Chapter 8:
Questions to guide your reading:
1. What must be included in a viable theory of the origin of the solar system? It must
include an explanation of the Sun and the planets. Specifically, it must explain
a. Why the planets all orbit the Sun in the same direction
b. Why we have 2 major categories of planets: terrestrial planets which are
small and rocky; and Jovians which are large, composed of
hydrogen/helium, and farther from the Sun;
c. The huge number of asteroids and comets
d. What are the exceptions (For example Uranus is spinning on its side and
Earth has a very large Moon)
2. Why do the Jovians have so much Hydrogen, Helium, and hydrogen compounds,
while the terrestrials don’t? (See number 9 below)
3. How do we know the age of the solar system? Astronomers determine the age of
the solar system by doing radioactive age dating on rocks that formed in the early
stages of the development of the solar system.
4. How do astronomers think the solar system formed? Astronomers think the solar
system formed from a large gas cloud called a nebula. The cloud had some
rotation and formed a disk then the Sun and planets coalesced from material in the
disk
5. Did all of the planets form in the same way? No, the terrestrial planets formed in
a hotter environment because they were nearer the Sun so they are composed of
metals and heavier elements which have high boiling points.
6. Are there planets orbiting other stars? How do astronomers search for other
planets? Yes astronomer know there are planets orbiting other stars because they
can see the wobble in the position of the stars, also they can see the Doppler shift
in the light from the star as it wobbles in its orbit due to gravitational effects from
the planet. They can see a change in the brightness if the planet does a transit
across the face of the star while they are observing it.
7. The graphite in your pencil is a form of carbon. Where were these carbon atoms
formed? In stars.
8. What is the interstellar medium and how does it become enriched over time with
heavy elements? The interstellar medium is the pace between stars. It becomes
enriched with heavy elements as various stars die and expel some of their material
(which includes heavy elements).
9. If hydrogen and helium account for 98% of the mass of all the atoms in the
universe why aren’t the Earth and Moon composed of them? Because the Earth
formed near the Sun where it is hot and hydrogen and helium can’t condense at
the high temperatures that existed in the inner part of the early solar system.
10. How do radioactive elements make it possible to determine the age of our solar
system? What are the oldest rocks that have been found in the solar system.
Radioactive elements change into another element at a fixed rate called a “halflife.” By measuring the amount of the radioactive element and the amount of the
resulting radioactive decay product in a particular rock, astronomers can
determine the age of the rock. The oldest rocks are meteoroids which are about
4.56 billion years old.
11. What is a planetesimal? How did planetesimals give rise to the terrestrial planets?
Planetesimals are rocks that formed in the early solar system.
12. What is accretion? It is the accumulation of more and more matter to form larger
and larger objects.
13. Why did the terrestrial planets form close to the Sun while the Jovians formed
farther away? Since it is hot near the sun, only certain elements can condense to
form dust particles and small rocks. The elements that can do this are heavier
elements like iron, silicon, magnesium, and others. When these kind of elements
form a planet, it is mostly rocks and iron, which is what the terrestrials are.
Farther from the Sun it is much cooler and water and other compounds can
condense then this matter can accrete until it is large enough to capture hydrogen
and helium gravitationally.
14. Explain how our current understanding of the solar system can account for the
fact that all planetary orbits lie in nearly the same plane. This is because in the
nebular hypothesis the planets formed from a flattened rotating disk of gas—the
matter was already moving in a plane when the planets formed.