1. Formation of the Solar System Comparative Planetology
Chapter 5

3. Formation of the Solar System
Uncovering the origin of the Solar system
Early days of the formation
Building the planets and other stuff

4. Comparative Planetology
Studying planets as worlds and compare them with each other is called comparative planetology
Planetology is applied to any noticeably large object in the system (planets, moons, asteroids, comets)
To start we need to seek clues to the origin of the Solar system

5. What does the solar system look like from far away?
NASA Figure
Sun, a star, at the center…
Inner Planets (Mercury, Venus, Earth, Mars) ~ 1 AU
They are all rocky planets…
Asteroid Belt, ~ 3 AU
Outer Planets (Jupiter, Saturn, Neptune, Uranus), ~ 5-40 AU
They are all gaseous planets..
Pluto: odd ball planet, more like a comet…
Kuiper Belt ~ 30 to 50 AU
Oort Cloud ~ 50,000 AU
Where comets come from…

6. Four Challenges Pattern of Motion
All planets orbit the Sun in the same direction (counterclockwise as seen from the Earth’s North Pole)
Planet orbits are nearly circular and co-planar
Planets rotate in the same direction which they orbit (with one exception)
Almost all moons orbit their planets in the direction of the planet rotation
The Sun rotates in the direction planets orbit it
Explain: Why is this order so good?

7. Four Challenges Different types of planets
Two distinct groups of planets:
Terrestrial planets (Mercury, Venus, Earth, Mars)
Small, rocky, abundant in metals, few moons
Jovian planets (Jupiter, Saturn, Uranus, Neptune)
Large, gaseous (made of hydrogen and its compounds), no solid surfaces, have rings, a lot of moons (made of low-density ices and rocks)
Explain: Why is the inner and outer Solar system divided so neatly?

8. Four Challenges Asteroids and Comets
Asteroids are small, rocky bodies that orbit the Sun mostly between Mars and Jupiter (the asteroid belt)
Almost 9,000 asteroids have been discovered
Comets are small and icy bodies that spend most of their lives beyond the orbit of Pluto
They occupy 2 regions: Kuiper belt and Oort cloud
Explain: The existence and general properties of the large number of these small bodies

9. Four Challenges Exception to the Rules
Mercury and Pluto have larger orbital eccentricities
Uranus and Pluto have tilted rotational axes
Venus rotates backwards (clockwise)
Earth has a large moon
Pluto has a moon almost as big as itself

10. Planetary Nebula or Close Encounter?
Historically, two hypothesis were put forward to explain the formation of the solar system….
Gravitational Collapse of Planetary Nebula (Latin for “cloud”)
Solar system formed form gravitational collapse of an interstellar cloud or gas
Close Encounter (of the Sun with another star)
Planets are formed from debris pulled out of the Sun during a close encounter with another star. But, it cannot account for
The angular momentum distribution in the solar system,
Probability for such encounter is small in our neighborhood…

11. The Nebular Theory
The Solar system was formed from a giant, swirling interstellar cloud of gas and dust
The hypothesis was originally suggested by Immanuel Kant (1755) and Pierre-Simon Laplas (~1790)
A cloud is called nebula - nebular hypothesis
The collapsed piece of cloud that formed our own solar system is called the solar nebula

12. The Nebular Theory* of Solar System Formation
Interstellar Cloud (Nebula)
*It is also called the ‘Protoplanet Theory’.
Protoplanetary Disk
Protosun
Gravitational Collapse
Metal, Rocks
Condensation (gas to solid)
Sun
Gases, Ice
Heating  Fusion
Terrestrial Planets
Accretion
Nebular Capture
Jovian Planets
Asteroids
Leftover Materials
Comets
Leftover Materials

13. Gravitational Collapse
A Pictorial History
Gravitational Collapse
Interplanetary Cloud
Condensation
Accretion
Nabular Capture

14. The Interstellar Clouds
The primordial gas after the Big Bang has very low heavy metal content
The interstellar clouds that the solar system was built from gas that has gone through several star-gas-star cycles.

