Flux-limited Diffusion Approximation Models of Giant Planet Formation by Disk Instability. II. Quadrupled Spatial Resolution

While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2021) used the β cooling approximation to calculate 3D models of disks with initial masses of 0.091 M ⊙ extending from 4 to 20 au around 1 M ⊙ protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as β cooling models with β ∼ 1 to ∼10, including the critical value of β = 3 for fragmentation proposed by Gammie.

Diurnal variations in the stratosphere of an ultrahot exoplanet

The temperature profile of a planetary atmosphere is a key diagnostic of radiative and dynamical processes governing the absorption, redistribution, and emission of energy. Observations have revealed dayside stratospheres that either cool [1,2] or warm [3,4] with altitude for a small number of gas giant exoplanets, while others are consistent with constant temperatures [5,6,7,8]. Here we report spectroscopic phase curve measurements for the gas giant WASP-121b,[9] which constrain stratospheric temperatures throughout the diurnal cycle. Variations measured for a water vapor spectral feature reveal a temperature profile that transitions from warming with altitude on the dayside hemisphere to cooling with altitude on the nightside hemisphere. The data are well explained by models assuming chemical equilibrium, with water molecules thermally dissociating at low pressures on the dayside and recombining on the nightside [10,11]. Nightside temperatures are low enough for perovskite (CaTiO3) to condense, which could deplete titanium from the gas phase [12,13] and explain recent non-detections at the day-night terminator [14,15,16,17]. Nightside temperatures are also low enough for refractory species, such as magnesium, iron, and vanadium, to condense. Detections [16,17,18,19] of these metals at the day-night terminator suggest, however, that if they do form nightside clouds, cold trapping is not as effective at removing them from the upper atmosphere. Note: Numbered references have been entered into the "Manuscript Comment" box.

Rapid Formation of Gas-giant Planets via Collisional Coagulation from Dust Grains to Planetary Cores

Gas-giant planets, such as Jupiter, Saturn, and massive exoplanets, were formed via the gas accretion onto the solid cores, each with a mass of roughly 10 Earth masses. However, rapid radial migration due to disk–planet interaction prevents the formation of such massive cores via planetesimal accretion. Comparably rapid core growth via pebble accretion requires very massive protoplanetary disks because most pebbles fall into the central star. Although planetesimal formation, planetary migration, and gas-giant core formation have been studied with a lot of effort, the full evolution path from dust to planets is still uncertain. Here we report the result of full simulations for collisional evolution from dust to planets in a whole disk. Dust growth with realistic porosity allows the formation of icy planetesimals in the inner disk (≲10 au), while pebbles formed in the outer disk drift to the inner disk and there grow to planetesimals. The growth of those pebbles to planetesimals suppresses their radial drift and supplies small planetesimals sustainably in the vicinity of cores. This enables rapid formation of sufficiently massive planetary cores within 0.2–0.4 million years, prior to the planetary migration. Our models shows the first gas giants form at 2–7 au in rather common protoplanetary disks, in agreement with the exoplanet and solar systems.

Russian gas giant shies away from new pipeline routes

Headline RUSSIA: Gazprom shies away from new pipeline routes

Modelling Mineral Snowflakes in the Atmospheres of Gas-Giant Exoplanets

<p>Exoplanets provide excellent laboratories to explore novel atmospheric regimes; using observations coupled with microphysical models we can probe our understanding of the formation and evolution of planets beyond those in the Solar System. However, clouds remain a key challenge in observation of exoplanet atmospheres, both altering the local atmospheric composition and obscuring deeper atmospheric layers. Currently, most observed exoplanet atmospheres are tidally locked gas-giants in close orbit around their host star. These hot and ultra-hot Jupiters have day-side temperatures in excess of 2500 K, and still above 400 K on the night-side, thus they form solid clouds made of minerals, metal oxides and metals. These clouds may form snowflake like structures, either through condensation or by constructive collisions (coagulation).</p><p>We explore the effects of non-compact, non-spherical cloud particles in gas-giant exoplanet atmospheres by expanding our kinetic non-equilibrium cloud formation model, to include parameterised porous cloud particles as well as cloud particle growth and fragmentation through collisions. We apply this model to prescribed 1D temperature - pressure Drift-Phoenix atmospheric profiles, using Mie theory and effective medium theory to study cloud optical depths, representing the effects of the non-spherical cloud particles through a statistical distribution of hollow spheres.</p><p>Finally, we apply our cloud formation model to a sample of gas-giants as well as ultra-hot Jupiters, using 1D profiles extracted from the 3D SPARC/MITgcm general circulation model. In particular, we take the example cases of gas-giant WASP-43b and the ultra-hot Jupiter HAT-P-7b, where we find dramatic differences in the day-/night-side distribution of clouds between these types of exoplanets due to the intensity of stellar irradiation for HAT-P-7b. Further an asymmetry in cloud coverage at the terminators of ultra-hot Jupiters is observable in the optical depth of the clouds, which affects the observable atmospheric column and thus has implication for detection of key gas phase species. Clouds also enhance the gas phase C/O which is often used as an indicator of formation history. With next-generation instruments such as the James Webb Space Telescope (JWST) such details will begin to be examined, but we find that a detailed understanding of cloud formation processes will be required to interpret observations.</p>