P32A-01 10:20h
The Condensation/Sublimation Front in the Solar Nebula
The condensation/sublimation front in the early solar system is investigated using analytical solutions of the nebula evolution equations. One-dimensional theory shows that the front evolves from beyond 40 AU to its final position over a period of 10 Myrs. The physical process governing this movement is a combination of local viscous heating (early in the evolution) and luminescent heating from the central star (in the later stages). The one-dimensional analysis is extended to two-dimensions using a vertical structure model of the solar nebula. This model, using a code supplied by C. P. Dullemond, computes the radial and vertical temperature-density distribution using both viscous and radiative transfer heating processes. The two-dimensional water condensation/sublimation front is computed and contrasted with that predicted by one-dimensional theory.
P32A-02 10:35h
Collisional Evolution of Terrestrial Planets
The terrestrial planets are generally thought to have formed via the collisional accumulation of rocky bodies. The characteristics of the planets produced by this process are, to a large degree, determined by their collisional evolution, and their associated differentiation and thermal evolution. Studies of planet formation and planetary collisional evolution have typically been conducted separately. Most works of late-stage planet formation use perfectly inelastic mergers to model collisions (e.g.\ Agnor, Canup & Levison 1999, Chambers 2001, Levison & Agnor 2003), with certain recognized inadequacies, notably prohibitively large spin angular momentum acquired as a planet grows. To date, studies of the collisional evolution of terrestrial planets has focused on determining the efficacy of single impacts to account for particular planetary characteristics and the formation of satellites (e.g.\ Benz et al.\ 1988, Canup & Asphaug 2001, Canup 2004). It has been recognized for some time (Wetherill 1985) that the final characteristics (e.g.\ spin state, bulk composition, isotopic age) of an accreting planet are determined not by the last or single largest collision but by all of the major collisional encounters in a planet's history (Agnor, Canup & Levison 1999). As demonstrated by our impact models, each major impact changes the silicate to metal ratio, the thermal state, and the spin state, and sets the stage for the subsequent collision. We are studying collisional dynamics and outcomes common to the late stage of terrestrial planet formation. We use smooth particle hydrodynamics model collisions in an effort to identify the range of impact dynamics that allow for accretion (i.e. mass growth instead of mass loss). In our initial study we found that for dynamical environments typical of most late stage accretion models, about half of all collisions between equal mass planetary embryos do not result in accumulation into a larger embryo (Agnor & Asphaug 2004). We will present new results of collisions for a variety of mass ratios and will discuss the cumulative affect of giant impacts and non-accretionary collisions on planetary characteristics (e.g.\ Mercury's collisional mantle loss and bulk composition, planetary spin states) and the extent to which collisional processes may account for planetary heterogeneity.
P32A-03 10:50h
Can Habitable Terrestrial Planets Co-exist With Hot Jupiters?
``Hot jupiters,'' giant planets with orbits very close to their parent stars, are thought to most plausibly form at large orbital distances and then migrate inward via interactions with the gaseous protoplanetary disk. If a giant planet forms and migrates inward with sufficient rapidity, then terrestrial planets may form in the system. We present results of simulations of terrestrial planet formation in the presence of hot jupiters, broadly defined as having orbital radii within 0.5 AU of the parent star. We show that terrestrial planets similar to those in the Solar System can form around stars with hot jupiters, and can have water contents equal to or higher than the Earth's. For small orbital radii of hot jupiters (e.g. 0.15, 0.25 AU) potentially habitable planets can form, but for semi-major axes of 0.5 AU or greater their formation is suppressed. We show that the presence of a giant planet external to the terrestrial planet zone does not necessarily enhance the water content of the terrestrial planets, but does descrease both their formation and water delivery timescales. We speculate that asteroid belts may exist interior to the terrestrial planets in systems with hot jupiters.
