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Chapter 28: Our Solar System

Ch. 28.1 Formation of the Solar System

Formation Theory

Theories of the origin of the solar system rely on direct observations and data from probes.

A Collapsing Interstellar Cloud

Stars and planets form from interstellar clouds, which exist in space between the stars. They consist mostly of hydrogen and helium gas with small amounts of other elements and dust.
Interstellar clouds look dark when blocking light from stars, or illuminated when reflecting light.

Gravity slowly draws matter together until it is concentrated enough to form a star and possibly planets. Astronomers think that the solar system began this way.

Collapse Accelerates

Collapse of a interstellar cloud gradually accelerates making the cloud become denser at the center.

If rotating, the cloud spins faster as it contracts by centripetal force. As the collapsing cloud spins, the rotation slows the collapse in the equatorial plane, and the cloud flattens; eventually becoming a rotating disk with a dense concentration of matter at the center.

Matter Condenses

The Sun formed when the dense concentration of gas and dust at the center reached a temperature and pressure high enough to fuse hydrogen into helium. The rotating disk surrounding the Sun became our solar system.
Depending on the distance from the Sun, a temperature gradient formed, causing a difference in elements and compounds condensing.
Inner planets are richer in the higher melting point elements and the outer planets are composed mostly of the more volatile elements; mostly gases and ices.


Eventually, smaller colliding particles merged to form planetesimals. Planetesimals- objects ranging from one km to hundred of kms in diameter.
Collisions continued building/destroying planetesimals eventually forming larger bodies- the planets.

Gas Giants Form

Jupiter formed first. Increasing in size through the merging of icy planetesimals that contained mostly lighter elements. It grew as it's gravity attracted more objects.
Saturn and the other gas giants formed similarly. As each gas giant attracted material from its surrounding, a disk formed in its equatorial plane to for rings and satellites.

Terrestrial Planets Form

Inner planets formed by the merging of planetesimals and are composed primarily of elements that resist vaporization. As a result, they are rocky and dense.


Material that remained after the formation of the planets and satellites is called debris.

  • Comets- icy debris
  • Asteroids- rocky debris

Most asteroids are found in the area between Jupiter and Mars; asteroid belt.

Modeling the Solar System

Early astronomers assumed that the Sun, planets, and stars orbited a stationary Earth. This geocentric model, or Earth-centered model, could not explain some other aspects of planetary motion.
Retrograde motion- the apparent backward movement of a planet. Every two years, Mars appears to move from east to west instead of the normal set to east movement.

We now know that Mars retrograde motion is due to Earth "catching up" to Mars and overtaking it as both planets orbit the Sun. Earth orbits the Sun faster than Mars so for every one orbit of Mars, Earth completes two.

Heliocentric Model

In 1543 Nicolaus Copernicus suggested that the Sun was the center of the solar system. In a heliocentric model, the increased gravity of proximity to the Sun causes the inner planets to move faster in their orbits than do the outer planets.
The model also explained retrograde motion.

Kepler's First Law

Copernicus's ideas were confirmed by other astronomers who found evidence supporting the heliocentric model.
Tyco Brahe designed and built very accurate equipment for observing the stars and used them to make very accurate observations of the planet's positions.

Johannes Kepler used Brahe's data to demonstrate that each planet orbits the Sun in a shape called an ellipse. Ellipse- an oval shape that is centered on two points instead of a single point, as in a circle.

  • The two points are called foci and the major axis is the line that runs through both foci at the maximum diameter of the ellipse.
  • The Sun is always at one focus for each planet.
  • Average distance between the Sun and the planet is the semimajor axis, which equals half the length of the major axis. Earth's average distance from the Sun referred to as 1 astronomical unit (AU). Distance in space is often measured in AU.


    A planet in an elliptical orbit does not orbit at a constant distance from the Sun. The shape of a planet's elliptical orbit is defined by eccentricity. Eccentricity- the ratio of the distance between the foci to the length of the major axis.

    The orbit of most planets are not very eccentric; some are almost perfect circles. Eccentricity of a planet can change.

    Kepler's Second and Third Laws

    Kepler's second law states that planets move faster when they are closer to the Sun.
    Orbital period: the time it takes for a planet or other body to travel a complete orbit around the Sun.
    Kepler's third law determined the mathematical relationship between the size of a planet's ellipse and its orbital period.


    Galileo Galilei was first person to use telescope to observe the sky. He made many discoveries supporting Copernicus's ideas. Galileo discovered four moons orbiting Jupiter, proving not all celestial bodies orbit Earth, and that Earth was not necessarily the center of the solar system.


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Ch. 28.2 The Inner Planets


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Ch. 28.3 The Outer Planets


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Ch. 28.4 Other Solar System Objects


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Page last updated January 2, 2017.