Planets of the Solar System

Chapter 27, Section 1 - Formation of the Solar System                                                                                                                            

Solar System - The Sun and all of the planets and other bodies that orbit it.
Planet - A celestial body that orbits the Sun, has enough mass for its gravity to pull it into a round shape, and has cleared the neighborhood around its orbit.

The Formation of the Solar System
  • According to the solar nebula hypothesis, the solar system started off as an interstellar cloud.  This means that it was a huge cloud of gas and dust - probably a couple of light-years across and made of mainly hydrogen (71%) and helium (27%) and small amounts of other elements (sound familiar?).  Such clouds also contain interstellar grains, which can range in size from big molecules to particles a few micrometers across.  These grains are thought to be made up of silicates, iron and carbon compounds, and water ice.

  • Gravity caused the densest parts of the cloud to collapse inward.  As it collapsed, it rotated around its center - this caused it to flatten out.  This formed the solar nebula - a rotating disk with a large bugle in the middle.  The bulge condensed into the Sun, while material in the disks formed the planets.  It's worth pointing out that the nebula would have been hot near the center (where the Sun formed), but very cold towards the edges.

  • Condensation is when a gas cools and changes into a liquid or solid.  As the solar nebula cooled, particles started to condense out of it.  At a temperature of about 1300K, iron flakes started to form.  Rocky silicate particles condensed out at about 1200K.  Water doesn't condense until about 373K.  Since the inner part of the solar system stayed hotter, ice particles did not condense inside of Jupiter's orbit.  This caused the early solar system to be separated into two zones: the inner zone contained iron and silicate dust particles, while the outer zone was rich in ice particles.

  • Accretion is when the particles clump together.  In this case, the particles eventually form into planetesimals (think "mini-planets").  The planetesimals gradually clump together to form planets.  As they collide, the force of their collisions generate heat, which melts the planets and allows them to differentiate into layers (heavier materials, such as iron, sink to form the core).  Moons form in similar ways from planetesimals that are pulled into orbits around the planets.

  • Atmospheres form in different ways depending on the size of the planet.  The larger Outer Planets had enough gravity to collect gas from the solar nebula, thus forming thick atmospheres.  The Inner Planets did not have enough gravity to do this, so their thin atmospheres come from volcanic activity and comets.

  • Finally, the remaining gas and dust were pushed out to the edges of the solar system by the heat of the Sun.  The Sun was more energetic in its youth, radiating more heat and stronger solar wind.

  • Oceans formed from water brought to Earth by comets and water condensing out of volcanic steam.  The ocean may have started out as freshwater, then became salty as minerals were dissolved.
See Astronomy: Chapter 7 for more information about the formation of the Solar System.

Chapter 27, Section 2 - Models of the Solar System                                                                                                                                

Geocentric Model - A model of the Solar System in which the Earth is the center and other bodies revolve around it

Heliocentric ModelA model of the Solar System in which the Sun is the center and other bodies revolve around it. 

Kepler's Laws of Planetary Motion
  1. Law of Ellipses
    • Planets move in elliptical orbits with the Sun at one focus of the ellipse.
  2. Law of Equal Areas
    • The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep over equal areas in equal time intervals.
  3. Law of Periods
    • The amount of time a planet takes to orbit the Sun is related to the orbit’s size, such that the Period (P) squared is proportional to the semimajor axis (a) cubed.  The formula for this is:  P2 = a3
Newton's Laws of Motion
  1. Newton's 1st Law - Inertia
    • An object in motion/at rest will stay in motion/at rest until acted on by an outside force.
    • inertia: a resistance to change in motion.
  2. Newton's 2nd Law
    • The change in motion of an object depends on the amount and the direction of the force put on that object.
    • Force = Mass x Acceleration
    • acceleration: change in velocity (speed and/or direction).
  3. Newton's 3rd Law
    • For every action, there is an equal and opposite reaction.

See Astronomy: Chapter 1 for more information about the history of Astronomy.
See Astronomy: Chapter 2 for more information about forces and motion.

Chapter 27, Section 3 - The Inner Planets                                                                                                                                                

Chapter 27, Section 4 - The Outer Planets                                                                                                                                               


Chapter Assignment:
  • Read Chapter 27.
  • Pg. 778-779 #9-22, 24,26.