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Chapter 6 Our Solar System and Its Origin Comparative Planetology By studying the differences and similarities between the planets, moons, asteroids and comets, we can gain a fuller understanding of the solar system as a whole. Side View of Our Solar System

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chapter 6

Chapter 6

Our Solar System and Its Origin


Comparative Planetology

  • By studying the differences and similarities between the planets, moons, asteroids and comets, we can gain a fuller understanding of the solar system as a whole.
the origins of the solar system four challenges
The Origins of the Solar System: Four Challenges
  • Patterns of Motion
  • Categorizing Planets
  • Asteroids and Comets
  • Exceptions to the Rules
challenge 1 patterns of motion
Challenge 1: Patterns of Motion
  • All planets orbit the Sun in the same direction --- counterclockwise when seen from high above the Earth’s North Pole.
  • All planetary orbits lie nearly in the same plane.
  • Almost all planets travel on nearly circular orbits, and the spacing between planetary orbits increases with distance from the Sun according to a fairly regular trend.
challenge 1 patterns of motion continued
Challenge 1: Patterns of Motion - Continued
  • Most planets rotate in the same direction in which they orbit- counterclockwise when seen from above the Earth’s North Pole – with fairly small axial tilts (i.e. < 25o)
  • Almost all moons orbit their planet in the same direction as the planet’s rotation and near the planet’s equatorial plane.
  • The Sun rotates in the same direction in which the planets orbit.

Terrestrial Planets: Earth-like planets.

  • These include Mercury, Venus, Earth and Mars.
  • Jovian Planets: Jupiter-like planets.
  • These include Jupiter, Saturn, Uranus, and Neptune.
challenge 2
Challenge 2
  • The second challenge for any theory of solar system formation is to explain why the inner and outer solar system planets divide so neatly into two classes.
  • Terrestrial Planets
  • Jovian Planets
challenge 3
Challenge 3
  • In order for a theory to be complete, it must also address the issue of Asteroids and Comets.
  • Asteroids are small, rocky bodies that orbit the Sun primarily in the asteroid belt.
  • Comets are small, icy bodies that spend most of their lives well beyond the orbit of Pluto. (Oort Cloud)
  • We generally recognize them only on the rare occasions when one visits the inner solar system.

We know today that comets are orbiting the Sun primarily in two broad regions.

    • The Kuiper (Koy-per) belt: (Begins in the vicinity of the orbit of Neptune ~ 30AU)
    • The Oort Cloud: Huge spherical region centered on the Sun and extending perhaps half way to nearest stars.
The Third Challenge for any theory of solar system formation is to explain the existence and general properties of the large numbers of asteroids and comets.
    • Why are there so many?
    • How did their existence come about?
challenge 4 exceptions to the rule
Challenge 4: Exceptionsto the Rule

Some objects don’t fit into the general pattern:

  • Mercury and Pluto have larger eccentricities.
  • Pluto and Uranus are substantially tilted.
  • Venus rotates backwards.
four major characteristics of the solar system
Large bodies in the solar system have orderly motions.

Planets fall into two main categories:

Small, rocky terrestrial planets.

Large, hydrogen rich gas giants (Jovian planets).

Several notable exceptions to these general trends stand out:

Planets with unusual axial tilts or surrounding large moons.

Moons with unusual orbits.

Four Major Characteristics of the Solar System
the nebular theory
The Nebular Theory
  • In the past few decades, a tremendous amount of evidence has accumulated in support of one model.
  • This model is called the Nebular Theory.
  • This Theory holds that our solar system formed from a giant, swirling interstellar cloud of gas and dust.

