Accumulation of a swarm of small planetesimals

Abstract The stage of planetesimal accumulation in which ∼ 10-km planetesimals in the vicinity of 1 AU grow into “planetary embryos” greater than 10 25 to 10 27 g in mass has been studied using the methods of gas dynamics. Particular attention is given to identifying the circumstances for which runaway growth results in the formation of a relatively small number (e.g., 30) of massive (∼4 × 10 26 g) embryos in the terretrial planet region on a time scale of 10 5 to 10 6 years. It is found that under the assumptions made by the Moscow (Safronov and others) and Kyoto (Hayashi and others) “schools,” no runaways are found, in agreement with the conclusions of these investigators. In contrast, when more plausible physical processes are included, e.g., the importance of equipartition of energy in gravitational encounters, the presence of the “seeds” in the initial distribution, enhancement of the gravitational cross section above the two-body value at low velocity, and fragmentation, runaways in the terrestrial planet region are found to be very probable on a time scale of about 10 5 years. The final stage of planetary accumulation may then consist of the accumulation of these embryos into the present planets on a time scale of 10 7 to 10 8 years. At larger heliocentric distances the planetesimal evolution could be different; circumstances may have existed in which runaway growth of Jupiter prevented runaways in the asteroid belt.