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6. Solar System Formation
Our Solar System contains
The Sun, Planets; two major types: Terrestrial Planets:(inner planets):
Mercury, Venus, Earth, Mars and Jovian Planets:(outer planets): Jupiter, Saturn, Uranus, Neptune.
Moons of the planets,Dwarf planets: Ceres, Pluto, Eris, Asteroids (minor planets). Kuiper Belt Objects
(dwarf planets in the Kuiper Belt are called plutoids). Comets (many in the Oort Cloud). Meteoroids.
Comparative Planetology
By studying the similarities and differences among the planets we can better
understand the Solar System, and gain clues about how the Solar System was formed.
Locations of the Terrestrial and Jovian Planets
Mercury, Venus, Earth, and Mars, the Terrestrial
Planets (Inner Planets), are located close to the Sun, and are closely spaced. Jupiter, Saturn, Uranus,
and Neptune, the Jovian Planets (Outer Planets), are located far from the Sun, and are spaced far
apart.
Size and Mass of the Terrestrial and Jovian Planets
Jovian Planets are much larger and more
massive than the Terrestrial Planets
Density of the Terrestrial and Jovian
High density of the Terrestrial Planets reveals a composition of
metals and rocks. Low density of the Jovian Planets reveals a composition of liquid hydrogen and helium
Surfaces of the Terrestrial and Jovian Planets
You could walk around on the hard surface of a
terrestrial planet. No landing is possible on the clouds and high pressure liquid of a Jovian planet
Rotation of the Terrestrial and Jovian Planets
The generally fast rotation of the Jovian planets
produces a stronger magnetic field than the Terrestrial Planets. The magnetic fields give information
about the interiors of the planets.
Rings and Moons of the Terrestrial and Jovian Planets
The Terrestrial Planets have few moons, and
no rings. The Jovian Planets all have rings, and many moons.
Terrestrial Planets (Inner Planets) are
Mercury, Venus, Earth, and Mars. Close to Sun. Closely
spaced. Small radius. Small mass. High density. Rock and metal. Solid surface. Slower rotation. Weak
magnetic field. Few moons. No rings. Jovian Planets (Outer Planets) are
Jupiter, Saturn, Uranus, and
Neptune. Far From Sun. Widely spaced. Large radius. Large mass. Low density. Liquid hydrogen and
helium (but called gaseous). No solid surface. Faster rotation. Strong magnetic field. Many moons. Many
rings.
It All Begins with an Interstellar Cloud
About 5 billion years ago, an interstellar cloud, made mostly of
hydrogen (71%) and helium (27%), plus small amounts of other elements, and perhaps 2 light years
across, was the start of our Solar System.
Gravitational attraction brings things together (scientific principle) because of this
The interstellar
cloud (nebula) collapses in on itself, becoming denser in the center. (The disturbance that starts the
cloud collapsing is still unclear—perhaps a nearby supernova, or the passage of another cloud.)
Energy is conserved in the universe (scientific principle) because of this
The kinetic energy of
collapsing material turns to heat in the center of the collapsing cloud, making the center hotter (where
the Sun will form).
Angular momentum is conserved (scientific principle) because of this
When rotating things get
smaller, they spin faster. So, if the collapsing cloud has any rotational motion at the start, it will spin
faster as it collapses. This produces a solar system with everything spinning, and all in the same
direction.
Force is needed to move toward the center of a rotating object because of this
It’s hard for material
to move toward the center from the side, while it’s easier to move toward the center along the axis. This
results in the solar system forming into a flattened shape.
Basic Steps of Planet Formation
(1) Material condenses on dust grains. (2) Tiny globs stick together
when they collide (accretion). (3) Big planetesimals draw in more mass by gravitational attraction. (4)
Self-gravity pulls the protoplanet into a spherical shape
Different materials condense at different temperatures because of this
Metals and rocks (with high
boiling points) condense everywhere in the solar system, but hydrogen and helium (with low boiling
points) condense only in the cooler, outer parts of the solar system.
(All the planets begin as rocky masses, but the cooler region toward the outer part of the Solar
System allows a huge amount of hydrogen and helium to condense around the outer planets.)
(Iron and nickel (which have high boiling points) can condense everywhere in the Solar System)
(Hydrogen and helium (which have very low boiling points) can condense only in the outer, cooler
parts of the Solar System)
Angular momentum is conserved because of this
The planets all orbit the Sun in the same direction,
and the planets (almost) all spin in the same direction, and their regular moons all revolve in the same
direction.
The Sun Emits a Flow of Energetic Particles because of this
Small particles are pushed away from
the Solar System by energetic, charged material emitted by the Sun. This leaves the Solar System
clean, with little debris that blocks the view.
Questions to be Answered:
•
What are the objects in our Solar System? How do they move?
•
What are the major differences between the Terrestrial Planets and the Jovian Planets?
•
What was the original composition of the material that formed into our Solar System?
•
How do the laws of physics explain the development of the solar system into a flat, spinning
disk?
•
Why did the early-stage Sun-material become so hot?
•
What are the beginning stages of planet formation?
– Why are the terrestrial planets “rocky,” and…
– … the Jovian planets “gaseous?”
•
How did the solar system become so clean?