Unit II: Properties of Our Solar System

• Describe the basic properties of the solar system.

Read This Material in Your Text:

• Seeds, 5th Edition: Section 7.2
• Seeds, 4th Edition: Section 5.2

Read These Notes:The properties of our solar system provide clues to how our solar system formed. Like a detective who must recreate a crime on the basis of existing clues, so too must an astronomer establish hypotheses of how the solar system formed on the basis of existing evidence. What is that evidence? Print out this Planetary Data Table and examine the data. From this and other data, we can make the following generalizations about our Solar System:

The composition of the solar system is typical of that of the universe.
Everything in the universe is made of atoms (def.). There are 92 different kinds of atoms or elements (def.). Elements are classified on the basis of the number of protons in the nucleus (the atomic number (def.)). The periodic table of elements is an arrangement of the 92 naturally occurring elements by atomic number. The 92 elements are not, however, equally abundant. Two elements, hydrogen (H) and helium (He) make up 99% of the universe. Hydrogen makes up 75% ; and helium, 24%. (Leaving the other 90 elements to make up 1% of the universe.) No matter where we look, we find that the composition of the universe is relatively uniform. ( THIS LAST LINK TO THE COMPOSITION OF THE UNIVERSE IS A SLIDE SHOW THAT CONSISTS OF ABOUT 6 SLIDES. YOU HAVE TO BE ONLINE TO VIEW IT. )

If you need a refresher in reading a periodic table, you should complete this tutorial.

99.87% of the mass of the solar system is in the Sun.
The data in the data table provides information only on the planets. The mass of the Earth, for example is 5.976e+24 kilograms (interpreted 5.976 times 10 raised to the 24th power kilograms). It is pretty humbling to think the combined mass of the planets, their moons, the comets and asteroids make up only .13% of the mass of the solar system. The Sun makes up the other 99.87%. Even more humbling is the fact that that .13% is the planet Jupiter.

The orbits of the planets are nearly circular.
Although we learned in the previous unit that the orbits of the planets are ellipses, with the Sun located at one of the two foci. In fact, the eccentricities of their orbits is nearly circular, with eccentricities close to "0". (Refer to the column labeled "Eccen." on your data table.)

The orbits of the planets lie in nearly the same plane.
The orbits of each of the planets lie in a plane. This plane is referred to as the "orbital planet" (def.) The inclination (def.)of the orbital planes of the planets is within 4 degrees of the ecliptic with the exception of two planets. (Refer to the column labeled "Incl." on your data table.)

All of the planets orbit the Sun in the same direction.
The planets all orbit the Sun in an eastward direction.

All of the planets, except two, rotate in the same direction as they revolve.
We saw in the discussion of rotation that the Sun and other celestial bodies rise in the east because of the Earth's eastward rotation. All but two of the planets rotate eastward.

Most of the planets are tilted on their axis by only a small amount.
(Refer to the column labeled "Tilt" on your data table.)

The planets fall into two distinct categories, the terrestrial planets and the jovian planets.
There is a tendency to think of a planet as being "earthlike". However, our solar system has two distinct and different types of planets. The terrestrial planets are those planets that are similar to the Earth. The jovian planets are those that are similar to Jupiter.

The Oort comet cloud
Surrounding the solar system is a spherical cloud having a radius of 100,000 AUs. It is thought to be the source of long period comets.

Complete These Self-check Questions: These questions and their answers are designed to help you determine how well you understand this objective and to provide additional instruction.

• You will notice that the planets that have a retrograde rotation are also the planets that are tilted more than 90 degrees. Which planets are they? Do you think that this is a coincidence? (ans.)

• Refer to this periodic table of elements. How many protons and neutrons are in the nucleus of lead (Pb)? If you need a refresher in reading a periodic table, you should complete this tutorial. (ans.).

• Identify one element that has the same number of protons and neutrons its nucleus. (ans.)

• What elements on the periodic table is the only element that has no neutrons in its nucleus? (ans.)

• The universe is essentially composed of 2 elements, hydrogen and helium. There is one major, familiar exception to this "rule". What is it? (ans.)

Homework Questions: Answer these homework questions after reading the on-line lecture and the readings in the text. Complete the self-check questions before attempting the homework. Follow the instructions for submitting the homework on this unit homework page.

Use this Planetary Data Table to answer the following questions:

7. Calculate to 3 decimal places the average eccentricity of the planet's orbits.
8. How would you describe the shape of the planets' orbits?
9. Which planet's orbit has the highest eccentricity? the lowest eccentricity?
10. Using the Planetary Data Table, determine which three planets have a retrograde rotation (def.). Retrograde rotation is represented by a negative or clockwise (CW) rotation period.
11. Using the Planetary Data Table, calculate the average inclination for the orbital planes of the planets. Because Pluto's status as a planet is questionable, you should exclude Pluto. You should also exclude the Earth.
12. Why should you exclude the Earth from your calculation?
13. What does your calculated answer imply about the orbits of the planets?
14. Which two planets have orbits inclined at an angle greater than 4 degrees?

Here are Some OPTIONAL Sites You May Want to Eplore:

• The Nine Planets: An Overview of the Solar System