15. The Formation of the Solar System

16. The Formation of the Solar System
Interstellar Clouds
By Mass
73% Molecular Hydrogen
25%Atomic Helium
2% Dust (Metals)

17. ConcepTest!
What process heated the early solar nebula as it slowly condensed towards a central protosun?
Atoms collided with increasing speed as they fell towards the center of the nebula, creating heat.
Heat was released by the formation of molecules such as CO2 and NH3 from atoms, and the condensation of ices from gases such as H2O.
Thermonuclear fusion in the protosun was followed by radiative heating of the material in the nebula

18. The Formation of the Solar System
Dense Molecular Cores
(“Bok Globules”)
≈ 1 Mo
≈ 50,000 AU
≈ 10 oK

19. The Formation of the Solar System

20. The Formation of the Solar System
Protoplanetary Disks
≈ 0.01 Mo
≈ 100 AU
≈ > 10oK

21. The Formation of the Solar System

22. The Formation of the Solar System
Condensation Sequence
Condensation Temperature
Temperature at which Solid Gas
T > 50 oK T > 200 oK T > 1000 oK
Hydrogen (H)
Helium (He)
H20, Methane (CH4)
CO2, Ammonia (NH3)
Iron (Fe), Silicon (Si)
Metal Compounds
Gas Gas Gas
Ice Gas Gas
Rock Rock Gas

23. The Formation of the Solar System
Condensation Sequence M V E M J
1000 K
200 K
Rock
Grains
Rock, Ice
Grains
No Grains

24. The Formation of the Solar System

25. The Formation of the Solar System
Grain Collisions  Planetesimals (100 km) random
100 km

26. The Formation of the Solar System
Planetesimal Accretion  Rocky Planets and Jovian Cores
gravity

27. The Formation of the Solar System
Gas Accumulation ==> H and He onto Jovian Cores
gravity
Protomoons

28. ConcepTest!
Most comets have orbits that take them well beyond Jupiter. You would expect their composition to be:
A) Rocks and heavy elements only
B) Rocks and ices only
C) Rocks, ices, and hydrogen and helium

29. The Formation of the Solar System

30. The Formation of the Solar System
Dispersal of Hydrogen and Helium Gas
Solar Wind?
Jets?

31. Collapse of the Solar Nebula
Three important processes gave form to our system, when it collapsed to a diameter of 200 A.U.
The temperature increased as it collapsed
The rotation rate increased
The nebula flattened into a disk (protoplanetary disk)

32. Building the Planets
Initial composition: 98% hydrogen and helium, and 2% heavier elements (carbon, nitrogen, oxygen, silicon, iron)
Condensation: the formation of solid or liquid particles from a cloud of gas
Different kinds of planets and satellites were formed out of different condensates

34. Ingredients of the Solar System
Metals : iron, nickel, aluminum, etc.
Condense into solid form at 1000 – 1600 K
0.2% of the solar nebula’s mass
Rocks : primarily silicon-based minerals
Condense at 500 – 1300 K, 0.4% of the mass
Hydrogen compounds : methane (CH4), ammonia (HN3), water (H2O)
Condense into ices below 150 K, 1.4% of the mass
Light gases: hydrogen and helium
Never condense in solar nebula; 98% of the mass

35. Accretion Accretion is growing by colliding and sticking
The growing objects formed by accretion – planetesimals (pieces of planets)
Small planetesimals came in a variety of shapes, reflected in many small asteroids
Large planetesimals (>100 km across) became spherical due to the force of gravity
Inner solar system: only rocks and metals condensed and only small bodies formed

36. Accretion

38. Nebular Capture
Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium
It led to the formation of the Jovian planets
Numerous moons were formed by the same processes that formed the protoplanetary disk
Condensation and accretion created mini solar systems around each Jovian planet

39. Formation of Gaseous Planets

40. Leftover Planetesimals
Planetesimals remained from the clearing became comets and asteroids
They were tugged by the strong gravity of the jovian planets and got more elliptical orbits
Rocky leftovers became asteroids
Icy leftovers became comets

41. Asteroids and Comets

42. Planetary Evolution - Geological
Internal heating leads to geological activity: volcanism, tectonics
As core cools and solidifies, activity slows, and eventually stops (Moon)
Earth and Venus are large enough to be active

43. Planet Activity

44. Summary: Collapse of the Solar Nebula
Gravitational Collapse
Denser region in a interstellar cloud, maybe compressed by shock waves from an exploding supernova, triggers the gravitational collapse.
Heating  Prototsun  Sun
In-falling materials loses gravitational potential energy, which were converted into kinetic energy. The dense materials collides with each other, causing the gas to heat up. Once the temperature and density gets high enough for nuclear fusion to start, a star is born.
Spinning  Smoothing of the random motions
Conservation of angular momentum causes the in-falling material to spin faster and faster as they get closer to the center of the collapsing cloud.  demonstration
Flattening  Protoplanetary disk.
The solar nebular flattened into a flat disk. Collision between clumps of material turns the random, chaotic motion into a orderly rotating disk.
This process explains the orderly motion of
most of the solar system objects!