CALTECH GE 133 - FINAL STAGES OF PLANET FORMATION (11 pages)

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FINAL STAGES OF PLANET FORMATION



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FINAL STAGES OF PLANET FORMATION

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Pages:
11
School:
California Institute of Technology
Course:
Ge 133 - The Formation and Evolution of Planetary Systems
The Formation and Evolution of Planetary Systems Documents

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The Astrophysical Journal 614 497 507 2004 October 10 2004 The American Astronomical Society All rights reserved Printed in U S A FINAL STAGES OF PLANET FORMATION Peter Goldreich 1 2 Yoram Lithwick 3 and Re em Sari 2 Receivved 2004 April 12 accepted 2004 June 22 ABSTRACT We address three questions regarding solar system planets What determined their number Why are their orbits nearly circular and coplanar How long did they take to form Runaway accretion in a disk of small bodies resulted in a tiny fraction of the bodies growing much larger than all the others These big bodies dominated the viscous stirring of all bodies Dynamical friction by small bodies cooled the random velocities of the big ones Random velocities of small bodies were cooled by mutual collisions and or gas drag Runaway accretion terminated when the orbital separations of the big bodies became as wide as their feeding zones This was followed by oligarchic growth during which the big bodies maintained similar masses and uniformly spaced semimajor axes As the oligarchs grew their number density decreased but their surface mass density increased We depart from standard treatments of planet formation by assuming that as the big bodies got bigger the small ones got smaller as the result of undergoing a collisional fragmentation cascade It follows that oligarchy was a brief stage in solar system evolution When the oligarchs surface mass density matched that of the small bodies dynamical friction was no longer able to balance viscous stirring so their velocity dispersion increased to the extent that their orbits crossed This marked the end of oligarchy What happened next differed in the inner and outer parts of the planetary system In the inner part where the ratios of the escape velocities from the surfaces of the planets to the escape velocities from their orbits are smaller than unity big bodies collided and coalesced after their random velocities became comparable to their escape velocities In the



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