Download Formation of the Solar System

Formation of the Solar System
Astronomy
Name:
Period:
Any model of the formation of the solar system must account for the motions, compositions and locations
of all the planets and their moons. In this lab, you will use the motions of objects in the solar system to
concoct a model of the formation of the solar system. Processes which were important to the formation of
our solar system are also important in star formation, and galaxy evolution, so we will be visiting many of
these concepts again.
Procedure
Part 1: Shapes of Planetary Orbits
Figure 1: The orbits of the inner planets
Figure 2: The orbits of the outer planets
1. Examine Figure 1, which shows the orbits of the inner planets. In general, what shape are the inner
planet orbits?
2. Are all the orbits centered in the same place? Which two orbits are most off-center? Are these also the
most elliptical (least round) of the orbits?
3. Examine Figure 2, which shows the orbits of the outer planets. In general, what shape are the outer
planet orbits?
© 1999 University of Washington
Revised: 3 January, 2001
4. Which of the planet orbits is different from the others? What are two ways in which the odd orbit is
different from the others?
5. In one sentence, describe the shapes of the orbits of the planets.
Part 2: Inclinations of Planetary Orbits
Inclination = If you drew a straight line from the sun through Earth, inclination is the angle of other
planets’ orbits above or below that of Earth’s (so Earth’s inclination = 0)
TABLE 1: Rotation and Revolution Data
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Planet
Revolution
CCW
CCW
CCW
CCW
CCW
CCW
CCW
CCW
CCW
Appx. Inclination
of Orbit
7
3
0
2
1
2
1
2
17
Planet Rotation
Moon Revolution
Planet Density
CCW
CW
CCW
CCW
CCW
CCW
CCW
CCW
CCW
No moons
No moons
CCW
CCW
CCW
CCW
CCW
CCW
CCW
5.4
5.2
5.5
3.9
1.3
0.7
1.3
1.6
2.1
1. Examine Table 1. Which object has the largest inclination?
2. In general, how do the inclinations of the Inner Planets compare with those of the Outer Planets (don’t
forget that Pluto isn’t a planet)
© 1999 University of Washington
Revised: 3 January, 2001
3. Why is the Earth's inclination exactly 0?
4. In one sentence, describe the shape of the solar system, using your answers from Part 1 and Part 2.
Part 3: Rotations of the Planets
1. Examine Table 1. In which direction is our solar system rotating and revolving?
2. Which planet is not rotating the same direction?
3. Do the rotations of solar system bodies seem to indicate that most of them formed 1) together at the
same time in the same way, or 2) separately under different conditions?
4. How could you tell if a moon or planet did not form with the rest? If a moon or planet did not form
with all the others in its vicinity, how might it have gotten there?
Part 4: The Densities of Planets
1. Examine Table 1. Compare the densities of the inner planets and outer planets.
2. Where in the solar system would you expect to find most of the iron and radioactive materials (the
heavy stuff!)---1) the inner or 2) outer solar system?
Part 5: The Standard Model
In the standard model of the formation of the solar system, we begin with an enormous cloud of
gas and dust (solar nebula), which is slowly rotating counterclockwise. Because there is mass in this
cloud, it begins to collapse under gravity. This spinning cloud has angular momentum (like and ice
skater), and so as it collapses, it must spin more rapidly.
During this time, the particles can slip past each other easily, since the cloud is not very dense.
The heavier particles, like iron and uranium, are more strongly attracted towards the center, and so the
fraction of heavy atoms becomes higher near the center of the cloud.
As the cloud collapses, it becomes denser. Eventually, it becomes dense enough for particles to
begin to collide, and sometimes stick together, forming larger particles. This is called condensation. These
larger particles are orbiting the center of the cloud counterclockwise, because the smaller particles were
traveling in that direction. As these larger particles begin to collide with other particles of the same size,
© 1999 University of Washington
Revised: 3 January, 2001
they 'regularize' the orbits. That is, the collisions with particles moving slightly outward in their orbits are
as common as collisions with particles moving slightly inward in their orbits, causing the orbit of the
growing body to become more and more circular, and less elliptical.
Similarly, particles which are traveling north are as common as those going south. As these
particles collide, their velocities average out, causing the cloud to flatten into a disk.
The cloud continues to collapse because of gravity, and to spin faster because of the conservation
of angular momentum. Larger and larger particles form, which are rotating and orbiting counterclockwise,
just like the original cloud. Eventually, most of the large particles have been gathered up into a few large
bodies, and continue adding mass by running into lots of smaller particles. This is called accretion.
We have now accounted for the shape of the orbits, the shape of the solar system, the rotations of
the planets, and the distribution of densities. All with a very simple model of a cloud which collapses
under gravity, and conserves angular momentum. As you will find out later in the term, however, this is
far from the whole story. It works pretty well for our solar system, but fails when applied to the dozens of
planets around other stars which have been discovered in the last decade.
1. What is the difference between condensation and accretion?
2. What would happen if the cloud were so thin that gravity didn’t affect it?
3. How does angular momentum help the solar nebula form into a solar system?
Extension Questions:
1. How might you explain Venus' counter-rotation? Feel free to engage in wild speculation. This is an
unanswered question in astronomy!
2. How might you explain the oddities of Pluto's orbit? Can you see why Pluto's status as a planet is
disputed by many astronomers?
© 1999 University of Washington
Revised: 3 January, 2001