1. Making a Solar System

2. Learning Objectives
 What are the features of our Solar System? Where are
asteroids, comets and each planet? Where is most of the
mass? In what direction do planets orbit and rotate?
 How does the Nebular Theory explain our Solar System?
 If the Solar System forms from a ball (sphere) of gas,
why do the planets orbit in a flat plane (the ecliptic)?
 What is the source of energy of the protosun?
 What key factor guides the types of planets that form at
each distance from the Sun? What is the Frost Line?
 How do planets form from planetesimals? How do
planetesimals relate to the Kuiper and Asteroid Belts?
The Oort Cloud? The Period of Heavy Bombardment?

3. What is the Age of
the Solar System?
 Earth: oldest rocks are 4.4 billion years
 Moon: oldest rocks are 4.5 billion years
 Mars: oldest rocks are 4.5 billion years
 Meteorites: oldest are 4.6 billion years
 Sun: fusion models based on energy output
and how much of the Sunʼs hydrogen has
been fused so far estimate an age of 4.5
billion years (next lecture)
 Age of Solar System is around 4.6 billion
years old

4. Solar Nebula Theory
 Proposed by the German
philosopher
Immanuel Kant
 The Solar System
formed from a
spinning cloud of
gas, dust, and ice
 Mostly hydrogen
and helium
 4.6 billion years ago

5. Solar Nebula Theory
 In these clouds are
small clumps that
become gravitationally
unstable
 The gas and dust has
mass (thus gravity)
 Gravitational attraction
between all particles
pull them toward the
center of the cloud, and
the cloud contracts

6. But..
 Not all the mass falls in directly. Why?
 The gas cloud (which is three-dimensional, of
course so think of it as a ball) is spinning
slightly. This causes the formation of a
flattened structure as it collapses
 Forms a pancake-like disk, concentrated at
the center, with a rotation in the original
direction of spin

7. At the center of this mess…
 …the Sun begins to
form
 As gas and dust fall
towards the middle of
the cloud of dust,
their gravitational
potential energy is
turned into heat
 The stuff nearest the
center starts to give
off light and heat.
 This is the protosun
Protosun forms in first 2-3 slides

8. But weʼre not finished yet…
 Because this is not
yet the Sun as we
know it. Its energy is
still coming entirely
from gravitational
contraction
 Eventually, it
becomes so hot and
dense, fusion begins
(hydrogen converts
to helium, and
energy is released)
 Initial contraction to
hydrogen ignition
takes about 100
million years Protosun forms in first 2-3 slides

9. Planet Formation in the Disk
 Heavy elements clump
 Form dust grains
 Dust grains collide,
stick together
 Form planetesimals
 Like asteroids & comets
 Big planetesimals
attract small ones
 Collisionsbuild up inner
planets, outer planet cores

10. Why are the Planets Different?
Temperature (and thus distance from the
center of the cloud) is the key factor!

11. Why are the Planets Different?
 Temperature is the key factor
 Inner Solar System: Hot
 Light elements (H, He) and “ices”
vaporized
 Blown out of the inner Solar System by the
solar wind
 Only heavy elements (iron etc.) left
 Outer Solar System: Cold
 Too cold to evaporate ices to space
 Rock & ice “seeds” grew large enough to
pull gasses (H, He) onto themselves

12. Formation of the Inner
Planets
 The inner Solar System was too hot for
ices and light gases to exist. Thus,
planetesimals consisted entirely of
heavy elements (they were just rocks)
 Planetesimals run into each other, then
coalesce to form protoplanets
 Protoplanets accrete (attract) more
planetesimals until almost all matter in
inner Solar System is swept up

13. Formation of the Inner
Planets
 Computer models show it takes a few hundred
million years to form four inner planets
 The Sun would have begun fusion by then

14. Formation of the Outer
Planets
 The process is initially very similar to the
formation of the inner planets
 Since it is colder at this distance, ices can exist,
and planetesimals consist of rock and ices. This
leads to larger protoplanets, which ultimately
become the rocky, icy cores of the outer planets
 Finally, because hydrogen and helium havenʼt
been cleared from the area, the outer planets
gravitationally attract huge amounts of these
gases. Planets become much bigger than inner
planets

15. Heavy Bombardment
 There were billions of
planetesimals in the
early Solar System
 Many collided with
the young planets
 Look at the Moon &
Mercury
 Period of heavy bombardment
 Lasted for about the first 800 million years of
the Solar System (after which most of the
planetesimals had hit something or gone into
stable orbits)

16. Fates of the Planetesimals
 Between Mars and
Jupiter
 Remain as the asteroids
 Near Jupiter & Saturn
 Ejected from the Solar
System
 Near Uranus & Neptune
 Ejected to the Oort Cloud
 Beyond Neptune
 Remain in the Kuiper Belt

17. Results
 Most stuff goes into the planets
 Asteroids and comets are left-over
planetesimals
 “Fossils” of Solar System birth
 The Solar System continues to evolve,
but slower
 Outer planets still contracting
 Earth and Venus are still volcanically active
 Impacts from left-over planetesimals
continue

18. The
The
Constellation
Orion
Orion
Nebula

19. Disks around Young Stars
are Common

20. Next Time
Our Beacon: The Sun