Solar System Review:

Our solar system consists of a star (average in size and luminosity) we call the Sun, the planets (in order of their distance from the Sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Kuiper Belt objects (e.g., dwarf planets like Pluto), the satellites or moons of the planets, numerous comets, asteroids, meteoroids and the interplanetary medium.

The planets, and most of the satellites of the planets, and the asteroids revolve around the Sun in the same direction (counterclockwise), in nearly circular orbits (ellipses, but close to circles). When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. A new class of dwarf planets was added in 2006, these objects are found mostly in the Asteroid and Kuiper Belts.

The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special dwarf planet in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets.

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined.

Terrestrial Worlds:

The four primary terrestrial worlds are the innermost planets in the solar system, Mercury, Venus, Earth and Mars. There are an additional 8 other terrestrial worlds; the Moon, Io, Europa, Ganymede, Callisto (the four Galilean moons), Titan (a moon of Saturn), Triton (a moon of Neptune) and Pluto.

They are called terrestrial because they have a compact, rocky surface like the Earth's and are spherical in shape. The other moons are not spherical and are more asteroid-like (i.e. irregular). Venus, Earth, Mars and Titan have significant atmospheres, the rest have little to zero.

Jovian Worlds:

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all similar in size and structure, i.e. gigantic compared with Earth (Earth is in the lower right corner above) and having a non-solid, gaseous nature.

The Jovian planets are also referred to as the gas giants, although all of them might have small semi-solid cores below their thick atmospheres.

Planet vs. Moon:

A planet revolves around the Sun. A moon or satellite revolves around a planet. The term planet or moon is not selected by mass or size of the body (for example, Titan, a moon of Saturn, is bigger than Mercury).

The period of revolution of a planet is determined by timing and astrometry (the science of measuring stellar and planetary positions). Periods of rotation are determined by either

  1. timing surface features
  2. timing clouds and atmospheric features
  3. reflected sunlight (light curves)
  4. Doppler radar measurements of planet limb
Note that timing atmospheric features reveals that Jovian planets have differential rotation (meaning their equators rotate faster than the pole regions, i.e. the planet is not solid).

Information about the planets is obtained by:

Solar System Formation:

Any model of Solar System formation must explain the following facts:

  1. All the orbits of the planets are prograde (i.e. if seen from above the North pole of the Sun they all revolve in a counter-clockwise direction).
  2. All the planets have orbital planes that are inclined by less than 6 degrees with respect to each other (i.e. all in the same plane).
  3. All the planets have low eccentricities.
  4. All planets have prograde rotation except Venus and Uranus.

Other details for any formation theory:

T=rocky and small <-> J=gaseous and large

Any theory of Solar System formation must explain the following properties and why there is a separation of the planets into two types

Common Elements/Compounds in the Solar System:

The most common elements on each planet tell us something about its formation process and later evolution. For example, small planets have gravitational fields that are too weak to maintain light elements such as H and He. Jovian worlds are cold and have many compounds as ice rather than gas or liquids.

Note that the Universe is mostly H and He (like the Sun), so the planets are very *unlike* the chemical composition of the Universe in general.