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Register a new userFormation of Silicate and Titanium Clouds on Hot Jupiters
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We present the first application of a bin-scheme microphysical and vertical transport model to determine the size distribution of titanium and silicate cloud particles in the atmospheres of hot Jupiters. We predict particle size distributions from first principles for a grid of planets at four representative equatorial longitudes, and investigate how observed cloud properties depend on the atmospheric thermal structure and vertical mixing. The predicted size distributions are frequently bimodal and irregular in shape. There is a negative correlation between total cloud mass and equilibrium temperature as well as a positive correlation between total cloud mass and atmospheric mixing. The cloud properties on the east and west limbs show distinct differences that increase with increasing equilibrium temperature. Cloud opacities are roughly constant across a broad wavelength range with the exception of features in the mid-infrared. Forward scattering is found to be important across the same wavelength range. Using the fully resolved size distribution of cloud particles as opposed to a mean particle size has a distinct impact on the resultant cloud opacities. The particle size that contributes the most to the cloud opacity depends strongly on the cloud particle size distribution. We predict that it is unlikely that silicate or titanium clouds are responsible for the optical Rayleigh scattering slope seen in many hot Jupiters. We suggest that cloud opacities in emission may serve as sensitive tracers of the thermal state of a planets deep interior through the existence or lack of a cold trap in the deep atmosphere.
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We determine the observability in transmission of inhomogeneous cloud cover on the limbs of hot Jupiters through post processing a general circulation model to include cloud distributions computed using a cloud microphysics model. We find that both the east and west limb often form clouds, but that the different properties of these clouds enhances the limb to limb differences compared to the clear case. Using JWST it should be possible to detect the presence of cloud inhomogeneities by comparing the shape of the transit lightcurve at multiple wavelengths because inhomogeneous clouds impart a characteristic, wavelength dependent signature. This method is statistically robust even with limited wavelength coverage, uncertainty on limb darkening coefficients, and imprecise transit times. We predict that the short wavelength slope varies strongly with temperature. The hot limb of the hottest planets form higher altitude clouds composed of smaller particles leading to a strong rayleigh slope. The near infrared spectral features of clouds are almost always detectable, even when no spectral slope is visible in the optical. In some of our models a spectral window between 5 and 9 microns can be used to probe through the clouds and detect chemical spectral features. Our cloud particle size distributions are not log-normal and differ from species to species. Using the area or mass weighted particle size significantly alters the relative strength of the cloud spectral features compared to using the predicted size distribution. Finally, the cloud content of a given planet is sensitive to a species desorption energy and contact angle, two parameters that could be constrained experimentally in the future.
Observations have confirmed the existence of multiple-planet systems containing a hot Jupiter and smaller planetary companions. Examples include WASP-47, Kepler-730, and TOI-1130. We examine the plausibility of forming such systems in situ using $N$-body simulations that include a realistic treatment of collisions, an evolving protoplanetary disc and eccentricity/inclination damping of planetary embryos. Initial conditions are constructed using two different models for the core of the giant planet: a seed-model and an equal-mass-model. The former has a more massive protoplanet placed among multiple small embryos in a compact configuration. The latter consists only of equal-mass embryos. Simulations of the seed-model lead to the formation of systems containing a hot Jupiter and super-Earths. The evolution consistently follows four distinct phases: early giant impacts; runaway gas accretion onto the seed protoplanet; disc damping-dominated evolution of the embryos orbiting exterior to the giant; a late chaotic phase after dispersal of the gas disc. Approximately 1% of the equal-mass simulations form a giant and follow the same four-phase evolution. Synthetic transit observations of the equal-mass simulations provide an occurrence rate of 0.26% for systems containing a hot Jupiter and an inner super-Earth, similar to the 0.2% occurrence rate from actual transit surveys, but simulated hot Jupiters are rarely detected as single transiting planets, in disagreement with observations. A subset of our simulations form two close-in giants, similar to the WASP-148 system. The scenario explored here provides a viable pathway for forming systems with unusual architectures, but does not apply to the majority of hot Jupiters.
We present the results of a search for planetary companions orbiting near hot Jupiter planet candidates (Jupiter-size candidates with orbital periods near 3 days) identified in the Kepler data through its sixth quarter of science operations. Special emphasis is given to companions between the 2:1 interior and exterior mean-motion resonances. A photometric transit search excludes companions with sizes ranging from roughly 2/3 to 5 times the size of the Earth, depending upon the noise properties of the target star. A search for dynamically induced deviations from a constant period (transit timing variations or TTVs) also shows no significant signals. In contrast, comparison studies of warm Jupiters (with slightly larger orbits) and hot Neptune-size candidates do exhibit signatures of additional companions with these same tests. These differences between hot Jupiters and other planetary systems denote a distinctly different formation or dynamical history.
The observed low densities of gas giant planets with a high equilibrium temperature can be simulated in models when a fraction of the surface radiation is deposited deeper in the interior. Meanwhile migration theories suggest that hot Jupiters formed further away from their host-star and migrated inward. We incorporate disk migration in simulations of the evolving interior of hot Jupiters to determine whether migration has a long lasting effect on the inflation of planets. We quantify the difference between the radius of a migrated planet and the radius of a planet that formed in situ as the radius discrepancy. We remain agnostic about the physical mechanism behind interior heating, but assume it scales with the received stellar flux by a certain fraction. We find that the change in irradiation received from the host-star while the planet is migrating can affect the inflation and final radius of the planet. Models with a high fraction of energy deposited in the interior ( > 5%) show a significant radius discrepancy when the deposit is at higher pressures than P=1 bar. For a smaller fraction of 1%, there is no radius discrepancy for any deposit depth. We show that a uniform heating mechanism can cause different rates of inflation, depending on the migration history. If the forthcoming observations on mean densities and atmospheres of gas giants give a better indication of a potential heating mechanism, this could help to constrain the prior migration of such planets.
We use results of shallow-water magnetohydrodynamics (SWMHD) to place estimates on the minimum magnetic field strengths required to cause atmospheric wind variations (and therefore westward venturing hotspots) for a dataset of hot Jupiters (HJs), including HAT-P-7b, CoRoT-2b, Kepler-76, WASP-12b, and WASP-33b, on which westward hotspots have been observationally inferred. For HAT-P-7b and CoRoT-2b our estimates agree with past results; for Kepler-76b we find that the critical dipolar magnetic field strength, over which the observed wind variations can be explained by magnetism, lies between $4mbox{ G}$ and $19mbox{ G}$; for WASP-12b and WASP-33b westward hotspots can be explained by $1mbox{ G}$ and $2mbox{ G}$ dipolar fields respectively. Additionally, to guide future observational missions, we identify $61$ further HJs that are likely to exhibit magnetically-driven atmospheric wind variations and predict these variations are highly-likely in $sim 40$ of the hottest HJs.
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