Class 2 : The Structure and Formation of the Solar System Basic constituents of the solar system The formation of the solar system What are the facts that we have to explain? The collapsing gas cloud model. The role of bombardment in the early solar system.

I : Basic structure of the solar system What makes up our solar system?

The Sun – our central star

Inner planets – small/rocky Mercury  Venus  Earth  Mars

Outer planets – large/gaseous Jupiter  Saturn  Uranus  Neptune  Pluto/Charon

What do small and large mean?

Asteroids – rocky debris Gaspra: 20 km

Mathilde: 59 km

Ida (58 km) & Dactyl (2 km)

Eros (33 km)

The Asteroid Belt Derichardson

Comets – dirty snowballs or snowy dirtballs

Comet Hale-Bopp (John Gleason)

Comet Ikeya-Zhang

II : Physical properties of the Solar System All planets (apart from Pluto) orbit in the same plane… solar system is flat

Properties, cont’d All planets orbit the Sun in same sense. The Sun and all planets (apart from Venus, Uranus, & Pluto) rotate in the same sense as they orbit the Sun. Inner planets are small and rocky, but outer planets are large and gaseous. There are also asteroids and comets All asteroids and short-period comets are usually in planetary plane and orbit in same sense Long-period comets are random in direction and sense.

III : Formation of the Solar System How did all of this come to be? A good theory needs to Explain the facts we just listed. Be as simple as possible (but consistent with the laws of nature). Be testable. Our best theory is the Collapsing Gas Cloud theory.

Formation, cont’d Phase I (started 5 billion years ago) Gas cloud starts to collapse in on itself due to its own gravitational pull. As it collapses, it rotates faster and flattens. Phase II The central part of the gas cloud forms the Sun – nuclear reactions start in its core. Phase III Dust grains and ice crystals start to condense out of flattened gas disk. Grains grow via collisions to form large planetesimals (objects about Moon-size).

Formation, cont’d Stage IV – 100 million yrs have passed Planets grow by accreting planetesimals. Eventually, there are hardly any large planetesimals left – planets stop growing. Outer massive planets grab the remains of the gas from the cloud. Stage V – 4.5 billion yrs ago to present Planets formed. Still some debris around (asteroids and comets) that occasionally hit the planets.

Questions… How would we ever test our theory for the formation of the solar system?