Astronomy Assignment #6
Solutions
Read Chapter 10 section 8 Solar System Formation and answer the following Review Questions:
1. What observed facts does the Condensation Model of the solar system formation explain?
1. All the planets' orbits lie roughly in the same plane.
2. The Sun's rotational equator lies nearly in this plane.
3. Planetary orbits are slightly elliptical, very nearly circular.
4. The planets revolve in a west-to-east direction. The Sun rotates in the same west-to-east direction.
5. The planets differ in composition. Their composition varies roughly with distance from the Sun: dense, metal-rich planets are in the inner part and giant, hydrogen-rich planets are in the outer part. Meteorites differ in chemical and geologic properties from the planets and the Moon. Comets sampled by spacecraft are made of a mixture of rock/metals and hydrogen-rich compounds.
6. The Sun and most of the planets rotate in the same west-to-east direction. Their obliquity (the tilt of their rotation axes with respect to their orbits) are small. Uranus and Venus are exceptions.
7. The rotation rates of the planets and asteroids are similar---5 to 15 hours, unless tides slow them down.
8. The planet distances from the Sun obey Bode's law---a descriptive law that has no theoretical justification. However, Neptune is a significant exception to Bode's "law" .
9. Planet-satellite systems resemble the solar system.
10. The Oort Cloud and Kuiper Belt of comets.
11. The planets contain about 90% of the solar system's angular momentum but the Sun contains over 99% of the solar system's mass.
2. From what did the solar system form?
The solar system formed from a large gas nebula (H2, He and other trace gases) that had some dust grains (Rick and Ice) in it.
3. Why are the inner terrestrial planets small and rocky while the outer jovian planets are large and gaseous?
ALL planets begin with the accumulation of solid material. In the Solar Nebula, close to the Sun temperatures were too high to allow ices to condense and only rock was solid. Far from the Sun temperatures were low enough that ices could condense as well as rock. The boundary between these regions is the “ice line”. Terrestrial planets formed inside the “ice line” and had only rock as a solid material from which to begin growing. Since rock is the least abundant material in the solar nebula they never grew large enough to gravitationally capture the hot hydrogen and helium in the nebula around them. Thus they remained small and rocky. Jovian planets formed beyond the “ice line” and had ice in addition to rock as a solid material from which to begin growing. Since ice is 100 times more abundant than rock, the planetary cores beyond the ice line could grow to much larger sizes. These larger planetary cores had sufficient gravity to gravitationally capture the cold hydrogen and helium around them and grow to a much greater final size.
4. Why does a disk form in the collapsing cloud?
As the solar nebula collapsed, its slight rotation increased. This is because of the conservation of angular momentum.
Centrifugal effects caused the outer parts of the nebula to flatten into a disk, while the core of the solar nebula formed the Sun. The planets formed from material in the disk and the Sun was at the center of the disk
5. What role do dust particles play in planet formation?
Dust particles are the seed of planets. ALL planets begin with the accumulation of solid material. The solid dust particles in the Solar Nebula grow by sticking together when the gently collide with each other in much the way a dust bunny grows from colliding dust particles under your bed. These dust particles grow from microscopic diameters to the diameter of a pea, then a golf ball, then a car and so on. This phase of growth is slow and gentle. When these planetesimals grow to about 100 km in diameter they have sufficient gravity to reach out and gravitationally attract other planetesimals to it. This begins a rapid violent phase of growth.
6. If the disk was moving so quickly, how did it create big enough clumps to make planets?
In the Solar nebula, although orbital velocities are quite high (in the km/sec range), the particles are all in nearly circular orbits and neighboring particles are moving at nearly the same speeds. So collisions between particles tend to have low relative velocities. The low relative collision velocities allow these particles to stick together without busting apart with each collision. As the planetesimals ger large enough their own gravity holds them together against all but the largest energy collisions.
7. What drove out the rest of the nebula after the planets formed?
When nuclear fusion first started in the Sun a vigorous stellar wind called the t-Tauri wind blew out of the Sun and this very strong outflow of protons and electrons from the Sun swept away the unbound gas and dust in the solar nebula. T-Tauri winds are observed in other very young stars. The removal of loose material in the solar nebula starved the growth of planets and finished the growth of the planets by accretion of gas and dust. Planets did continue to grow slightly by the accretion of comets and asteroids (Period of Early Heavy Bombardment).
8. Why are the planet interiors made of layers of increasing density closer to their cores?
During the growth of planets, the energy deposited by in-falling material was large enough to melt the planets. When the planets became molten the denser materials could sink to the center and displace the less dense material upward towards the outer surface of the planet. This process of differentiation created the iron and nickel cores we know are in the centers of the terrestrial planets.
9. How do we test the Condensation Model of the solar system?
The best way to test the Condensation Model is to observe other planetary systems around other stars. If these exoplanet systems look like our own, then the Condensation Model would be verified.
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