1. The Formation of Our Solar System The Nebular Hypothesis

2. Watch this video for an overview!

3. 4 things that need to be explained.. 1.Pattern of motion 2.Two different types of planets 3.Asteroids & Comets 4.Exceptions to the rules

4. 1. 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

5. 2. Two 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)

6. Two Kinds of Planets Planets of our solar system can be divided into two very different kinds: Terrestrial (earthlike) planets: Mercury, Venus, Earth, Mars Jovian (Jupiter-like) planets: Jupiter, Saturn, Uranus, Neptune

7. Terrestrial Planets Four inner planets of the solar system Relatively small in size and mass (Earth is the largest and most massive) Rocky surface Surface of Venus can not be seen directly from Earth because of its dense cloud cover.

8. The Jovian Planets Much larger in mass and size than terrestrial planets Much lower average density All have rings (not only Saturn!) Mostly gas; no solid surface

9. 3. Asteroids & 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

10. 4. Exceptions 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

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

12. Collapse of the Solar Nebula Three important processes gave form to our system, when it collapsed to a diameter of 200 A.U. 1.The temperature increased as it collapsed 2.The rotation rate increased 3.The nebula flattened into a disk (protoplanetary disk)

13. Evolution of the Solar System

14. 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

15. 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

16. 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

17. 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

18. Leftover Planetismals 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

19. Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of planet formation right now!

20. The Age of the Solar System Sun and planets should have about the same age. Ages of rocks can be measured through radioactive dating: Measure abundance of a radioactively decaying element to find the time since formation of the rock. Dating of rocks on Earth, on the moon, and meteorites all give ages of ~ 4.6 billion years.

21. What could have “sparked” our nebula of gas and dust to start collapsing in the first place? Watch this video!