James Sikora

Near-ultraviolet (NUV) measurements of exoplanet transits offer a means to probe atmospheric escape, cloud formation, and planetary magnetic fields. We examine a 2024 XMM-Newton Optical Monitor NUV observation of the transit of XO-3 b, a massive hot Jupiter on an eccentric orbit with a previously observed abnormally large NUV-absorbing atmosphere. We analyze these NUV data jointly with a concurrent ground-based optical observation and all Transiting Exoplanet Survey Satellite transit observations, and find a NUV transit depth of R p , NUV / R ⋆ = 0.137 1 − 0.019 + 0.016 , which is 30%–70% deeper than the optical transit. Although the optical transits do not show signs of transit timing variations, the transit center in the NUV is 2 2 − 11 + 13 minutes late relative to the optical ephemeris. We investigate atmospheric escape as a potential explanation for the properties of this NUV transit by examining X-ray data from XMM-Newton, characterizing the X-ray luminosity of XO-3 for the first time and estimating an extremely small mass-loss rate of ∼10 ⁴ g s ⁻¹ (∼10 ⁻¹⁹ M _jup yr ⁻¹ ). Finally, we investigate the likelihood of an NUV-absorbing bow shock by estimating the magnetic field of the planet. While such a mechanism can in principle produce NUV transit offsets on the order of tens of minutes, our analytic approximations predict an early rather than late transit, indicating the need for further magnetohydrodynamic simulations.

MASCARA-5 b/TOI-1431 b is an ultra-hot Jupiter ( P _orb = 2.650237 ± 0.000003 dayd, T _eq = 2370 ± 70 K, M _p = 3.12 ± 0.18 M _Jup , R _p = 1.49 ± 0.05 R _Jup ) orbiting a bright Am star ( V = 8.0 mag). Recent time-series observations obtained with PEPSI@LBT during the planet’s posteclipse phases have revealed Fe i emission lines indicative of a thermally inverted atmosphere. These observations demonstrate that MASCARA-5 b is well suited to atmospheric characterization via emission spectroscopy, thereby motivating further follow-up observations covering additional orbital phases to constrain the planet’s atmospheric chemistry, thermal structure, and dynamics. Here we present preeclipse time-series observations obtained with the high-resolution optical spectrograph EXPRES@LDT. Our analysis confirms the previous detection of gas-phase Fe i on MASCARA-5 b’s dayside (with a 5.5 σ significance obtained from two nights of observations), and the fact that the thermal profile is inverted with lower and upper temperatures ∼2000 K and ∼4500 K, respectively. A search for Fe ii and Cr i did not yield any plausible detections. We also find that the preeclipse signal exhibits a nonnegligible blueshift of −3.2 ± 1.4 km s ⁻¹ potentially caused by winds.

Stellar activity remains one of the primary challenges in the detection and characterization of low-mass exoplanets, as it can induce radial velocity (RV) variations that mask or mimic planetary signals. Identifying reliable activity proxies is essential in order to distinguish stellar variability from genuine planetary signatures. In this study, we examine the variability of the chromospheric He I D3 line in high-resolution solar spectra and assess its potential as an activity indicator. We find a strong correlation between the He I D3 line intensity variation and the Sun’s unsigned magnetic flux derived from Solar Dynamics Observatory/Helioseismic and Magnetic Imager data as well as with the solar RVs. Our results suggest that the He I D3 line offers a promising and straightforward proxy for magnetic activity, which may complement existing stellar activity indicators. Its inclusion could help disentangle stellar signals in RV measurements and ultimately improve the detection of Earth-like exoplanets.

We present the JWST/NIRSpec G395H transmission spectrum of the young (10–30 Myr) transiting planet V1298 Tau b (9.85 ± 0.35 R _⊕, T _eq = 670 K). Combined Hubble Space Telescope and JWST observations reveal a haze-free, H/He-dominated atmosphere with a large scale height (∼1500 km), allowing detection of CO₂ (35σ), H₂O (30σ), CO (10σ), CH₄ (6σ), SO₂ (4σ), and OCS (3.5σ). Our observations probe several scale heights (∼4.4 in the CO₂ 4.3 μm band and ∼3 in the 2.7 μm water band). The planet’s mass, inferred from atmospheric scale height using free retrieval and grid modeling, is 12 ± 1 M _⊕ and 15 ± 1.7 M _⊕, respectively, which is significantly lower than previous radial velocity estimates and confirms it as “gas-dwarf” sub-Neptune progenitor. We find an atmospheric metallicity (log Z = 0.6 − 0.6 + 0.4 × solar) and subsolar C/O ratio (0.22 − 0.05 + 0.06 ). The atmospheric metallicity is low compared to mature sub-Neptunes by an order of magnitude. The CH₄ abundance ([CH₄] = −6.2 − 0.5 + 0.3 ]) is ∼7σ lower than the equilibrium chemistry prediction. To adjust for the low methane abundance, the self-consistent grids favor a high internal temperature (∼500 K) and vertical mixing (K _zz ∼ 10⁷−10⁸ cm² s^–1). These internal temperatures are inconsistent with predictions from evolutionary models, which expect ∼100–200 K at the current system age. We estimate a gas-to-core mass fraction between 0.1% and 8%, with a core mass of 11–12 M _⊕, consistent with in-situ gas-dwarf formation. A deep atmospheric metallicity gradient may explain both the high internal temperature and low observable metallicity. Over time, mass loss from such an atmosphere could enhance its metallicity, potentially reconciling V1298 Tau b with mature sub-Neptunes.

