1. Formation of the Solar System
Uncovering the origin of the Solar system
Early days of the formation
Building the planets and other stuff
Other planetary systems

2. 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

3. 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
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?

4. 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?

5. Four Challenges 3. 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

6. Four Challenges 4. 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
Allow for these exceptions

7. 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

8. 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)

9. Evolution of the Solar System

10. 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

11. 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

12. Condensation

13. 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

14. 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

15. The Solar Wind
Solar wind is a flow of charged particles ejected by the Sun in all directions
It was stronger when the Sun was young
The wind swept out a lot of remaining gas and interrupted the cooling of the nebula
If the wind were weak, the ices could have condensed in the inner solar system

16. 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

17. 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

18. Planet Activity

19. Planetary Evolution - Atmosphere
Atmospheres are formed by:
gases escaping from interior
- impacts of comets (volatile-rich debris)
Fate of water depends on temperature (distance from the Sun)
Atmospheres changed chemically over time
Life on Earth substantially changed the atmosphere

20. Other Planetary Systems
Over 100 extrasolar planets have been discovered since The Extrasolar Planet Encyclopedia
Stars are too far away from the Sun, and direct imaging cannot detect planets near them
Current strategy involves watching for the small gravitational tag the planet exerts on its star
The tag can be detected using the Doppler effect

21. Extrasolar Planets in the Sky

22. Planet Transits

23. The Nature of Extrasolar Planets
The discovery of extrasolar planets gives us an opportunity to test the solar system formation theory
Most of the discovered planets are different from those of our system
They are mostly Jupiter-size and located closer to their stars
But: possible planet migration
discovered planets are exceptions

24. Summary
All the planets were formed from the same cloud of dust and gas
Chance events may have played a large role in the formation and evolution of individual planets
Planet-forming processes are apparently universal