Dr. Paul Estrada

December 21, 2006

"Formation and Evolution of Giant Planet Satellite Systems"


The main objective of this research is the continued advancement in our understanding of the formation and evolution of giant planet satellite systems. Our research is directly applicable to satellite systems that will almost certainly be found around the growing reservoir of observed extrasolar giant planets (EGPs) using forthcoming missions such as COROT (ESA, 2005), Kepler (NASA, 2007), and Eddington (ESA, 2008).

Previously, we have developed a consistent model for the regular satellites of Jupiter, Saturn and Uranus (Mosqueira and Estrada, 2003a,b; hereafter MEab). Which includes satellitesimal migration due to gas drag and tidal torques, and forms satellites by a combination of Safronov-style binary accretion and drift-augmented accretion in an extended, two-component planetary subnebula. The inner disk is set by the location of the centrifugal radius, while the outer disk extends to the location of the irregulars. The transition occurs at the location of the centrifugal radius r c ~ R H/48 (R H = a P (M P/3M *)1/3, where a P and M P are the planet semi-major axis and mass, and M * the mass of the central star), which results from equating the centrifugal and gravitational forces on a parcel of gas whose specific angular momentum is conserved as it enters the planet's Hill sphere (MEa). MEb explicitly address the survival of regular satellites in a minimum mass subnebula enhanced in solids by a factor of ~ 10, which leads to a gas surface density ~ 10 4 g cm -2. In particular, the formation time for Ganymede and Titan (set by the gas drag timescale of satellitesimals) is comparable to their Type I migration time provided one allows for a factor ~ 10 slower migration due to 3-D effects. This surface density is consistent with that obtained using the inviscid gap-opening formula of Rafikov (2002) in a disk with aspect ratio ~ 0.1, which corresponds to a temperature of ~ 250 K and ~ 100 K at Ganymede and Titan, respectively. This can work provided turbulence subsides and Type II migration ceases. We have also developed an alternative model for formation of satellites around gas giant planets under the assumption that an unknown mechanism (e.g., gas turbulence) removes the gas disk in timescale shorter than that for satellite formation (insuring satellite survival). This model thus entails the formation of satellites in a gas-poor environment (of unspecified surface gas density, though the presence of some gas may help to explain the observations). In this model (Estrada and Mosqueira 2005; hereafter EM), which follows along the lines of the work of Safronov (1969; et al. 1986), sun-orbiting planetesimals collide within the planet's Hill sphere and generate a disk primarily of solid material from which the satellites will form. Unlike the previous model, all of the satellites form in a timescale determined by the timescale for planetesimal feeding (> 105 years).

We have investigated the final masses of giant planets in disks with one or more than one giant planet cores (Estrada and Mosqueira 2004; Mosqueira and Estrada 2004). In the core accretion model of giant planet formation (Pollack et al. 1996), when the core reaches critical mass, hydrostatic equilibrium is no longer possible and gas accretion ensues (Mizuno 1980). If the envelope is radiative, the critical core mass is nearly independent of the boundary conditions and is roughly ~ 10 ME (with weak dependence on the rate of planetesimal accretion and the disk opacity; Stevenson 1982). Given that such a core may form at the present location of Jupiter in a time comparable to its Type I migration time (105-106 years; Bate et al. 2003) provided that the nebula was significantly enhanced in solids with respect to the minimum mass solar nebula (Inaba et al. 2003) and stall at this location in a weakly turbulent (α < 10-4) disk (Rafikov 2002), it may be appropriate to assume that such objects inevitably form and drive the evolution of late-phase T Tauri star disks. In this proposal we pursue several projects (7) based on our previous work that relate to each other in a logical and systematic fashion. The main objective is the continued development of a framework for the formation and evolution of giant planet/satellite systems that can be applicable to extrasolar planetary/satellite systems. In particular, a goal of this work is to help to provide constraints on observations of extrasolar planet/satellite systems that will almost certainly be detected in the coming years with upcoming missions such as COROT. Section 2.1 involves a study of the concomitant growth and migration of giant planets. A detailed study will allow us to characterize the diversity of planetary systems, which can be directly compared with statistics of the growing catalogue of EGPs. EGPs, like the gas giants of our own solar system, likely will possess moon systems. Coupled migration of satellites within the circumplanetary gas disk due to the gas tidal torque along with migration of the giant planet can lead to a variety of outcomes that may be constrained using the model of MEab. This task is outlined in sec. 2.2. Our goal is to characterize the diversity of these satellite systems in order to provide potentially valuable constraints on the detectability of these moons in time for upcoming missions designed to detect EGP moons. In sec. 2.3, we study the consequences of increasing solar luminosity on Ganymedean and super-Ganymedean moons in orbit around migrating giant planets. In sec. 2.4, we continue development of our recently submitted gas-poor planetesimal capture model for the formation of satellites around gas giant planets (EM). In sec 2.5, we describe a scenario for the impact origin of Titan's eccentricity that could potentially explain the inner icy saturnian satellites as well as provide an explanation for Iapetus consistent with the model of EM. In sec. 2.6, we study the compositions of satellites and in the in-situ formation of Iapetus in the context of the model of MEab. Finally, in sec. 2.7, we describe our development of a dust-coagulation/thermal evolution code that we shall use to model the thermal environment of the circumplanetary disk.