The rotating ball collapsed into a thin disk about 4.6 billion and years ago, with the Protosun (cloud of gas that will develop into the Sun) located at the center. In the disk, solid material accumulates into larger and larger particles, forming planetesimals, protoplanets, and  planets.and allowed it to differentiate, namely, to separate materials according to their density. Heavy metals, such as iron and nickel, settled in the core, while the lightest silicates "floated" to the surface.

An important clue to the origin and formation of the solar system is the clear division of the planets into two categories: Terrestrial, such as the Earth, and Jovian, such as Jupiter. The terrestrial planets are: Mercury, Venus, Earth, and Mars. They lie in the inner solar system and are small, dense, rocky worlds with less atmospheres than the Jovian planets: Jupiter, Saturn, Uranus, and Neptune. The Jovian planets lie in the outer solar system - beyond the asteroid belt - are large, gaseous, low-density worlds. Poor Pluto does not fit either category very well: it is small, like the terrestrial planets, but lies far away from the Sun and has a low density just like the Jovian planets. It is now not even considered to be a planet.

Planet	Distance to the Sun (AU)	Period of the Orbit (years)	Diameter (km)	Mass/Mass(Earth)	Density (g/cm3)
Mercury	0.3871	0.24084	4,878	0.055	5.4
Venus	0.7233	0.61515	12,104	0.82	5.3
Earth	1.0000	1.00000	12,756	1.00	5.5
Mars	1.5237	1.88080	6,794	0.107	3.9
Jupiter	5.2028	11.8670	142,800	317.8	1.3
Saturn	9.5388	29.4610	120,540	94.3	0.7
Uranus	19.180	84.0130	51,200	14.6	1.2
Neptune	30.061	164.793	49,500	17.2	1.6
Pluto	39.440	247.700	2,200	0.0025

In what is the outer solar system, four very large masses formed and became the four Jovian planets. All of these planets grew to have masses of at least 15 times the mass of the Earth. Once a planet has grown to such a large mass, it has a strong enough gravitational pull to begin capturing gas directly from the solar nebula, the material from which the Sun was formed: 74% Hydrogen, 24% Helium, and 2% of the heavier elements. Thus, the Jovian worlds are low-density worlds rich in hydrogen and helium. 

In the inner part of the solar system there were simply too many collisions for large planets to form; from the four terrestrial planets Earth is the most massive one - with only 1/320 times the mass of Jupiter - Venus (with 82% percent of the Earth's mass) is the next, and then Mars (10% of the Earth's mass) and Mercury (5% of the Earth's mass) are the lightest. The terrestrial planets are dense worlds - with low hydrogen and helium content - because they are composed mostly from the 2% of the heavier elements contained in the solar nebula.

The solar system is filled with smaller bodies, such as satellites (moons), asteroids, and comets. Our own moon was probably created in a catastrophic collision between the Earth and a Mars-size planet. Some of these smaller bodies, however,  have settled in the asteroid belt between Mars and Jupiter, while a few were captured as the moons of planets.. 

Rings around the giant planets, such as the ones in Saturn:

are probably the result of stray planetesimalshen they ventured too close to the planet.