Exoplanet atmospheres

Abstract:

Abstract The Hubble Space Telescope collected imaging data spanning the full lifetime of Neptune's dark spot NDS‐2018, which is the sixth large, persistent dark spot seen in the planet's atmosphere. Neptune's dark spots are thought to be anticyclonic vortices, although internal flows have never been directly measured to confirm their rotation. Previous reports covered the formation and evolution of the mature NDS‐2018, while here we report that the contrast weakened over the 2021 to 2022 period, while the feature rapidly drifted equatorward from 14 deg N to 6–7 deg N planetographic latitude. The persistence of the spot so close to the equator is surprising, and represents a challenge for numerical models which find anticyclones to be disrupted within 15 deg of the equator. The changing contrast of the dark spot constrains changes in the aerosols over time, but the link between dynamical and microphysical properties of dark vortices is not known. Plain Language Summary The Hubble Space Telescope collected images spanning the full lifetime of Neptune's dark spot NDS‐2018, which is the sixth large, persistent dark spot seen in the planet's atmosphere. Neptune's dark spots are thought to be high‐pressure rotating storms, although internal flows have never been directly measured to confirm the direction of their rotation. Previous reports covered the formation and evolution of the mature NDS‐2018, while here we report that the contrast weakened over the 2021 to 2022 period, while the feature rapidly drifted equatorward from 14 deg N to 6–7 deg N planetographic latitude. The persistence of the spot so close to the equator is surprising, and represents a challenge for numerical models which find these storms to be disrupted within 15 deg of the equator. The changing contrast of the dark spot provides information on changes in the cloud particles over time, but it is not known what is the link between how the storm works and what are the properties of cloud particles in the dark spots. Key Points Dark spot NDS‐2018 was observed on Neptune over its full lifespan, between 3.7 and 4.9 years In 2022, NDS‐2018 remained within 8 deg of the equator as it faded away Secondary dark features had morphologies of discrete spots in 2019–2020 and extended arms in 2022

Abstract:

ABSTRACT Photochemical haze, a common aerosol type expected to form in the atmospheres of hot-Jupiters, can become concentrated to different extents between the morning and evening limbs depending on the balance between advection, gravitational settling, and radiation pressure. We present a analytical framework incorporating the effect of gravity, planetary radius, and stellar flux, alongside the particle size of the haze on its resulting relative distribution between the two limbs. Using this framework and further comparing with 3D climate simulations, our framework provides a reasonable first-order estimate of the maximum radius of haze particles which would reach the morning limb and subsequently be trapped by the nightside gyres, resulting in a higher or comparable concentration of haze over the morning limb compared to the evening limb for a given hot-Jupiter atmosphere. We find that the framework performs best for higher-gravity planets, where the transport of haze particles is more strongly controlled by gravitational settling and therefore less sensitive to the approximations made in describing the atmospheric circulation. We further show that for low-gravity hot-Jupiters, even large haze particles can be readily transported to the morning limb before being removed by gravitational settling, whereas for high-gravity hot-Jupiters only small particles can survive transport to the morning limb. Our novel framework provides a rapid way to understand the transport of haze and plan limb asymmetry observations with James Webb Space Telescope, constraining the parameter space exploration for full-scale computationally expensive 3D simulations.

Abstract:

We report the direct imaging discovery of a third exoplanet in the β Pictoris (β Pic) system. We detect β Pictoris d (β Pic d) in noncoronagraphic observations obtained with the Very Large Telescope (VLT) Enhanced Resolution Imager and Spectrograph (ERIS), as well as multi-epoch archival datasets from the JWST Near Infrared Camera (NIRCam) and VLT/SPHERE. Astrometric measurements over an 11 yr baseline demonstrate that it is consistent with a gravitationally bound source with orbital motion. Joint multi-planet orbit fits of all three planets in the system yield a semimajor axis of 26.0−6.1+2.2 au and inclination 89.0−0.6+0.7 deg for planet d. β Pic d has a larger orbital semimajor axis than the other known planets in the system, but is coplanar with the inner two planets, and its orbit is consistent with sculpting the inner edge of the debris disk. β Pic d has a contrast of ΔL′=12.11±0.15 mag, with colors and luminosity that closely match those of 51 Eri b, another exoplanet in the β Pic moving group. Its VLT/ERIS and JWST/NIRCam colors are distinct from those of free-floating planetary-mass objects of a similar age and temperature. Its red F410M − F444W color indicates strong CO2 absorption in its atmosphere and suggests significant enhancement in metals compared to free-floating objects. From the ATMO hot-start evolutionary models, we estimate an effective temperature of 600−60+45 K and mass of 2.4 ± 0.6 MJup, which also closely matches similar estimates for 51 Eri b. β Pic d is among the lowest-mass exoplanets imaged from the ground. This discovery highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope.

Abstract:

Discovered deep within the “Neptunian desert,” LTT9779b remains the only known ultrahot Neptune, prompting significant speculation regarding its unique formation and evolutionary history. Its exceptionally high geometric albedo has previously been attributed either to the presence of clouds or to an extremely metal-rich atmosphere. Here, we present a comprehensive panchromatic analysis of its dayside atmosphere using JWST NIRISS and NIRSpec/G395H observations to characterize its atmospheric structure and composition. Leveraging the exceptional signal-to-noise ratio (SNR) in the observed spectra, we report a 3σ-to-5σ detection of dayside clouds, with strong evidence for Mg2SiO4(s) (silicate) condensation. This constitutes the first statistically significant detection of clouds on the dayside of a Neptunian-mass exoplanet. We demonstrate that a highly reflective cloud deck, rather than an extremely high-metallicity atmosphere, is the most likely explanation for the planet’s anomalously high optical albedo. Furthermore, our atmospheric retrievals yield robust detections of both CO (∼4.88σ) and CO2 (∼8.76σ), while providing tentative constraints on the H2O abundance and upper limits on SiO, TiO, and VO. Finally, our analysis places a robust constraint on the C/O ratio of 0.984 ± 0.019. This aligns LTT9779b with other known ultrahot Jupiters exhibiting supersolar C/O ratios, suggesting a broader trend driven by the sequestration of oxygen-bearing condensates in ultrahot atmospheres.