Nebular Model

This results in a



disk, with mass


near the center

The cloud heats

up and spins


and faster as

it contracts

Original Cloud

is large and

diffuse with




Beta Pictoris (Hubble)

Twin Dust disks around a binary star system., taken by the VLA at radio wavelengths



disks around


in the



building the planets
Building The Planets
  • Condensation: Sowing the Seeds of Planets
    • Condensation is the formation of solid or liquid particles from a cloud of gas.
    • We refer to such solid particles as condensates.
The Ingredients of the Solar Nebula Fell Into Four Categories Based on Their Condensation Temperatures:
  • Metals
  • Rocks
  • Hydrogen compounds
  • Light gases



Temperature Differences in the Solar Nebula


Metals: Include mostly iron, nickel, aluminum

  • Rocks: materials common on the surface of the Earth. Primarily silicon based minerals.
  • Hydrogen Compounds: Molecules such as methane (CH4), ammonia (NH3), and water (H2O) that solidify into ices below 150K.
  • Light gases: (hydrogen and helium) never condense under solar nebula conditions.
accretion assembling the planetesimals
Accretion: Assembling the Planetesimals
  • The process of growing by colliding and sticking is called accretion.
  • The growing objects that formed by accretion are called planetesimals, which means “pieces of planets”.

Early in the accretion process, there are many Moon sized planetesimals on crisscrossing orbits


Only the largest planetesimals avoid being destroyed. These bodies will become the planets of this newly formed solar system

nebular capture making the jovian planets
Nebular Capture: Making the Jovian Planets
  • Large icy planetesimals of the outer solar system act as seeds for capturing large amounts of hydrogen and helium gas. This is called nebular capture.
  • This explains the large sizes and low densities of the Jovian planets.
  • Nebular capture also explains the formation of the diverse satellite systems of the jovian planets.
the solar wind clearing away the nebula
The Solar Wind: Clearing Away the Nebula
  • The remaining gas of the solar nebula was blown into interstellar space by the solar wind (a flow of charged particles ejected by the Sun in all directions.)
  • The Solar wind is believed to have been much stronger in the past than it is today.
leftover planetesimals
Leftover Planetesimals
  • Origin of Asteroids and Comets:
  • The strong wind from the young Sun cleared excess gas from the solar nebula, but many planetesimals remained scattered between the newly formed planets.
  • These leftovers became the comets and asteroids.
the early bombardment a rain of rock and ice
The Early Bombardment: A Rain of Rock and Ice
  • The collision of a leftover planetesimal with a planet is called an impact, and the responsible planetesimal is called the impactor.
  • On planets with solid surfaces, impacts leave the scars we call impact craters.
  • Impacts were extremely common in the young solar system.
captured moons
Captured Moons
  • Some moons have unusual orbits – orbits in the “wrong” direction or with large inclinations to the planet’s equator.
  • These unusual moons are probably leftover planetesimals that were captured into orbit around a planet.
giant impacts and the formation of our moon
Giant Impacts and the Formation of Our Moon.
  • The largest planetesimals remaining as the planets formed may have been as large as Mars.
  • It is believed that a glancing collision with a Mars sized planetesimal was reponsible for forming our Moon.
earth impact with mars sized planetesimal
Earth Impact with Mars sized Planetesimal
  • Artist’s Conception of the impact of a Mars sized object with Earth which caused the formation of the Moon.
summary meeting the challenges
Summary: Meeting the Challenges
  • The nebular theory explains the great majority of important facts contained in our four challenges.
  • However, planetary scientists are still struggling with more quantitative aspects, seeking reasons for the exact sizes, locations, and compositions of the planets.
other planetary systems
Other Planetary Systems
  • How common are planetary systems?
  • Observations have confirmed that protoplanetary disks are common.
  • Rapid advances in observational astronomy have allowed us to search for actual planets around other star systems.
  • In the beginning of 1990, there was no conclusive proof that planets existed around any star but the Sun.
  • By 2001, dozens of planet like objects have been detected around other stars.

Doppler shifts allow for the detection of slight motions of a star due to perturbations caused by the orbiting planet.


First 55 Extra-Solar Planets Discovered

Approximate masses in terms of the mass of Jupiter

Closer than the Earth-Sun distance