We present Keck Planet Imager and Characterizer (KPIC) high-resolution (R ∼35,000) K-band thermal emission spectroscopy of the ultrahot Jupiter WASP-33b. The use of KPIC’s single-mode fibers greatly improves both blaze and line-spread stabilities relative to slit spectrographs, enhancing the cross-correlation detection strength. We retrieve the dayside emission spectrum with a nested-sampling pipeline, which fits for orbital parameters, the atmospheric pressure–temperature profile, and the molecular abundances. We strongly detect the thermally inverted dayside and measure mass-mixing ratios for CO ( logCO MMR = − 1.1 − 0.6 + 0.4 ), H2O ( logH 2 O MMR = − 4.1 − 0.9 + 0.7 ), and OH ( logOH MMR = − 2.1 − 1.1 + 0.5 ), suggesting near-complete dayside photodissociation of H2O. The retrieved abundances suggest a carbon- and possibly metal-enriched atmosphere, with a gas-phase C/O ratio of 0.8 − 0.2 + 0.1 , consistent with the accretion of high-metallicity gas near the CO2 snow line and post-disk migration or with accretion between the soot and H2O snow lines. We also find tentative evidence for 12CO/13CO ∼ 50, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2–15 × solar). These observations demonstrate KPIC’s ability to characterize close-in planets and the utility of KPIC’s improved instrumental stability for cross-correlation techniques.

Exoplanets with radii between those of Earth and Neptune have stronger surface gravity than Earth, and can retain a sizable hydrogen-dominated atmosphere. In contrast to gas giant planets, we call these planets gas dwarf planets. The James Webb Space Telescope (JWST) will offer unprecedented insight into these planets. Here, we investigate the detectability of ammonia (NH3, a potential biosignature) in the atmospheres of seven temperate gas dwarf planets using various JWST instruments. We use petitRadTRANS and PandExo to model planet atmospheres and simulate JWST observations under different scenarios by varying cloud conditions, mean molecular weights (MMWs), and NH3 mixing ratios. A metric is defined to quantify detection significance and provide a ranked list for JWST observations in search of biosignatures in gas dwarf planets. It is very challenging to search for the 10.3–10.8 μm NH3 feature using eclipse spectroscopy with the Mid-Infrared Instrument (MIRI) in the presence of photon and a systemic noise floor of 12.6 ppm for 10 eclipses. NIRISS, NIRSpec, and MIRI are feasible for transmission spectroscopy to detect NH3 features from 1.5–6.1 μm under optimal conditions such as a clear atmosphere and low MMWs for a number of gas dwarf planets. We provide examples of retrieval analyses to further support the detection metric that we use. Our study shows that searching for potential biosignatures such as NH3 is feasible with a reasonable investment of JWST time for gas dwarf planets given optimal atmospheric conditions.

We present a detailed characterization of 2MASS J04435750+3723031, a low-mass companion orbiting the young M2 star 2MASS J04435686+3723033 at 7.″6 (550 au) with potential membership in the 23 Myr β Pictoris moving group (βPMG). Using near-infrared (NIR) spectroscopy of the companion from IRTF/SpeX, we have found a spectral type of M6 ± 1 and indications of youth through age-sensitive absorption lines and a low surface gravity index (VL-G). A young age is supported by Hα emission and lithium absorption in the host. We reevaluate the membership of this system and find that it is a marginally consistent kinematic match to the βPMG using Gaia parallaxes and new radial velocities for the host and companion. If this system does belong to the βPMG, it would be a kinematic outlier and the companion would be overluminous compared to other similar ultracool objects like PZ Tel B; this would suggest that 2M0443+3723 B could be a close binary (≈52+52 M Jup if equal-flux, compared with 99 ± 5 M Jup if single), and would make it the sixth substellar companion in this group. To test this hypothesis, we acquired NIR adaptive optics images with Keck II/NIRC2, but they do not resolve the companion to be a binary down to the diffraction limit of ∼3 au. If 2M0443+3723 AB does not belong to any moving group, then its age is more uncertain. In this case it is still young (≲30 Myr), and the implied mass of the companion would be between ∼30 and 110 M Jup.

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