0.0046}^{+0.0027}$ . In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios ofq= 0.150. For TOI-5610b, the best-fitting parameters are periodP = 7.95346 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.887_{-0.031}^{+0.031}$ R_J, and moderate eccentricity $e=0.354_{-0.012}^{+0.011}$ . Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the JohnsonKband. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods (P < 13 days) exhibit a dearth of 13M_J < M_BD < 40M_Jcompanions (q < 0.1) compared to those at slightly wider separations. 

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