Jupiter and Saturn represent a challenge to theories of planet formation. Being gas giants, they must have formed in less than 10 million years to be able to accrete gas from the solar nebula before it dissipated. The current leading theory for giant planet formation is the core accretion model, in which a large planetary embryo forms first by accretion of solid planetesimals, and subsequently accretes gas when it has grown to approximately 10 Earth masses. However, this model has a weakness, in that the initial accretion of a large embryo is hindered by a variety of physical processes. In this renewed project, the Principal Investigator, a postdoctoral researcher, and collaborators will continue to develop an extensive computer model of core accretion in order to determine whether this theory is viable. The team will perform a series of simulations of the evolution of a system of embryos and planetesimals embedded in the solar nebula. The models will include: (i) direct gravitational interaction of the planetesimals and embryos, (ii) aerodynamic drag on the planetesimals, (iii) migration of the embryos through gravitational interaction with the protoplanetary disk, (iv) embryo fragmentation, (v) the effects of embryos' atmospheres, (vi) the buildup of solids at the snow-line, and (vii) turbulence-driven migration of the embryos. The team's N-body code will be coupled with state-of-the-art models of the solar nebula in order to determine how (ii), (iii) and (vi) change with time. The project is broadly relevant to the history of the Solar System and has implications for Earth?s acquisition of water. The project will support the early career development of a woman scientist. The Principal Investigator will continue his involvement with public outreach and broadcast and print media.

Project Report

It is ironic that the most massive planets in the Solar System must have formed inthe least amount of time. The giant Hydrogen and Helium rich planets, Jupiter and Saturn, must have formed within the few million years; they must have accreted their gaseous envelpoes before the gas rich disk around the young Sun dispersed. This is in contrast to the much smaller Earth, which we know by radiometric dating must have taken well over 10 million years to form. Forming these giant planets in such a short amount of time has been a long standing problem in planet formation theory. The first step in forming these giants is to form a large rocky/icy proto-planet, know as a core, onto which gas from the solar nebula can accrete. However, forming these approximately 10 Earth mass cores in the short time alotted is very difficult. During this NSF funded research, Dr. Levison led a team undertaking the more comprehensive investigation to date on giant planet core formation. Using state-of-the-art computational tools, Levison and his team demonstrated that the standard idea that these cores could be formed from 1 to 100 km-sized planetesimals does not work if one accounts for the full dynamical interactions between the growing cores and the planetesimals. However, they discovered that it is possible to form the Solar System's giant planets if they begin from more humble beginnings, small centimeter- to decimeter- sized objects refereed to as "pebbles". In their new model Levison's team shows that initial seed planetesimals efficiently accrete these pebbles as they are continually formed throughout the lifetime of the gas rich disk. These planetesimals can easily grow large enough to form giant planet cores within the alloted time span, while simulatously scattering their smaller competitors out of the pebbles, preventing them from growing. In this way the largest planets likely grew with the help of small pebbles. This work serves as an important part of the puzzle to help us understand how our planetary system came to look as it does today.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Maria Womack
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Southwest Research Institute
San Antonio
United States
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