NASA Astrobiology Institute (NAI)

1. Formation of Planetesimals in a Dynamically Evolving Nebula

   Project Investigators: Nader Haghighipour

   Summary

   The current model of the formation of planetesimals through gravitational instability cannot account for the growth of particles from a few mm to several cm in size. The shear-induced turbulence in such systems prevents small solid objects to accumulate and grow larger. The focus of this project is to show that the appearance of gas density/pressure enhanced regions will facilitate this process and speed up the growth of small solid objects.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets

   Project Progress

   It has been suggested that km-sized objects can form through the fragmentation of a gravitationally unstable layer of solid materials in a nebula. However, the difference between the rotational velocities of gas and dust particles in a gaseous disk produces a shear between these layers, which in turn results in turbulence. This turbulence and prevents solid objects from accumulating and increasing their local density to the necessary value for starting gravitational instability. To overcome this problem, models of gravitational instability consider objects with initial sizes larger than 50 cm and do not account for the growth of mm-sized particles (which are formed though hitting and sticking of micron-sized objects) to several cm in size. In this project, we have shown that the key in resolving this issue and filling the gap between mm-sized particles and 50 cm-sized objects is the interactions of dust particles with gas in the vicinity of pressure-enhanced regions. The combined effect of gas-drag and pressure-gradients causes micron-sized dust particles to migrate towards pressure-enhanced regions, while growing to a few millimeter in size (Figure 1).

   
   

   Growth of a 10 micron dust particle to a few cm in size in the vicinity of a pressure enhanced region.

   These objects accumulate in the location of the maximum gas pressure, where gas and dust rotate with similar Keplreian velocity and no shear-induced turbulence exists. Settling dust particles in the location of maximum gas pressure form clumps of several centimeter in size (Figure 2).

   
   

   Settling and accumulation of cm-sized objects in the region of maximum gas pressure (the red dashed line). No shear-induced turbulence exists in this region.

   The appearing and disappearing of pressure enhancements attracts these clumps and forms larger accumulations. These large accumulations, in turn form km-sized bodies by coalescing with one another and/or undergoing gravitational instability.

Publications

Haghighipour, N.  (2007).  Planetesimal Formation via Gravitational Instability in Gas-Density Enhanced Regions.  Gordon Research Conference, Origins of Solar Systems.  Mount Holyoky, Connecticut.

Haghighipour, N.  (2008).  Gas Density/Pressure Enhanced Regions as Favorable Places for the Formation of Planetesimals via Gravitational Instability.  Workshop on Planet Formation Processes and the Development of Prebiotic Conditions.  Caltech, Pasadena, California.