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Basics
The Solar System consists of the Sun; the nine planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto; sixty one satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. The orbits of the planets are ellipses with the Sun at one focus. The orbits of all the planets are more or less in the same plane, called the ecliptic plane.
There are numerous smaller bodies that inhabit the Solar System; the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also everywhere else, and the comets (small icy bodies) which come and go from the inner parts of the Solar System in highly elongated orbits at random orientations to the ecliptic plane.
Discovery
We have observed the cosmic ocean for thousands of years. Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers; they named them after Roman deities - Jupiter, King of the Gods; Mars, the god of war; Mercury, the messenger of the gods; Venus, the goddess of love and beauty and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky.
Science flourished during the European Renaissance. Fundamental physical laws governing planetary motion were observed, and the orbits of the planets around the sun were calculated. In the 17th Century, astronomers pointed a new device called the telescope at the heavens and made startling discoveries.
However, the years since 1959 have amounted to the golden age of Solar System exploration. Advances in rocketry after World War II enabled our machines to break the grip of Earth's gravity and travel to the Moon and to other planets. The United States has sent automated spacecraft, then human-crewed expeditions, to explore the Moon. Our automated machines, have orbited and landed on Venus and Mars; explored the Sun's environment; observed comets and asteroids and made close-range observations while flying past Mercury, Jupiter, Saturn, Uranus and Neptune.
These travellers brought a quantum leap in our knowledge and understanding of the Solar System. Through the electronic sight and other "senses" of our automated spacecraft, colour and complexion have been given to worlds that for centuries appeared to earth bound senses as fuzzy discs or indistinct points of light.
Classification
The classification of these planetary objects is a matter of minor controversy. Traditionally, the Solar System has been divided into planets (the big objects orbiting the Sun), their satellites (moons, variously sized objects orbiting the planets) asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the Solar System has been found to be more complicated than this would suggest:-
* There are several moons larger than Pluto and two larger than Mercury
* There are several moons that are probably captured asteroids
* Comets sometimes fizzle out and become indistinguishable from asteroids
* The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets"
The nine bodies conventionally referred to as planets are often classified in several ways:-
By composition:-
Terrestrial or rocky planets: Mercury, Venus, Earth and Mars.
The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.
Jovial or gas planets: Jupiter, Saturn, Uranus and Neptune.
The gas planets are composed primarily of Hydrogen and Helium and generally have low densities, rapid rotation, deep atmosphere, rings and lots of satellites.
Pluto:- a small planet of rock and ice. It is most like the moons of the gas planets.
By size:-
Small planets: Mercury, Venus, Earth, and Mars and Pluto.
The small planets have diameters less than 13000 km.
Giant planets: Jupiter, Saturn, Uranus and Neptune.
The giant planets have diameters greater than 48000 km
Mercury and Pluto are sometimes referred to as lesser planets. The giant planets are sometimes referred to as the gas planets.
Note: The asteroids are sometimes referred to as the minor planets.