P32A-04 11:05h
Accumulation of Giant Planet Atmospheres Around 5 -- 10 M$_{\oplus}$ Cores
\documentstyle [titlepage,11pt]{article} \setlength{\textwidth}{6.5in} \setlength{\textheight}{8.5in} \setlength{\topmargin}{.2in} \setlength{\oddsidemargin}{.0in} \setlength{\evensidemargin}{\oddsidemargin} \setlength{\parskip}{7pt plus 2pt} \baselineskip=24pt \def\baselinestretch{1.0} \def\ref{\par \hangindent 2em\noindent} \newcounter{cureqno} \newenvironment{mathletters}{\refstepcounter{equation}% \setcounter{cureqno}{\value{equation}}% \let\curtheeqn\theequation% \edef\cur@eqn{\csname theequation\endcsname}% \def\theequation{\cur@eqn\alph{equation}}% \setcounter{equation}{0}}% {\let\theequation\curtheeqn% \setcounter{equation}{\value{cureqno}}} \begin{document} Observations of protoplanetary disks imply that gas giant planets form very quickly ($\le$ 10 Myr). Recent interior models of Jupiter suggest smaller core masses (0 -- 10 M$_\oplus$) than had been previously predicted (10 to 30 M$_\oplus$). We have computed evolutionary simulations of Jupiter based on the core accretion model of gas giant planet formation where we vary the grain opacity and the planetesimal surface density of the solar density of the solar nebula. We also explore the implications of halting the solid accretion at selected core mass values during the protoplanet's growth, thus simulating the presence of a competing embryo. The core accretion model states that a solid core is formed from the accretion of planetesimals in the solar nebular followed by the capture of a massive envelope from the solar nebula gas. Our simulations based on this model (Pollack et al. 1996) have been successful in explaining many features of the giant planets. Our most recent results (Hubickyj et al. 2004) demonstrate that decreasing the grain opacity reduces the formation time by more than half of that for models computed with full interstellar grain opacity values. In fact, it is the reduction of the grain opacity in the upper portion of the envelope with T $ < $ 500 K that governs the lowering of the formation time. Decreasing the surface density of the planetesimals lowers the final core mass of the protoplanet but increases the formation timescale. Finally, a core mass cutoff results in the reduction of the time needed for a protoplanet to evolve to the stage of runaway gas accretion provided the cutoff mass is not too small. Our models show that with reasonable parameters it is possible to form Jupiter by means of the core accretion process in 3 Myr or less. \ref Hubickyj, O., P. Bodenheimer, & J. J. Lissauer 2004. Accumulation of giant planet atmospheres around 5 -- 10 M$_{\oplus}$ cores. In preparation. \ref Pollack J. B., O. Hubickyj, P. Bodenheimer, J. J. Lissauer, M. Podolak, and Y. Greenzweig 1996. Formation of the giant planets by concurrent accretion of solids and gas. {\it Icarus \bf 124}, 62--85. This work was supported in part by NASA grant NAG5--9661 and NASA grant NAG 5--13285 from the Origins of Solar Systems Program. \end{document}
P32A-05 INVITED 11:20h
Gas Flow Near a Young Gas Giant Planet
The mass acquired by a gas giant planet has long been believed to result from accretion of material within the surrounding gaseous disk (nebula). Bate, D'Angelo, and I have analyzed the properties and consequences of this flow by means of three-dimensional hydrodynamical simulations. Most of the accretion occurs after the planet has partially cleared a gap in the surrounding disk. The flow through the gap leads to accretion within the planet's Hill sphere. In that region, a circumplanetary disk forms which can serve as a site for satellite formation. The flow onto this disk is fully three-dimensional and involves shock production. The nebula exerts a torque on the planet that causes it to migrate radially. We have recently examined the torques produced by the material within the gap and within the Hill sphere.
P32A-06 11:40h
On Type III Protoplanet Migration
Planet migration due to disk torques from the corotation region (type III) is considered. There is a fast regime where the drift rate increases with the planet's mass and generally exceeds the type I migration rate, and a slow regime where the rate decreases with mass. The transition between these regimes or to type II motion is examined, and the range of validity of this migration mechanism explored. Type III migration depends on a sustained surface density deficit between material trapped in the corotation region and the ambient disk. Sufficient disk viscosity could suppress the motion by removing the deficit. Implications for the formation and survival of planets embedded in disks will be discussed.
P32A-07 INVITED 11:55h
Satellite Formation
The multitude of satellites in our solar system suggests that satellite formation was a natural byproduct of planet formation. Several processes appear promising candidates for producing large satellites or satellite systems. The Moon is thought to have resulted from what was perhaps the largest impact of Earth's accretion, and a giant collision may have also led to the formation of the Pluto-Charon system. The large regular outer planetary satellites are believed to have co-formed with their parent planets, with satellites accumulating within circumplanetary accretion disks during the final stages of their planet's own growth. Recent work on satellite formation and its potential implications for planet formation processes will be presented. Support from NASA and NSF is gratefully acknowledged.