PES 105        Fall 2001 General Astronomy I

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Lecture Notes:

Solar System Overview

text: Overview 4, Chapter 7

Observations ----> Model

We developed models based on our observations of Earth and Moon.

Now we will test those models with observations on each planet.

New observations from:

1. reflected light from Sun
2. infrared radiation from objects glowing
3. space craft
   1. fly by planet (all except Pluto)
   2. orbit planet (Venus, Earth, Moon, Mars, Jupiter)
   3. land on planet (Venus, Earth, Moon, Mars, Jupiter (?))

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General Properties

9 planets:

SUN

• Mercury
• Venus
• Earth
• Mars
  • (Asteroid Belt)
• Jupiter
• Saturn
• Uranus
• Neptune
• Pluto

ALL orbit in same direction around Sun

Most spin in this same direction.

Distances from Sun:

58 million km --------------> 6 billion km

1/3 AU -----------------------> 40 AU

Orbital Planes:

edge on view:

Most orbits within 3 degrees of the Earth's orbital plane => "flat" solar system

range of angles is 0 -> 17 degrees

Figure 7.2 also shows this property. [Link to Figure 7.2]

Tilts of spin axes:

 

Most similar to Earth. . . . . . . Venus, Uranus and Pluto are exceptions

Range: 0 -> 177 degrees

Figure 7.2 also shows this property. [Link to Figure 7.2]

Average Temperatures:

Sizes:

diameters:

• smallest: Pluto
  • 2300 km, about 1/5 the diameter of the Earth
  • smaller than the Moon
  • 2300 km = Colorado Springs to Pittsburg
• largest: Jupiter
  • 138,000 km, about 11 x the diameter of the Earth

  

  Figure 7.3 shows all of the planets to scale. [Link to Figure 7.3]

Masses:

• least: Pluto
  • about 1/50 the mass of the Earth
• greatest: Jupiter
  • 318 x the mass of the Earth

  

Densities:

density = mass/volume

use this for rough composition information

inner planets are rocky

outer planets are gas and liquid

Number of moons:

0	0	1	2	|	16	18	15	8	|	1
Mercury	Venus	Earth	Mars		Jupiter	Saturn	Uranus	Neptune		Pluto

Chemical Composition:

Mercury, Venus, Earth, Mars: . . . . . heavy elements (metals, silicon ...), almost NO hydrogen or helium

Jupiter, Saturn, Uranus, Neptune . . . . . . mainly hydrogen and helium

Galaxy over-all: . . . . 74% Hydrogen, 24% Helium, 2% other stuff

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Planet types

 Terrestrial Planets: Mercury, Venus, Earth, Mars

1. small
2. high density
3. solid
4. closer to Sun (warm)
5. some have atmospheres
6. few moons, no rings

Gas Giant (Jovian) Planets: Jupiter, Saturn, Uranus, Neptune

1. big (4 -> 11 times the Earth)
2. low density
3. mainly gas (probably solid cores)
4. farther from Sun (cooler)
5. all have atmospheres
6. many moons and rings

Pluto - very strange !

NASA has put together a nice JAVA animation which summarizes what we see in the solar system. It is at: http://kids.msfc.nasa.gov/SolarSystem/SolarSystemJava.asp You can drag the coordinate system in the lower left to change the angle that you view the solar system from. Clicking on the "next" label in the upper right part of the animation will move you to the next part of the animation.

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Formation

To put things in perspective, we will briefly examine a model of the formation of the Solar System. We will see some of the supporting evidence for this model as we examine the planets individually.

We need to answer questions about several issues that have arisen in this overview of our observations:

• Why are all orbits in the same direction ?
• Why are most orbits in the same plane ?
• Why do we have terrestrial and gas giant groups of planets ?

History of the Solar System

Start around 5 billion years ago.

This number comes from radioactive dating of rocks, computer simulations and cooling rates of planets.

This model is also sketched in Figure 7.11. [Link to Figure 7.11]

Planets forming close to Sun:

1. warmer
2. strong gravitational forces from Sun (tidal)

   hard for light elements to condense from gas to solid (no water condensed)

   hard for solids to collect together

   1. planets made of heavy elements only => tend to be smaller

   fewer moons because

   1. small mass of planets => weaker gravity

      close to Sun => competition with Sun's gravity

Planets farther from Sun:

1. colder
2. less effects of Sun's gravity

   light and heavy elements included in planet (also water and ice) - mainly hydrogen

   1. by trapping all elements, planets become large and attract even more gas

   more moons because

   1. large mass of planets => strong gravity

      far from Sun => little competition with Sun

We see some evidence for this model from looking at the formation of other stars.

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