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1. The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256–1257 b

   https://doi.org/10.3847/2041-8213/acb04a  

   Miles, Brittany E.; Biller, Beth A.; Patapis, Polychronis; Worthen, Kadin; Rickman, Emily; Hoch, Kielan K.; Skemer, Andrew; Perrin, Marshall D.; Whiteford, Niall; Chen, Christine H.; et al (March 2023, The Astrophysical Journal Letters)

   Abstract We present the highest fidelity spectrum to date of a planetary-mass object. VHS 1256 b isa<20M_Jupwidely separated (∼8″,a= 150 au), young, planetary-mass companion that shares photometric colors and spectroscopic features with the directly imaged exoplanets HR 8799c, d, and e. As an L-to-T transition object, VHS 1256 b exists along the region of the color–magnitude diagram where substellar atmospheres transition from cloudy to clear. We observed VHS 1256 b with JWST's NIRSpec IFU and MIRI MRS modes for coverage from 1 to 20μm at resolutions of ∼1000–3700. Water, methane, carbon monoxide, carbon dioxide, sodium, and potassium are observed in several portions of the JWST spectrum based on comparisons from template brown dwarf spectra, molecular opacities, and atmospheric models. The spectral shape of VHS 1256 b is influenced by disequilibrium chemistry and clouds. We directly detect silicate clouds, the first such detection reported for a planetary-mass companion. 

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2. TESS Hunt for Young and Maturing Exoplanets (THYME). X. A Two-planet System in the 210 Myr MELANGE-5 Association

   https://doi.org/10.3847/1538-3881/ad4993  

   Thao, Pa Chia; Mann, Andrew W; Barber, Madyson G; Kraus, Adam L; Tofflemire, Benjamin M; Bush, Jonathan L; Wood, Mackenna L; Collins, Karen A; Vanderburg, Andrew; Quinn, Samuel N; et al (June 2024, The Astronomical Journal)

   Abstract Young (<500 Myr) planets are critical to studying how planets form and evolve. Among these young planetary systems, multiplanet configurations are particularly useful, as they provide a means to control for variables within a system. Here, we report the discovery and characterization of a young planetary system, TOI-1224. We show that the planet host resides within a young population we denote as MELANGE-5. By employing a range of age-dating methods—isochrone fitting, lithium abundance analysis, gyrochronology, and Gaia excess variability—we estimate the age of MELANGE-5 to be 210 ± 27 Myr. MELANGE-5 is situated in close proximity to previously identified younger (80–110 Myr) associations, Crius 221 and Theia 424/Volans-Carina, motivating further work to map out the group boundaries. In addition to a planet candidate detected by the TESS pipeline and alerted as a TESS object of interest, TOI-1224 b, we identify a second planet, TOI-1224 c, using custom search tools optimized for young stars (NotchandLOCoR). We find that the planets are 2.10 ± 0.09R_⊕and 2.88 ± 0.10R_⊕and orbit their host star every 4.18 and 17.95 days, respectively. With their bright (K= 9.1 mag), small (R_*= 0.44R_⊙), and cool (T_eff= 3326 K) host star, these planets represent excellent candidates for atmospheric characterization with JWST. 

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3. Are These Planets or Brown Dwarfs? Broadly Solar Compositions from High-resolution Atmospheric Retrievals of ∼10–30 M _Jup Companions

   https://doi.org/10.3847/1538-4357/ad4796  

   Xuan, Jerry_W; Hsu, Chih-Chun; Finnerty, Luke; Wang, Jason; Ruffio, Jean-Baptiste; Zhang, Yapeng; Knutson, Heather_A; Mawet, Dimitri; Mamajek, Eric_E; Inglis, Julie; et al (July 2024, The Astrophysical Journal)

   Abstract Using Keck Planet Imager and Characterizer high-resolution (R∼ 35,000) spectroscopy from 2.29 to 2.49μm, we present uniform atmospheric retrievals for eight young substellar companions with masses of ∼10–30M_Jup, orbital separations spanning ∼50–360 au, andT_effbetween ∼1500 and 2600 K. We find that all companions have solar C/O ratios and metallicities to within the 1σ–2σlevel, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets withm< 10M_Jup, which show tentative atmospheric metal enrichment compared to their host stars. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions withT_eff∼ 1700–2000 K (κAnd b and GSC 6214–210 b), our best-fit models prefer a nongray cloud model with >3σsignificance. The cloudy models yield 2σ−3σlowerT_efffor these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1σlevel. Finally, we constrain¹²CO/¹³CO for three companions with the highest signal-to-noise ratio data (GQ Lup b, HIP 79098b, and DH Tau b) and report $v\sin i$and radial velocities for all companions. 

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4. The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

   https://doi.org/10.3847/1538-3881/ad81d7  

   Thao, Pa Chia; Mann, Andrew W; Feinstein, Adina D; Gao, Peter; Thorngren, Daniel; Rotman, Yoav; Welbanks, Luis; Brown, Alexander; Duvvuri, Girish M; France, Kevin; et al (December 2024, The Astronomical Journal)

   Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5μm transmission spectrum of the 17 Myr, Jupiter-size (R∼10R_⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneousg-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20M_⊕irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0M_⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm⁻³, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H₂O and CO₂(≥7σ), a modest detection of CO (3.5σ), and weak detections of H₂S and SO₂(≃2σ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO₂feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03M_⊕Myr⁻¹), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence. 

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5. Revealing the Elusive Companion of the Red Giant Binary 2MASS J05215658+4359220 from UV HST and Astrosat/UVIT Data

   https://doi.org/10.3847/1538-4357/ad712f  

   Bianchi, Luciana; Hutchings, John; Bohlin, Ralph; Thilker, David; Berti, Emanuele (November 2024, The Astrophysical Journal)

   Abstract Black hole (BH) demographics in different environments is critical in view of recent results on massive star binarity, and of the multimessenger detectability of compact object mergers. But the identification and characterization of noninteracting BHs are elusive, especially in the sparse field stellar population. A candidate noninteractive BH + red giant (RG) binary system, 2MASS J05215658+4359220, was identified by T. A. Thompson et al. We obtained Astrosat/UVIT far-ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV−optical imaging and spectroscopy of the source to test possible scenarios for the optically elusive companion. HST/STIS spectra from ≈1600 to 10230 Å are best fit by the combination of two stellar sources, a RG withT_eff= 4250 ± 150 K, logg= 2.0,R_RG∼ 27.8R_⊙(assuming a single-temperature atmosphere), and a subgiant companion withT_eff= 6000 K,R_comp= 2.7R_⊙, orT_eff= 5270 K,R_comp= 4.2R_⊙using models with one-tenth or one-third solar metallicity, respectively, logg= 3.0, extinctionE_B−V= 0.50 ± 0.2, adopting the Data Release 3 Gaia distanceD= 2463 ± 120 pc. No FUV data existed prior to our programs. STIS spectra give an upper limit of 10⁻¹⁷erg cm⁻²s⁻¹Å⁻¹shortwards of 2300 Å; an upper limit of ≳25.7 ABmag was obtained in two UVIT FUV broad bands. The nondetection of FUV flux rules out a compact companion such as a hot white dwarf. The STIS spectrum shows strong Mgiiλ2800 Å emission, typical of chromospherically active RGs. The masses inferred by comparison with evolutionary tracks, ∼1M_⊙for the RG and between 1.1 and 1.6M_⊙for the subgiant companion, suggest past mass transfer, although the RG currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters. 

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