We examine formation conditions for the Galilean satellites in the context of models of late-stage giant planet accretion and satellite-disk interactions. We first reevaluate the current standard, in which the satellites form from a ‘‘ minimum mass subnebula ’’ disk, obtained by augmenting the mass of the current satellites to solar abundance and resulting in a disk mass containing about 2% of Jupiter’s mass. Conditions in such a massive and gas-rich disk are difficult to reconcile with both the icy compositions of Ganymede and Callisto and the protracted formation time needed to explain Callisto’s apparent incomplete differentiation. In addition, we argue that disk torques in such a gas-rich disk would cause large satellites to be lost to inward decay onto the planet. These issues have prevented us from identifying a self-consistent scenario for the formation and survival of the Galilean satellites using the standard model. We then consider an alternative, in which the satellites form in a circumplanetary accretion disk produced during the very end stages of gas accretion onto Jupiter. In this case, an amount of gas and solids of at least � 0.02 Jovian masses must still be processed through the disk during the satellite formation era, but this amount need not have been present all at once. We find that an accretion disk produced by a slow inflow of gas and solids, e.g., 2 � 10 � 7 Jovian masses per year, is most consistent with conditions needed to form the Galilean satellites, including disk temperatures low enough for ices and protracted satellite accretion times of � 10 5 yr. Such a ‘‘ gas-starved ’’ disk has an orders-of-magnitude lower gas surface density than the minimum mass subnebula (and for many cases is optically thin). Solids delivered to the disk build up over many disk viscous cycles, resulting in a greatly reduced gas-to-solids ratio during the final stages of satellite accretion. This allows for the survival of Galilean-sized satellites against disk torques over a wide range of plausible conditions.

/pdf/formation-of-the-galilean-satellites-conditions-of-accretion-2kugnli5at.pdf

Formation of the Galilean Satellites: Conditions of Accretion

We investigate the driving of orbital eccentricity of giant protoplanets and brown dwarfs through disc-companion tidal interactions by means of two dimensional numerical simulations. We consider disc models that are thought to be typical of protostellar discs during the planet forming epoch, with characteristic surface densities similar to standard minimum mass solar nebula models. We consider companions, ranging in mass between 1 and 30 Jupiter masses $M_{\rm J},$ that are initially embedded within the discs on circular orbits about a central solar mass. We find that a transition in orbital behaviour occurs at a mass in the range 10-20$ M_{\rm J}.$ For low mass planetary companions, we find that the orbit remains essentially circular. However, for companion masses $\gtrsim 20 M_{\rm J},$ we find that non steady behaviour of the orbit occurs, characterised by a growth in eccentricity to values of $0.1 \lesssim e \lesssim 0.25$. Analysis of the disc response to the presence of a perturbing companion indicates that for the higher masses, the inner parts of the disc that lie exterior to the companion orbit become eccentric through an instability driven by the coupling of an initially small disc eccentricity to the companion's tidal potential. This coupling leads to the excitation of an $m=2$ spiral wave at the 1:3 outer eccentric Lindblad resonance, which transports angular momentum outwards, leading to a growth of the disc eccentricity. The interaction of the companion with this eccentric disc, and the driving produced by direct resonant wave excitation at the 1:3 resonance, can lead to the growth of orbital eccentricity, with the driving provided by the eccentric disc being the stronger. Eccentricity growth occurs when the tidally induced gap width is such that eccentricity damping caused by corotating Lindblad resonances is inoperative. These simulations indicate that for standard disc models, gaps become wide enough for the 1:3 resonance to dominate, such that the transition from circular orbits can occur, only for masses in the brown dwarf range. However, the transition mass might be reduced into the range for extrasolar planets if the disc viscosity is significantly lower enabling wider gaps to occur for these masses. Another possibility is that an eccentric disc is produced by an alternative mechanism, such as viscous overstability resulting in a slowly precessing non axisymmetric mass distribution. A large eccentricity in a planet orbit contained within an inner cavity might then be produced.

/pdf/orbital-eccentricity-growth-through-disc-companion-tidal-hdimmn6qj1.pdf

Orbital eccentricity growth through disc-companion tidal interaction

The tilt of Saturn's spin axis to its orbit plane is 267, while that of Jupiter is only 31. We offer an explanation for this puzzling difference owing to gravitational perturbations of Saturn by the planet Neptune. A similarity between the precession period of Saturn's spin axis and the 1.87 × 106 yr precession period of Neptune's slightly inclined orbit plane implicates a resonant interaction between these planets as responsible for tilting Saturn from an initially more upright state. We make a case that Saturn was captured into this resonance during the erosion of the Kuiper belt, which decreased the rate of regression of Neptune's orbit plane. Penetrating the resonance pumped up Saturn's obliquity to its current value. The spin axis may also be librating in the resonance with an amplitude ψ 31°, and we discuss possible causes of this and the implied constraint on Saturn's moment of inertia. Matching the current pole position to the predicted outcome could place constraints on early solar system processes.

/pdf/tilting-saturn-i-analytic-model-26xbtnxecr.pdf

Tilting Saturn. I. Analytic Model

Hot Jupiters, giant extrasolar planets with orbital periods shorter than ~10 days, have long been thought to form at large radial distances, only to subsequently experience long-range inward migration. Here, we propose that in contrast with this picture, a substantial fraction of the hot Jupiter population formed in situ via the core accretion process. We show that under conditions appropriate to the inner regions of protoplanetary disks, rapid gas accretion can be initiated by Super-Earth type planets, comprising 10-20 Earth masses of refractory composition material. An in situ formation scenario leads to testable consequences, including the expectation that hot Jupiters should frequently be accompanied by additional low-mass planets with periods shorter than ~100 days. Our calculations further demonstrate that dynamical interactions during the early stages of planetary systems' lifetimes should increase the inclinations of such companions, rendering transits rare. High-precision radial velocity monitoring provides the best prospect for their detection.

/pdf/in-situ-formation-and-dynamical-evolution-of-hot-jupiter-4p8tfxlq33.pdf

In Situ Formation and Dynamical Evolution of Hot Jupiter Systems

Secular resonances from 2 to 50 AU

Secular resonance, solar spin down, and the orbit of Mercury

On the azimuthal brightness variations of Saturn's rings

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Secular resonance, solar spin down, and the orbit of Mercury

On the azimuthal brightness variations of Saturn's rings