Solar System Formation

1
Solar System Formation

2
NOT Haphazard Accumulation!

• There is too much order.
• There is no mechanism to capture the planets.

3
Not Always to be Trusted
4
Digression How to Capture a Passing Object

• You must slow it down!
• A good treatment the movie 2010

5
NOT Catastrophic Formation

• Once a popular theory!
• Invoked close passage of two stars, plus tidal
  effects.

6
Heres How
7
On the Positive Side

• It would explain
• Why planetary orbits are in the same plane
• Why planets orbit in the same direction
• Why biggest planets are in the middle of the range

8
Fatal Flaws

• Gas would be too hot to condense.
• There are angular momentum problems.
• There are too many planetary systems! (near
  collisions are very rare!)

9
YES The Nebular Hypothesis
10
A Snapshot of Progress

• 50 years ago nebular theory developed
• 30 years ago indirect evidence found (disks of
  gas)
• 15 years ago direct evidence was found (planets
  around many nearby stars)

11
Questions to be Answered

• What starts the collapse?
• Why are there planets of different compositions
  and sizes, and why the observed dependence on
  location?
• Can we also explain the details of motions?

12
The Mosh Pit
13
What Triggers a Collapse?

• Consider the air in the room temperature resists
  the effects of gravity. So too in interstellar
  space.
• Need to cool or compress the gas. (How?)

14

• Cool,
• Dense
• Clouds
• Abound

15
Composition

• Note that the proto-solar system is a gas cloud
  which is
• Big (millions of times the volume of the present
  solar system)
• Made of Hydrogen (2/3) and Helium (1/3), with
  traces (a few ) of other material

16
The Proto-Sun

• Potential Energy
• Kinetic Energy (inward-directed motions)
• Heat (as particles collide at center)
• This yields a hot proto-sun

17
But not everythingfalls right tothe centre!

18
What Happens as it Gets Denser?
19

• A General Fog

20
Condensation Made Spectacular
21
No unnecessary restrictions!

• The condensation could be more than just water!
• And it would not have to be on a surface
• (e.g. consider a hot vapour of iron atoms)

22
What will condense?
23
What Minerals Will Form
24
Continued Growth

• Pebbles ? Planetesimals ? Planets
• (distinguished by the moment when the gravity of
  a particular object starts to dominate the
  surroundings)

25
Big Fish, Little Fish

26
Important Points So Far

• When it forms, a given planet has the same
  composition throughout its entire interior.
  (That changes later.)
• The inner planets are small and rocky because
  90 of the material never condenses that close
  to the sun. (Uses only a fraction of the
  available building material!)
• The outer planets are big and gaseous because all
  the material condenses there -- and its mostly H
  and He. (All the building material gets used up!)

27
Motions Explained
28
Damping Out

• Objects moving in oblique directions suffer
  collisions, get caught up with the crowd.
• Detailed considerations also explain the general
  sense of the spin of the sun, planets, moons, etc

29
What We Dont See Now

• The planets
• actually travel
• through mostly
• empty space
• so any leftover
• gas is long gone

30
Where Does The Leftover Gas Go?
31
Enter the Magic Broom
32
Evidence of This Sweeping

• Other young stars (strong stellar winds)
• Earths atmosphere -- which is in fact secondary.
  The original atmosphere was completely swept
  away!

33
Is This Model Correct? If so

• Most stars should have planets
• There should be disks of gas and dust around
  stars which are forming only now
• The various parts of the SS should have the same
  age
• There should be evidence for some pretty big
  collisions in the early days (when numerous big
  lumps were still hurtling about)