1. Formation of the Solar System

2. A model of the solar system must explain the following: 1.All planets orbit the sun counterclockwise 2.All planets orbit in about the same plane 3.Most planetary orbits are nearly circular 4.The distance between the planets increase in a regular trend with distance from the sun

3. Why is solar system so orderly? 5. Most planets orbit counterclockwise with little axis tilt (<25 degrees) 6.Almost all moons orbit in the same direction their planet rotates 7.Almost all moons orbit near their planet’s equatorial plane 8.The sun rotates in the same direction as the planets orbit (counterclockwise)

4. 2 nd challenge: Why two types of planets? 1.Terrestrial/rocky: Mercury, Venus, Earth, Mars Small 2.Jovian/gas giants: Jupiter, Saturn, Uranus, Neptune large, Hydrogen-rich, many moons, rings of rock and ice

5. 3 rd Challenge: asteroids and comets Asteroids mostly in asteroid belt between Mars and Jupiter Comets mostly in Kuiper Belt and Oort cloud

6. 4 th challenge: exceptions to the rule Mercury has a more eccentric orbit Mercury’s orbit is more inclined Uranus’ axis of rotation is tilted almost on its side Venus rotates backwards (clockwise) Earth has a very large moon A few Jovian moons orbit opposite direction from their planet’s rotation

7. New stars like our sun, and their planetary systems, form from the gas and dust remains of older exploded stars.

9. Flattening of planetary disk explains why: Planets all rotate in nearly the same plane Planets all orbit in same direction Most planets spin in the same direction Collisions within the disk tend to make elliptical orbits more circular

12. Contraction: Interstellar cloud contracts due to gravity Cloud heats up, spins Condensation: Rocks and metals condense in inner solar system H and He gas collect to make gas giants Accretion: Materials collide and stick together Clearing: Other gases lost to space Leftovers become asteroids Comets, Kuiper belt

13. Explaining the exceptions: Jovian planets captured some moons Giant impacts caused: 1.Earth’s large moon to break off from Earth 2.Backward rotation of Venus 3.Axis tilts of Earth and Uranus