Survey of Solar System

The Solar System consists of a star (the Sun = 99.85% Mass of entire solar system) and planets, asteroids, and comets, which orbit it in a broad, flat disk. All the planets circle the Sun in the same direction and, with a few exceptions, spin in the same direction. Their moons also form flattened systems, generally orbiting in the same direction. The planets fall into two main categories:

Inner - Terrestrial

[Mercury, Venus, Earth, Mars] - rock and metal, 3.9 - 5.5 g/cm³

(Small, high-density bodies, rich in rock and iron)

Outer - Jovian

[Jupiter, Saturn, Uranus, Neptune] - liquids and gases, 0.7 - 2.2 g/cm³

( large, low-density bodies, rich in hydrogen and ice)

And then there is Pluto, a small, outer planet?

Satellites

Small bodies called Satellites orbit planets. Satellites come in a wide range of sizes from 10 km across to 5000 km (Titan, satellite of Saturn). Our Moon has a diameter of 3500 km. Many larger satellites are really small planets which means that gravity is responsible for giving them a spherical shape and that at one time they experienced internal geologic activity.

Asteroids

Relatively small rock bodies most of which revolve around the sun in a position between the orbits of Mars and Jupiter (Asteroid Belt). Asteroids are similar to planets in that they revolve around the sun but in shape and size they are more like satellites. The largest asteroids - Ceres 1020 km in diameter, Vesta (550 km) and Pallas 538 km are spherical and probably have undergone some geological activity. Many smaller lumps with no geological activity.

Comets

Comets are totally different class of objects moving on elliptical orbits. Ice objects with minor amounts of rock fragments, ranging in diameter from 1 to 10 km. Comets are thought to originate in the Oort Cloud or the Kuiper Belt.

Interplanetary medium

Essentially a vacuum.

Formation of the Solar System

These features of the Solar System can be explained by the solar nebula hypothesis. In this hypothesis, the Solar System was born from a cloud of interstellar gas that collapsed to a disk called the solar nebula. The center of the nebula became the Sun, and the disk became the planets. This explains the compositional similarities and the common age of the bodies in the system.

Time of formation of solar system is approximately 4.6 GA (4.6 billion yrs. ago).

The flat shape of the system and the common direction of motion around the Sun arose because the planets condensed within the nebula's rotating disk.

Gravitational collapse > Disk contracts and spins faster near centre.

Conservation of angular momentum – depends on 3 factors:

Mass – more mass – more momentum (mass of solar system constant)

Size – larger – more momentum | Rotational speed – faster – more momentum

Reduce Size > Increased Speed

Planet growth occurred in two stages: dust condensed and clumped (very slowly 0.3km/hr) to form planetesimals; and then later the planetesimals aggregated to form planets and satellites.

Inner part of disk heats up (conservation of energy – gravitational/kinetic to thermal)

Heat is important because ultimately decides the composition of the planets.

Two kinds of planets formed because lighter gases and ice could condense easily in the cold outer parts of the nebula, but only rocky and metallic material could condense in the hot inner parts (temp in this inner part may have been as high as 2000 deg. K). Planets seem to have formed very fast, in approximately 50 million years. Early sun blows all remnant gases out of inner parts of solar system.

Cosmic Abundance curve shows a large concentration of Fe.

This explains why terrestrial planets have Fe-rich cores.

Impacts of surviving planetesimals late in the formation stages cratered the surfaces and may have tilted the rotation axes of some planets. Density of cratering is the principal method by which we can date the surfaces of most planets/satellites.

Some planetesimals survive to this day as the asteroids and comets.