13-03
Formation of the Solar System
What is the origin of our planetary system?
- Star formation and the solar nebula
Conservation of angular momentum and the protoplanetary disk (proplyd)
- Planets have compositions different from the Sun and the solar nebula:
1) Planets begin growing by sticking together solid grains.
2) Only when much more massive than the earth do planets have enough gravity to
capture gas directly from the solar nebula.
- Condensation of solid grains
Under low pressure, matter exists either as a solid or as a gas.
The temperature of the original cold isothermal solar nebula is about 50 K.
The original solar nebular consists of mainly hydrogen and helium
gases seeded with many solid grains including
dusts and ices of water, methane, ammonia etc.
The protosun during its Kelvin-Helmholtz contraction heated the solar nebula
[Water, methane, ammonia etc condensate only in the outer part of the solar nebula, while
rock-forming substance can also condensates in the inner part.
The line beyond which water can condensate is called the snow line.]
- Growth of a planet
Solid grains sticked together to form larger chunks, and then planetesimals (1 km or so).
Planetesimals in a rotating disk then coalesced into objects of 100 km or so.
Large planetesimals further fused together to grow and attract smaller objects.
They became protoplanets of 1000 km or so.
Protoplanets then collided, accretted, and coalesced to form planets.
The process completed within about 108 years,
the same as the time for the protosun
to ignite its core nuclear burning.
- On planets: heating (by violent impacts, decay of radioactive elements) -->
melting --> chemical differentiation
- In the outer region of the solar nebula, more solid grains were available
to form planetesimals.
More massive planets were formed, which could trap gases
to form an even larger planet.
(It seems, however, that Jovian planets required much longer time to form this way,
unless they were formed at a smaller distance from the Sun and then
migrated to their current location. An alternative model to this issue
is the disk instability model.)
- Even farther away, icy bodies formed as KBOs we see today.
Gravitational
influence of large planets is important on the population properties of KBOs.
-
Explaining the characteristics of the solar system:
-- Disk shape (however, the rotation of Venus and Uranus)
-- Distinction between the terrestrial and Jovian planets
-- A common age
-
Clearing the nebula:
Radiation pressure
Solar wind
Sweeping up of space debris by planets
Ejection by close encounter with planets
This ALMA image of HL Tau seems supporting the scenario described above.