Survey of the Solar System

The Solar System consists of our Sun, and all the objects that orbit it.  These include the planets, dwarf planets, moons, asteroids and comets.

For an overview of the Solar System, see the attached PowerPoint slideshow below.

The PlanetQuest activity uses this website.
You can also check out NASA's Solar System Simulator here.

If you were looking down on the solar system from above (in the direction of Earth's North Pole), the planets would move counterclockwise around the Sun, and all rotate in a counterclockwise motion, as well.  (The exceptions are Venus - which rotates backwards - and Uranus and Pluto - which are tipped over on their sides.)  The planets all orbit in about the same plane - except for Pluto, the orbit of which is tilted 17 degrees to the rest.

The planets are categorized into two groups:
1. Inner Planets (aka - Terrestrial Planets)
  - Mercury, Venus, Earth, Mars
  - small, rocky bodies with thin or no atmosphere
2. Outer Planets (aka - Jovian Planets)
  - Jupiter, Saturn, Urnaus, Neptune (Pluto)
  - gaseous, liquid or icy
  - much larger than the inner planets, and have thick, hydrogen-
     rich atmospheres

Asteroid - a rock floating in space
Meteor - a rock falling through an atmosphere
Meteorite - a meteor that reaches the planet surface
Comet - a body of ice and dust with a long, oval orbit around the Sun

The Kuiper Belt - a band of icy objects that reaches from just past Neptune's orbit (30AU from the Sun) to about 60AU from the Sun.

The Oort Cloud - a sphere of icy objects that surrounds the solar system, reaching from about 40,000AU to 100,000AU.

Composition of Solar System
The Sun
 - 71% hydrogen
 - 27% helium
 - small amounts of almost all other elements
The Outer (Jovian) Planets
 - composition almost identical to the Sun
 - indicates cores of rock and iron, about the size of the Earth.
The Inner (Terrestrial) Planets
 - composition similar to Sun, but without the hydrogen and helium
 - indicates that they could have had thick atmospheres, like the
   Jovian Planets, but that something caused this material to be

Age of the Solar System
 - thought to be about 4.6 billion years
 - based on radioactive dating of rocks from Earth, Moon, and
 - based on brightness and temperature readings of Sun, and the
    rate at which it's using up its fuel (hydrogen).

The Formation of the Solar System
There's a good animation of the formation of the solar system here.

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.



Extrasolar planets (aka - exoplanets) are planets that exist outside of our solar system - meaning that they orbit stars other than our Sun.

Because the distances involved are so huge, it is very difficult to see exoplanets directly.  Two ways that we can look for them are:

  1. By looking for a tiny "wobble" in the host star, caused by the planet's gravity as it orbits around the star.
  2. By looking for faint "dimming" effect of the host star, caused when the planet passes between its star and Earth.  (This can only happen if the planet's orbit happens to fall across our line of view to the star.)

Most of the exoplanets found so far are large gas giants that orbit very close to their planets.  This is probably because these types of planets are the easiest to detect.  To check out an interactive map of some exoplanets that have been found, click here

Exoplanets are important to astronomers, because they can give us clues about how our own solar system formed.



Homework from the Text:

  • Read Chapter 7 (pg. 215-236). 
  • Review Questions #1-3, 5-8, 11-12, 16. (pg. 236)