1. The Solar System
An Inventory

2. What is the Solar System?
Answer: The system of objects in the solar neighborhood (near the Sun)
What are these objects?
One Star
Nine Planets
Six Planets
Dozens of moons
Thousands of asteroids
Trillions of comets

3. The Discovered Planets
All planets through Saturn known since the ancients – all you have to do is look up to see them
Uranus discovered in 1781 by William Herschel
He wanted to name the planet “Georgium Sidus” after his king and patron, George III of England
Neptune was first seen in 1846 by Johann Galle using predictions by Urbain Jean Joseph Leverrier and John Couch Adams
Pluto was discovered in 1930 by Clyde Tombaugh at Lowell observatory

4. Size and Scale

5. Planets The first step to studying planets? Compare and contrast
What are important quantities?
You have:
A stick
A tree
A car
A house
What are the important quantities?

6. Planetary Properties

7. Density and Mass What is mass? What is density?
Mass is similar to weight, it measures how much stuff an object is made of
Example: A bowling ball and a soccer ball are about the same size, but have different masses
What is density?
Density is mass per volume. It helps to tell you what kind of stuff an object is made of
Example: A log and a tree have different masses (and sizes), but the same density because they are made of the same stuff

8. Terrestrial Planets Close to the sun Small High density
Mass
Radius
High density
Primarily rocky
Solid surface
Weak magnetic field
Few moons
No rings

9. Jovian Planets Far from the sun Large Low density
Mass
Radius
Low density
Primarily gaseous
No solid surface
Strong magnetic fields
Many moons
Many rings

10. What About Pluto? Pluto does not easily fit into either category
Far from the sun (jovian)
Small (terrestrial)
Neither rocky nor gaseous (icy)
One moon
No rings
It is similar is composition to some moons in the outer solar system and its orbit is similar to a group of objects called “Kuiper Belt Objects” or KBOs

11. Pluto
Only planet in our Solar System that has not been visited by a NASA (or any other) spacecraft

12. Charon
Largest of any moon in relation to the planet it orbits (1/2 the size of Pluto)
Pluto and Charon are tidally locked to each other (always show the same face)
Charon discovered in 1978 by astronomers at the US Naval Observatory

13. Formation Where did the Solar System come from?
First, what observations can we make that will constrain the origin of the Solar System?

14. Model Requirements Planet’s are isolated
Planetary orbits are nearly circular
All planetary orbits lie in the same plane
All planets orbit in the same direction as the Sun’s rotation
All planets rotate in the same direction as the Sun
Most moons rotate in the same direction as the planet they orbit
The planetary system is highly differentiated

15. Differentiation
In general, planets get less dense as they get further from the Sun
They go from being composed of metals, to rocks, to ices, to gases
In other words, they go from being made of things with high melting temperatures to things with low melting temperatures

16. Highlights of the current theory
Nebular contraction
Mutual gravity causes contraction
Conservation of momentum increases speed
Planetary formation (accretion)

17. Differentiation revisited
As the solar nebula contracted, the center became hotter than the rest of the cloud
As elements condensed out of the nebulae, temperature determined which could form

18. Clearing of the nebula
After the planets formed, some small debris still remained. All of these small objects were affected by the gravity of the much larger planets. The debris either:
Hit a planet
Hit the Sun
Was thrown out of the area near the planets – becoming KBOs

19. Explaining observation
Matching model requirements:
Point (1) is due to planetesimal growth
Points (2), (3), (4), (5), and (6) are due to conservation of angular momentum and gravitational collapse
Point (7) is due to the heating in the nebula
Anomalies:
Retrograde rotation of Venus
Uranus’ axial tilt
The Earth’s moon
All can be explained by impacts of protoplanets into the planet soon after its formation