The 41/2 Steps of Terrestrial Planet Formation in the Solar System. (cont)
Step III: Oligarchic
Growth (Kokubo & Ida; Goldreich et al.; Chambers)
- Recall that as long as is not affected by M.
- As the big guys grow, they have a larger fraction of system mass stir small guys.
- is roughly constant with mass, so .
- So, growth slows and smaller oligarchs can catch up with larger ones.
- Studies have shown that (Kokubo & Ida):
- The largest few objects at any given time are:
- of comparable mass
- orbital separation is ~10RH, (Earth: RH=0.01AU)
where RH is the gravitational sphere of influence and is f(M,a).
- So, as the system evolves:
- Number of big guys decreases.
- Their masses increase (mainly by eating planetesimals, but some mergers happen).
- Their separation increases.
- Process works fine when Type I migration is included. (McNeil et al.)
......... Just need slightly more massive disks
- Oligarchic growth stops when small guys or gas can no longer damp big guys.
- Roughly the size of the Moon or Mars at 1AU. (Kenyon & Bromley)
- This takes roughly a million years.