Formation of Planetary Systems

Protoplanetary disk around the star Beta Pictoris

Fig. 15.2

Model Requirements

Each planet is relatively isolated

Orbits of planets are nearly circular

Orbits of planets all lie in nearly the same plane

The planets revolution about Sun is counterclockwise

The planets rotation is generally counterclockwise

Model Requirements (cont.)

Most moons revolve about their planets in the same sense as the planets orbit the Sun

Planetary system is highly differentiated

Asteroids are very old and appear more like planets than moons

Comets are primitive, icy fragments that do not orbit in ecliptic plane and reside at large distances from Sun

Planetary Irregularities

System can evolve after formation

Mercury's spin orbit resonance with Sun

Moon's synchronous rotation,

Life on Earth etc...

Model must be flexible enough to allow irregularities

Example: Venus, Uranus, and Pluto have retrograde rotation

Past is integral part of the picture

Old asteroids and comets tell us much about initial composition

Pieces of Model

Nebular contraction

Role of dust

Accretion and fragmentation

Role of heat-condensation theory

Nebular Contraction

Nebular contraction

Flattening of cloud

Angular momentum conservation

Role of Dust

Nebular contraction alone cannot describe the formation of inner planets -too warm for formation of planets

Dust is present throughout galaxy and arises from long dead stars

Dust forms clumps and act as condensation nuclei

Dust helps cool gas cloud which warms from gravitational collapse- radiates away the heat in the form of IR radiation

Evidence of Dust

Fig. 15.3

Accretion

Gradual growth of small objects by collision and sticking

Fig. 15.6

Condensation Theory

Molecules and solids form as regions of the solar system cool

Outer part of solar system cooled off first

Explains differentiation of solar system

Fig. 15.8

Model of Early Planet Formation

Fig. 15.4 and 15.7

Cleaning up Debris

Planetesimal ejection

Populates Kuiper belt and Oort cloud

Fig. 15.9

Details

Outer planets formed first, since cooler in outer regions

Outer planets are bigger, more massive because they had a head-start in formation and accretion

Jupiter and outer outer planets may have migrated inward after formation

Primary atmospheres have composition of Sun and early protoplanetary system

Terrestrial planets are deficient in lighter elements

Where did water in inner region come from- comets?

Role of Catastrophes

Catastrophes that occurred after initial solar system formation can explain irregularities and oddities

Mercury's large metal core- 2 protoplanets merged

2 large bodies merged to form Venus explaining low rotation rate

Earth-Moon system formed by collision

Uranus's tilt- grazing collision

Pluto perhaps is Kuiper belt object knocked into closer orbit

Etc...