Exoplanets and Stellar Physics

Abstract:

Magnetic fields are a key factor in the evolution of planets and their atmospheres, but they are still poorly constrained for exoplanets owing to limited direct observations. Ultra-hot Jupiters provide a new avenue to probe magnetic effects, as the circulation of their highly ionized atmospheres could be directly sensitive to the atmospheric magnetic field. However, it remains unclear whether the impact of these magnetic effects can be observed directly and used to constrain the magnetic field strength. With high spectral resolution observations targeting the planetary iron lines, we measure the Doppler shift and thus the wind speed of seven transiting ultra-hot Jupiters. Here we find a clear decrease of wind speed with increasing planetary temperature, which is a trend inconsistent with purely hydrodynamic mechanisms but naturally reproduced by magnetic drag. From this relationship, we estimate the possible strength of magnetic fields of hot giant planets to at most a few gauss, which is comparable with the Jovian equatorial field. Our results support the idea that magnetic fields affect the atmospheric circulation of ultra-hot Jupiters and could provide a crucial benchmark for scaling laws used to predict magnetic fields in exoplanets, from hot Jupiters to rocky Earths, with additional implications for future direct observations.

Abstract:

Abstract The impact of anthropogenic aerosols on the mean, spatial, and temporal distribution of precipitation remains a persistent source of uncertainty in climate research. In particular, absorbing aerosols are known to influence cloud formation and precipitation in ways that are not yet fully understood. On average, warming induced by absorbing aerosols is balanced by reduced latent heating from precipitation, so the atmospheric energy budget constrains mean precipitation. This constraint does not apply to spatial or temporal patterns, making the impact of absorbing aerosols on these aspects more uncertain. A recent idealized study suggests that absorbing aerosols can trigger a transition to episodic precipitation, where rainfall occurs in intense, short‐lived events followed by extended dry periods. This transition resembles a previously reported shift under hothouse climate conditions. Specifically, lower tropospheric radiative heating from absorbing aerosols decouples the lower and upper troposphere, suppressing convection for multiple days. During these dry periods, instability builds up until a strong rain event occurs. In this paper, we build on this previous work to further investigate the effects of absorbing aerosols on precipitation characteristics and extremes. We conduct a hierarchy of model simulations that incorporate online aerosol–radiation coupling, the diurnal cycle of solar radiation, convective aggregation in a large‐domain, and large‐scale tropical circulation in a mock Walker simulation. Our results show that the transition to episodic precipitation events under absorbing aerosol perturbation is robust and occurs across all model configurations. We also examine the role of diurnal solar radiation variations and large‐scale circulation in shaping this transition.

Abstract:

Context . Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during critical early phases of evolution when atmospheric escape and contraction are most active. HIP 67522 b, a 17 Myr old hot Jupiter with an extraordinarily low bulk density (<0.20 g cm −3 ), represents an ideal target for high-resolution transmission spectroscopy. Aims . We aim to constrain the mass and characterise the atmospheric composition, thermal structure, and dynamics of HIP 67522 b using ground-based high-resolution near-infrared spectroscopy with VLT/CRIRES+, complementing recent JWST observations. Methods . We obtained 92 high-resolution spectra ( R ≈ 10 5 ) with VLT/CRIRES+ in the K2166 band during a transit on 30 January 2025. We applied cross-correlation techniques and Bayesian nested sampling retrievals to constrain molecular abundances, temperature structure, and atmospheric dynamics. Results . We detected H 2 O at 20σ and CO at 5σ, confirming the extremely extended atmosphere of this low-mass giant. A velocity offset of −2.9 ± 0.2 km s −1 indicates day-to-night winds. The rotation velocity has been constrained to <1.8 km s −1 at 3σ, consistent with tidal locking. The retrieval analysis suggests a planetary mass of 27.7 −5.5 +5.9 Earth masses and statistically favours a two-temperature atmospheric structure with a discrete change at mbar pressures over an isothermal profile. This mass is twice as high as the mass estimated from JWST atmospheric observations and inconsistent at 3 σ , casting doubt on the actual planetary density of the planet. No matter the choice of atmospheric model, we derived a supersolar C/O ratio that is about 1.5 times solar, along with a supersolar metallicity that might further increase if the atmosphere is cloudy, which is a degeneracy that our data alone cannot resolve. We report a tentative 2 σ detection of HDO with an extreme enrichment factor of ∼1000 relative to the protosolar D/H ratio. If confirmed, this would be the first detection of deuterium in an exoplanet atmosphere and would require an intense escape rate to confirm its presence.