We present an analysis on the detection viability of refraction effects in Kepler’s exoplanet atmospheres using binning techniques for their light curves in order to compare against simulated refraction effects. We split the Kepler exoplanets into subpopulations according to orbital period and planetary radius, then search for out-of-transit changes in the relative flux associated with atmospheric refraction of starlight. The presence of refraction effects—or lack thereof—may be used to measure and set limits on the bulk properties of an atmosphere, including mean molecular weight or the presence of hazes. In this work, we use the presence of refraction effects to test whether exoplanets above the period–radius valley have H/He atmospheres, which high levels of stellar radiation could evaporate away, in turn leaving rocky cores below the valley. We find strong observational evidence of refraction effects for exoplanets above the period–radius valley based on Kepler photometry; however, those related to optically thin H/He atmospheres are not common in the observed planetary population. This result may be attributed to signal dampening caused by clouds and hazes, consistent with the optically thick and intrinsically hotter atmospheres of Kepler exoplanets caused by relatively close host star proximity.

High-eccentricity gas giant planets serve as unique laboratories for studying the thermal and chemical properties of H/He-dominated atmospheres. One of the most extreme cases is HD 80606 b—a hot Jupiter orbiting a Sun-like star with an eccentricity of 0.93—which experiences an increase in incident flux of nearly 3 orders of magnitude as the star–planet separation decreases from 0.88 au at apoastron to 0.03 au at periastron. We observed the planet’s periastron passage using JWST’s NIRSpec/G395H instrument (2.8–5.2 μm) during a 21 hr window centered on the eclipse. We find that, as the planet passes through periastron, its emission spectrum transitions from a featureless blackbody to one in which CO, CH₄, and H₂O absorption features are visible. We detect CH₄ during postperiapse phases at 4.1–10.7σ depending on the phase and on whether a flux offset is included to account for NRS1 detector systematics. Following periapse, H₂O and CO are also detected at 4.2–5.5σ and 3.7–4.4σ, respectively. Furthermore, we rule out the presence of a strong temperature inversion near the IR photosphere based on the lack of obvious emission features throughout the observing window. General circulation models had predicted an inversion during periapse passage. Our study demonstrates the feasibility of studying hot Jupiter atmospheres using partial phase curves obtained with NIRSpec/G395H.

We present an analysis on the detection viability of refraction effects in Kepler's exoplanet atmospheres using binning techniques for their light curves in order to compare against simulated refraction effects. We split the Kepler exoplanets into sub-populations according to orbital period and planetary radius, then search for out-of-transit changes in the relative flux associated with atmospheric refraction of starlight. The presence of refraction effects - or lack thereof - may be used to measure and set limits on the bulk properties of an atmosphere, including mean molecular weight or the presence of hazes. In this work, we use the presence of refraction effects to test whether exoplanets above the period-radius valley have H/He atmospheres, which high levels of stellar radiation could evaporate away, in turn leaving rocky cores below the valley. We find strong observational evidence of refraction effects for exoplanets above the period-radius valley based on Kepler photometry, however those related to optically thin H/He atmospheres are not common in the observed planetary population. This result may be attributed to signal dampening caused by clouds and hazes, consistent with the optically thick and intrinsically hotter atmospheres of Kepler exoplanets caused by relatively close host star proximity.

Observations of auroras on exoplanets would provide numerous insights into planet–star systems, including potential detections of the planetary magnetic fields, constraints on host-star wind properties, and information on the thermal structures of planets. However, there have not yet been any discoveries of auroras on exoplanets. In this paper, we focus on the search for infrared auroral emission from the molecular ion H 3 + , which is common in the atmospheres of solar system planets Jupiter, Saturn, and Uranus. Using Keck/NIRSPEC high-resolution spectroscopy, we search for H 3 + emission from two hot Jupiters, WASP-80b and WASP-69b. We do not see any evidence of emission in the observed spectra when cross correlating with an H 3 + spectral model or when using an autocorrelation approach to search for any significant features. We therefore place upper limits on the total emission of 5.32 × 10¹⁸ W for WASP-80b and 1.64 × 10¹⁹ W for WASP-69b.These upper limits represent the most stringent limits to date and approach the regime of emission suspected from theoretical models.