Astronomy 105G Lecture Notes, 26 Apr. 2004

Announcements

• Reading: Ch. 14 pp. 323-335
• Homework 4 DUE RIGHT NOW
• Final Exam is on Monday May 10, 10:30 am - 12:30 pm. NO MAKE-UPS! Study Guide is here
• Final Extra Credit of the semester: telescopes at Campus Observatory will be open Mon. - Thurs., 8-9 pm

Last Time:

• The Sun's Interior
• The Sun's Atmosphere
• Active Regions on the Sun

Today:

• Patterns in the Solar System
• Nebular Theory of Solar System Formation

From Last Time...

We can put a lot of the information we heard last time about solar activity into a single image to remind ourselves of the various aspects of the Sun that we discussed:

Image courtesy of NASA/ESA.

Patterns in the Solar System

We have now learned about all of the contents of our solar system: the Sun, planets, and small bodies. We now know that these objects all formed out of the same cloud of dust and gas. Yet today, the objects seem so different from one another! By comparing these worlds to each other, we can learn more about the processes that caused each one to look as it does today. This approach is known as comparative planetology. Here is an analogy: you can learn more about a person by studying his or her family, friends, lifestyle, etc., than studying that person in great detail. We have completed our study of all the components of the solar system in some detail - now we will take a step back and look at the big picture.

Take a few minutes in your groups to describe (in general terms) the characteristics of our solar system. Imagine what you would see if you arrived here from an alien planet for a visit. Things you might want to consider include:

• make-up of the planets (wood? styrofoam?)
• clumping/spacing of the planets (recall the lab we did on the football field)
• sizes and shapes of objects in the solar system (cubes?)

This is a good review exercise, but also, any theory that describes how the solar system formed must be able to describe our observations of our own system. It is the only solar system that we have been able to study (so far) in a lot of detail.

Everything you said (and more) can be lumped into 4 general features of our solar system that any formation theory must be able to explain:

1. Patterns of Motion
   
   
   • all planets orbit the Sun in the same direction: counter-clockwise if viewed from above the North Pole looking down. The Sun also rotates in this direction.
   • orbits are elliptical, but most are nearly circular
   • most planets rotate in the same direction that they orbit the Sun: CCW as seen from the N. Pole
   • most axial tilts are fairly small (less than 25 degrees)
   • most moons orbit planets in the same direction as the planets' rotation, near the plane of the planets' equators













2. Two Basic Categories of Planets
   
   

   Terrestrial Planets	Jovian Planets
   smaller mass, size	larger mass, size
   rock, metal composition	light gases, hydrogen compounds
   solid	no surface
   closer to Sun (and closer together)	farther from Sun (and spaced farther apart)
   warmer	cooler
   few moons, no rings	many moons, rings

   
   Why do we have 2 basic kinds of planets? (why not 4 kinds?)





3. Asteroids and Comets
   
   

   Asteroids	Comets
   small rocky bodies	small icy bodies
   mostly between orbits of Mars and Jupiter	two populations: Kuiper Belt and Oort Cloud
   orbit in same direction as planets (CCW)	KB: orbit in same direction as planets
   some have fairly elliptical orbits	OC: random orbits, not in plane of planets
   probably 100,000 or more	millions or billions of them

   
   These are the most numerous objects in the solar system - we need to be able to explain their existence!





4. Exceptions
   
   
   • Mercury and Pluto have pretty elliptical orbits (not close to circular)
   • Uranus and Pluto have very large axial tilts
   • Venus rotates "backwards" (CW as seen from above N. Pole)
   • Earth is the only terrestrial planet with a large moon
   • Pluto and its moon Charon are close to the same size - a "double planet" system
   • some moons of the giant planets orbit "backwards"

Any theory of solar system formation must be able to explain of the observations we've just described. There have been many theories put forward, but only one (so far) has been able to explain all of the general features of our solar system.

Nebular Theory of Solar System Formation

Our solar system formed out of a cloud of gas and dust. The solar nebula is the cloud from which the solar system formed. The following steps are believed to have occurred:

• the gas cloud started collapsing due to ?? Perhaps a shock wave from an exploding star nearby?
• the temperature of the gas cloud increased as it collapsed (temperature measures gas motion, and the atoms and molecules will move around a lot faster if they are confined to a smaller space)
• the temperature at the center is hottest, and eventually became hot enough to start nuclear fusion. This resulted in a star (the Sun) forming at the center of the collapsing nebula
• the solar nebula rotated faster and faster as the size shrank due to conservation of angular momentum (think about an ice skater who draws her arms in while she is spinning - what happens?)
• the solar nebula flattened into a disk (the protoplanetary disk) due to the rotation (think about clay on a pottery wheel)