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Chapter 7: The Birth and Evolution of Planetary Systems

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1 Chapter 7: The Birth and Evolution of Planetary Systems

2 Where did the solar system come from? How was it made?

3 “Facts” that must be accounted for in any theory of solar system formation
All the major planets orbit in almost the same plane All the planets orbit in the same direction Almost all the planets rotate in the same direction as they orbit The inner planets are rocky bodies while the outer planets are gaseous and/or icy bodies 99% of the mass of the solar system is in the Sun Most of the angular momentum of the solar system is in the planets, not the Sun Look at ClassAction Solar System Properties Explorer in the Solar System Characteristics module

4 We start with a cold cloud of gas and dust
Because of the internal motions of the gas and dust, the cloud almost always has some slight overall rotation

5 The cloud starts to collapse due to gravity
Angular momentum causes the cloud’s initial slow rotation to spin faster and flatten out

6 Angular momentum is what causes a skater to “spin-up”
Angular momentum depends on both the velocity, V, and the size, R. If R decreases, V must increase. It is also what causes the pizza dough to flatten out when tossed

7 The “Spin-up” causes the cloud to flatten out
Angular momentum keeps stuff from falling straight in. Instead, it spirals down onto a disk. This is the pizza toss effect

8 At this point we have something that looks like a star surrounded by a disk of gas and dust
The protostellar Sun is getting its energy from gravitational collapse, not from fusion like “normal” stars.

9 The temperature in the protoplanetary disk falls off as you get farther from the protosun
Check out planet Formation Temperature Plot on ClassAction website Solar System Characteristics module

10 The solar nebula is composed mostly of hydrogen and helium
The most common things to condense will be hydrides of carbon (CH4…methane), nitrogen (NH3…ammonia) and oxygen (H2O…water). These condense at fairly low temperatures. Elements like silicon and iron condense at higher temperatures.

11 What is found at different distances from the protosun depends on temperature and abundance

12 Condensation begins to form dust grains
The dust grains are tiny: about the size of particles in smoke. They are also charged with static electricity

13 The dust grains quickly start sticking together
Close to the protosun the grains are exclusively silicon, iron and other heavy elements: “rocky” materials. Farther out there are more grains of “icy” materials than rocky ones. Static electricity also plays an important part in making the grains stick together

14 Accretion is a snowball effect that builds larger and larger objects

15 Eventually Planetesimals are formed
Close to the Sun the planetesimals look like asteroids Far from the Sun the planetesimals are a mix of ice and rock

16 Planetesimals merge to form protoplanets
The larger the planetesimal, the stronger its gravity is. The stronger its gravity, the more it attracts stuff and the more violent the collisions become.

17 The gas giants form a large core of ice and rock and then grow by sweeping up large amounts of gas

18 When the gasses get blown away, the condensation phase ends

19 The Solar Nebula Stage Condensation starts and planetesimals begin growing

20 The Accretion Stage Planetesimals grow bigger by collisions. There may be hundreds of moon sized protoplanets form in the inner solar system. The outer planets have grabbed up the last of the gas

21 The accretion stage was a violent period with planet smashing collisions

22 Finally, we have a new star and new planets
The new planets at this stage are nothing like the planets we see today. They will evolve over time to become the eight planets we see now

23 The gas giants were like mini solar systems, forming a system of moons

24 Finding extra-solar planets
Our theory was designed to explain the formation of our solar system. How does it match up with other planetary systems around other stars?

25 We have seen lots of disks around forming stars confirming some of the nebular theory

26 Actually seeing a planet has only recently been done

27 Newton’s 3rd Law applies to the Sun and planets
If the Sun tugs on Jupiter, keeping it in orbit, then Jupiter tugs on the Sun, making it orbit. The two actually orbit a common point just outside the surface of the Sun Watch ClassAction Extrasolar Planet module Influence of Planets on the Sun animation

28 The Doppler Effect technique detects the motion of a star caused by a planet
Watch ClassAction Extrasolar Planet module Radial Velocity Graph animation

29 The transit method measures a planet directly if it passes in front of its star
The planet will be a dark spot passing across the face of the star. The dimming of the light from the star may be tiny but it is measurable if the planet is large enough.

30 OGLE detects gravitational microlensing caused by a planet

31 The Doppler method is the most prolific but it finds large mass planets close to their star

32 So what do we do about our solar nebula model?
Our model predicted small rocky planets close to the star We are finding large gas giants close to their star! The basic modification is that things move, sometimes they move a lot: Migration theory

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