THE SOLAR SYSTEM 

The nine planets of the Solar System in an artistic composition.   (Bill Arnett)

The Solar System is a set of celestial bodies that rotate around the Sun. The Sun itself, 9 planets, 61 satellites, some thousand asteroids, and an indefinite number of comets are part of it. Moving away from the Sun, we would first encounter the inner planets, Mercury and Venus, then the Earth and finally the outer planets: Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Between the orbits of Mars and Jupiter there is the band of the  asteroids.

From the dynamic point of view, the Solar System is a very complex and peculiar set. All the planets rotate in the same direction, that is anticlockwise with respect to a hypothetical observer placed on the North Pole of the Sun. 

The orbits of the inner planets  (Calvin J. Hamilton)

Their orbits lie almost on the same plane, in other words their inclination is 2.5 degrees at most with respect to the plane of the ecliptic, with the exception of those of Mercury (7 degrees) and Pluto (17 degrees).   The orbits are almost circular, with the exception of that of Pluto (which has an eccentricity of 0.25) and Mercury (0.20).

The total extension of the Solar System is approximately 6 billion Km, which equals 39.3 A.U.. The bodies of the Solar System actually occupy a very small volume with respect to the overall size. The Solar System is therefore practically "empty": if the Sun were a sphere with a diameter measuring 1 metre, the Earth would have the size of a pea and its distance from the star would be 108 metres, Jupiter would have the size of an orange at 550 metres, and finally Pluto would be at a distance of 4 Km and its diameter would measure less than one millimetre.

CLASSIFICATION OF THE PLANETS OF THE SOLAR SYSTEM

The Sun and the planets, approximately in the scale of real dimensions.  (Calvin J. Hamilton)

The characteristics that distinguish the various planets are the physical ones and those related to their revolution motion around the Sun and to their rotation around their axis.  The physical characteristics are: size, mass, average density; intensity of a possible magnetic field; chemical composition; presence and composition of the atmosphere.   The characteristics relative to the motion are:  the size of the orbit, the eccentricity, the inclination of the orbital plane and of the rotation axis with respect to the plane of the ecliptic, the orbital period and the rotation period.  Eventually, the various planets are distinguishable by the presence and the number of the satellites.

From both aspects, the planets of our Sun seem to form two distinct systems: a sort of internal solar system, composed of TELLURIC or ROCKY PLANETS (Mercury, Venus, the Earth and Mars) and an external system, which includes the GIANT PLANETS (Jupiter, Saturn, Uranus and Neptune). Pluto seems from many aspects an anomalous planet, which is not part of any of the two subsets.
 

Rocky of telluric planets

The rocky planets have a relatively small size (the diameter is less than 15,000 Km) and a quite high density (from 3 to 5, 1 being the density of water).
They are composed of an iron nucleus surrounded by a basaltic mantle.
With respect to the giant planets, their revolution motion is faster and their rotation is slower.
The rocky planets are rather different from each other as regards the atmosphere (when present), the surface of the soil, the magnetic field and the orbital parameters, in contrast with the relative uniformity of the giant planets. 
 

Giant planets

The giant planets owe their name to the large size (their major diameters measure 50,000 Km). Their densities are approximately 1 and they are divided in gaseous planets (Jupiter and Saturn) and ice planets (Uranus and Neptune).
The gaseous planets are composed by a rocky nucleus surrounded by a liquid mantle, which is in turn covered with a thick layer of gas.
The ice planets on the other hand are composed by a rocky nucleus, covered with a layer of ice, and the whole is surrounded by an atmosphere. The revolution periods of the giant planets are much longer compared to those of the telluric planets, and range between 12 years (Jupiter) and almost 165 (Neptune).  
Vice versa, they rotate faster than the rocky planets: hence a remarkable centrifugal force at the equator, and therefore a flat shape.

Besides, Jupiter, Saturn and Uranus have a set of rings composed of dust and fragments of rocks and ice of various sizes. Finally, all giant planets have a large number of satellites, while the rocky planets have two at most.
 

Jupiter and Saturn have the peculiarity that they emits 2 and half times more energy than they receive from the Sun. Such energy derives from a slow gravitational contraction, which heats up their nucleus.
Besides, in their atmosphere the Hydrogen-Helium ratio is very similar to that of the Sun; this suggests that the two planets be two "missed stars": if they were more massive, the pressure and the temperature of the gas within them would be sufficient to trigger the thermonuclear reactions and they would become stars. 
 

Pluto

Pluto and its satellite Caron (NASA/HST)

Pluto is the least known of all planets. It looks like a case apart respect to the others, both for its anomalous orbit, and for its type and size. In fact, although it is situated in the region of the giant planets, it is very small and it belongs to the rocky type.  Scientists think that Pluto could be a former satellite of Neptune, that escaped its gravitational attraction to place itself on an independent orbit around the Sun.

Distances of the planets

The distances of the planets from the Sun follow the Titius-Bode law, which was empirically determined in 1772. According to such law, the distances of the various planets from the Sun can be obtained by multiplying the distance of the previous planets by a coefficient equal to 1.75.
If we take as unit of measurement the distance between the Earth and the Sun, the distances of the other planets will be, respectively: 

planet         theory          real distance 
---------       ------          -------------
Mercury         0.4               0.387  
Venus           0.7               0.723
Mars            1.6               1.524
Asteroids       2.8               2.767        
Jupiter         5.2               5.203
Saturn         10.0               9.539 
Uranus         19.6              19.189
Neptune        38.8              30.060
Pluto          77.2              39.439

MINOR BODIES

Besides the planets there are a multitude of minor bodies in the Solar System, divided in three classes. 
 

The asteroids are small rocky objects with a size between a few cm and 1,000 Km. Thousands of them orbit in a zone between the orbits of Mars and Jupiter.

The Gaspra asteroid photographed by the Galileo spacecraft. '(NASA/JPL)

The comets are celestial bodies that rotate at a long distance from the Sun, on very eccentric orbits. They are basically made of an aggregate of rock and ice, with the size of less than 10 Km. Their typical aspect is due to the fact that, when they travel near the Sun, the surface of their frozen nucleus vaporizes due to the high temperature. The produced gas forms a diffused halo, almost spherical, called coma.
The radiation of the Sun and the solar wind exert a pressure on such gas. It deforms the coma pushing the gas in the opposite direction as the Sun, thus originating the typical tail, a stripe of gas with the length of dozens or even hundreds of millions of kilometres. 

The West comet during its passage in the vicinity of the Sun, in 1976.  (John Labord)

The meteorites are the remains of solid bodies, metallic or rocky, which penetrated in the terrestrial atmosphere at high speed. The friction with the atmosphere causes their heating and disintegration: the smaller are reduced to dust, while the larger are not completely destroyed and can reach the ground.

A chondritic meteorite found in the Antarctic. (NASA/JPL)