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Homework 4
October 4, 2011
Chapter 3
12. Briefly describe the characteristics of each of the following types
of worlds: terrestrial planets, jovian planets, moons, asteroids, comets,
and large Kuiper belt objects.
terrestrial planets are the smaller and less massive (but more dense) of the
two types of planets. They are located closer to the sun and are composed
primarily of rock and metal. They have fewer moons than jovian planets
and no rings. They also have a solid surface.
jovian planets are larger, more massive, and less dense than terrestrial planets. They are composed primarily of hydrogen, helium, and hydrogen
compounds like water and methane. They have many moons as well as
rings of smaller satellites. They do not have a solid surface.
moons are a lot like planets in composition but they do not orbit the sun
directly, they orbit planets. The composition of moons varies with distance
from the sun in essentially the same way as planets, moons closer to the
sun are made of rock and metal while moons further away are composed
mainly of ice (the same stuff the jovian planets are made of, but frozen).
asteroids are composed of metal and rock like terrestrial planets, but they are
much smaller. Unlike moons however, asteroids do orbit the sun directly
instead of a planet.
comets are like asteroids in that they are relatively small objects that orbit
the sun. However, their composition is closer to that of the jovian planets
and their moons - they are made of ice. Note that, as we would expect,
asteroids orbit close to the sun while comets tend to be farther away in
the Kuiper belt and Oort cloud.
large Kuiper belt objects are very similar to comets in that they are made
of the same stuff and orbit the sun from far off in the Kuiper belt. However, since they are much larger than the average comet astronomers are
reluctant to refer to them as comets, hence the term Kuiper belt object.
14. Briefly explain why we think our solar system ended up with
rocky worlds in its inner regions and icy or gaseous worlds in its
outer regions. How do we explain the small bodies that populate the
asteroid belt, Kuiper belt, and Oort cloud? According to the nebular
model, the solar system formed from the collapse of a spinning diffuse gas of
particles. All of the planets would have formed from accretion (a bunch of particles sticking together after collisions). This process begins with condensation,
when the first small particles form out of the gas. The key point is that different
materials condense at different temperatures, and in particular, hydrogen and
helium only condense at colder temperatures. So, close to the sun where it is
warmer only the rock and metal could condense and eventually form planets
made of those materials. But, farther away the hydrogen and helium condensed
as well, so planets in that region are composed of these elements as well (in fact
their composition is dominated by these elements since they were much more
abundant than the others in the nebula).
The smaller bodies in the solar system were formed in the same fashion as
the planets - they just did not grow large enough to be planets. The asteroids
are concentrated in the asteroid belt because Jupiter’s orbit ensures that the
asteroids there are least likely to collide with other planets. Objects in the
Kuiper belt are there because that is probably where they formed and they had
no reason to move. However, the objects in the distant Oort cloud were probably
formed near the jovian planets and jettisoned further out by a “gravitational
sling shot” effect.
Does it seem reasonable?
32. Astronomers have discovered another star system that is virtually
the reverse of ours: It has all its gaseous planets, icy moons, and
comets in its inner regions, and its rocky planets and asteroids in its
outer regions. This is not reasonable unless the solar system were formed
in a different manner than ours. The parts of the nebular model that predict
gas giants in the outer region and terrestrial planets in the inner region should
not be specific to our solar system. Hydrogen should condense at the same
temperatures everywhere in the universe.
33. A noted physicist today announced that he has found evidence
that gravity operates only on Earth and nowhere else in the universe.
This does not seem reasonable either. Aside from the fact that he would have
to explain why matter on Earth is unique in the sense that it is the only matter
in the universe to exhibit gravitation (or at least gravitation as we know it),
he would also have to explain why countless observations of distant matter
agree with predictions of the theory of gravity (either Newtonian or relativistic
depending on the situation).
Multiple Choice
39. According to observations, the overall chemical composition of
our solar system and other similar star systems is approximately...
(a) 98% Hydrogen and Helium, 2% other elements
40. The age of our solar system is about...
(a) one-third the age of the
43. Which of the following is not a general difference between terrestrial planets and jovian planets? (c) Terrestrial planets have oceans of
liquid water and jovian planets do not.