1. Origin of the Solar System
Astronomy 311
Professor Lee Carkner
Lecture 8

2. Quiz 1 Monday Covers lectures 1-8 and associated readings
About half multiple choice (~20 questions), half short answer/problems (~4 questions)
Study:
Notes
Can you write a paragraph explaining each major concept?
Exercises
Can you solve all the exercises with no resources?
Readings
Can you do all the homework with no book?
Bring pencil and calculator!

3. The Solar System
The solar system is not just a random collection of planets
The solar system has a structure
Rocky planets – gas giants – small icy bodies
The structure of the solar system is explained by its formation

4. Where Did the Solar System Come From?
Radioisotope dating of the Earth, moon and meteorites indicates that the solar system formed about 4.6 billion years ago
We can’t look back in time to see how the Sun and planets formed, but we can look at young stars that are forming today

5. Star Formation
Stars are formed in clouds of gas and dust when a clump of material starts to contract
The mutual gravity of the particles in the clump causes the contraction to continue
Gravity causes the core to contract to a star
Conservation of angular momentum makes the clump spin faster
Rapid rotation causes the outer layers to form a disk

6. Star Formation in Action

7. Star Formation in Orion

8. Protoplanetary Disks in Orion

9. Protostellar Jet

10. Circumstellar Disks
Disks are fairly cool and can be detected with infrared and millimeter telescopes
We also can see them silhouetted against a bright background in Hubble images
Disks are common around young stars

11. From Disks to Planets
Many stars between 1-50 million years old have disks, but stars slightly older generally do not
Where does the disk go?
Formed into planets
A disk has more surface area than a group of planets with the same mass, so it radiates more light

12. How Do Planets Form? There are 4 stages to planet formation
 dust grains settle to the center of the disk
 grains stick together to form planetesimals
 planetesimals collide to form planets
 gas and leftover planetesimals are cleared from solar system

13. What Was the Solar Nebula Made of?
Solar Nebula -- Initial disk of material that the solar system formed from
From studying meteorites and star forming regions we hope to discover what the solar nebula was made of
Two basic components
Gas -- mostly hydrogen with some helium
Dust -- made of silicates (rock) and ices (water, methane, ammonia)

14. Solar System Dust Grain

15. Accretion of Grains
Dust grains are very small (< 1 mm), how do they form planets?
Grains get larger by sticking together and settle to the center of the disk
If dust grains are fractal they may stick together more easily
Eventually the grains form into larger bodies (a few km in size) called planetesimals
At the end of this stage the solar system is populated by a few thousand planetesimals, such a system is invisible to telescopes

16. Accretion in a Protoplanetary Disk
High Density
Star
Low
Density
Larger Grains
move to center
Accretion in a Protoplanetary Disk

17. Temperature and the Solar Nebula
Two basic types of material in solar nebula:
Volatiles -- material with a low boiling point (ices such as water and methane)
Refractory Material -- high boiling point (silicates)
Temperatures were higher in the inner solar system and lower in the outer solar system
Near the Sun the volatiles boiled off leaving only the refractory material behind
Inner solar system -- rocky planetesimals
Outer solar system -- icy planetesimals

18. Planetesimals to Planets
Due to gravity and intersecting orbits the planetesimals collide with each other
The collisions produce larger and larger objects until the planets are formed
Planet formation happens differently in inner and outer solar system

19. Formation of Gas Giants
In the outer solar system you have more material (both volatiles and refractory material), so planets are larger
Icy planetesimals are “stickier” and form planets faster
These large bodies have enough gravity and form fast enough to collect the gas before it all is dissipated
Thus, in the outer system where the temperatures are lower you have gas giants

20. Formation of Terrestrial Planets
In the inner solar system planets grew more slowly out of less material, most of it refractory
Result is small rocky planets with no large gassy outer layers

21. Accretion of the Inner Planets

22. Beyond the Gas Giants
Beyond Neptune the densities are so low and the orbital timescales are so long no planets form at all
The left over planetesimals form the Kuiper Belt
Other icy planetesimals are ejected by a close encounter with Jupiter or another gas giant
Some end up in long orbits with aphelion far from the Sun and form the Oort Cloud

23. Cleaning Up
Some of the material in the solar nebula does not end up in the final solar system
Some planetesimals are gravitationally ejected from the solar system completely
The solar wind will eventually blow most of the unaccreted gas and small particles out of the system

24. The Final Solar System
Our picture of planet formation is driven by an attempt to explain our own solar system and its three regions
Inner or Terrestrial region
Outer or Gas Giant region
Trans-Neptunian or Cometary Region
We have also found other types of planetary systems different from our own
Our picture of planet formation is in a state of flux

25. Icy
Rocky
Gas
Temperature
Regions of Formation

26. Summary
By looking at stars with protoplanetary disks and the structure of our own solar system and we can develop a theory of how planetary systems form
Next step to use data from extrasolar planets
Our current picture of planet formation explains the 3 regions of the solar system

27. Summary
Inner solar system -- volatiles boil off, resulting in small rocky planets
Outer solar system -- large planet cores form rapidly from refractory and icy material, acquire large gas envelopes
Edge of solar system -- leftover and ejected icy planetesimals form Kuiper belt and Oort cloud