Unit II: Answers to Self-Check Questions
The Solar System, Its Origin and Terrestrial Planets

1. What is the difference between a star and a planet?

The major difference is that a star can create its own energy via nuclear reactions (specifically fusion), This will be gone into more detail in unit 4. Planets cannot create their own energy via nuclear fusion. They are visible to us not by the energy they create, but by they solar energy that they receive from the Sun and reflect to us. Some of the super massive planets in our solar system, Jupiter and Saturn are known to transmit more energy than they receive from the Sun. Thus they do have some internal mechanism or source of energy. However, this internal energy is not in the visible part of the spectrum and it is questionable whether it is created by nuclear fusion (which is the mechanism available to stars). It would be interesting to research on the source of this energy.

2. What is the difference between a planet and a moon?

In most cases a planet is more massive than a moon. However, a more important distinction is that a planet orbits a star whereas a moon orbits a planet.

3. Refer to the text link in figure 1. To what does the term "EGGS" refer. What is their significance?

The term EGGS stands for Evaporating Gaseous Globules. The are dense globules of dust and gas some of which may become large enough and hot enough to form stars.

4. Refer to the text link in figure 1. What are the two primary constituent materials which comprise the globules in the Eagle nebula ?

The globules are composed of molecular hydrogen and dust.

5. Refer to the text link in figure 2. What is the significance is the Orion nebula?

The Orion nebula is a region of active star formation that is relatively close to us. The nebula is a giant gas cloud illuminated by bright, young hot stars. Many of the fainter young stars are surrounded by disks of dust and gas that are slightly more than twice the diameter of the Solar System.

6. Refer to the text link in figure 3. What is the range in the masses of the stars forming in the Orion nebula?

The mass of the stars ranges between 30 percent and 150 percent of the mass of the Sun.

7. Refer to the text link in figure 3. What is the composition of "proplyds". What is their approximate size? How do astronomers explain the different shapes observed for proplyds?

Proplyds are 99% gas and dust. The disks range in size from two to eight times the diameter of our solar system. The researchers explain the different circular or elliptical shapes as being due to the fact that each disk is tilted toward Earth by different degrees.

8. 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?

There are three planets that are tilted more than 90 degrees. They are Venus, Uranus and Pluto. It is probably not a coincidence that they are also the planets that exhibit retrograde rotation. Try spinning a globe and look at its direction of rotation from the north and south poles. You will notice that they will appear to rotate in opposite directions. Over one pole the planet will appear to rotate clockwise and over the other pole it will appear to rotate counterclockwise. For planets tilted more the 90 degrees, we are looking at them from a different perspective than for planets tilted less than 90 degrees.

9. Refer to this periodic table of elements. How many protons and neutrons are in the nucleus of lead (Pb)?

The atomic weight of lead (pb) is is 207, meaning that there is a total of 207 protons and neutrons in its nucleus. Since its atomic number is 82, there are 82 protons in its nucleus. Therefore, there are 125 (207 - 82) neutrons in its nucleus.

10. Identify one element that has the same number of protons and neutrons its nucleus.

Both oxygen (8 protons and 8 neutrons) and nitrogen (7 protons and 7 neutrons) have the same number of protons and neutrons.

11. What elements on the periodic table is the only element that has no neutrons in its nucleus?

Hydrogen (H) has an atomic weight that is equal to its atomic number (1). Therefore it can only have 1 proton in its nucleus and no neutrons.

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

The Earth and other terrestrial planets. The density of the Earth is 5.515 grams/cubic centimeter. The average density of the terrestrial planets is 5.03 grams/cubic centimeter. The high density of the terrestrial planets suggests that they are dense, rocky planets composed of heavy elements. In fact, the most abundant element in the Earth is iron. Therefore, the terrestrial planets are the exception to the rule that the universe is composed of heavy elements. This is significant because a theory of the origin of the solar system has to explain this observation.

13. The Earth's atmosphere is composed of oxygen and nitrogen. It is more correct to say that it is composed of molecular oxygen and nitrogen. Most atoms have the tendency to bond with other atoms. Thus, it is rare to find single atoms of oxygen or nitrogen. The oxygen and nitrogen in the Earth's atmosphere is diatomic (def.), ) O₂. Use this periodic table of elements to determine the molecular weight of of diatomic oxygen.

According to the periodic table the atomic weight of oxygen (O). Therefore diatomic oxygen (two paired oxygen atoms) would yield a molecular weight of 32.

14. Jupiter's radius is how much larger than that of the Earth.

The radii of the two planets can be expressed as:

Therefore Jupiter is:

15 Why can't Pluto be classified as either a terrestrial or jovian planet?

Pluto has properties of both. It is small and is low in mass like the terrestrial planets. However, it has a low density and is far from the Sun like the jovian planets. In addition, it has the unique properties of having a very eccentric orbit and an orbital planet that is highly inclined with respect to the plane of the ecliptic.

16. How do the stages described in the solar nebula hypothesis (above) help to explain the general properties of our solar system.