An Overview of The Solar System
From the dawn of history until the origin of the 17th century the known universe consisted of only 8 planets:
1 Earth, 2. Sun, 3. Moon, 4. Mercury, 5. Venus, 6. Mars, 7. Jupiter, 8. Saturn and the “Fixed” stars. These are the ones that can be seen easily without any optical instruments. In Europe, the prevailing view was the Ptolemaic system with the Earth at the center and the other bodies revolving around it.
In 1610, Galileo first turned a telescope on the heavens and the universe was explored. By the end of the 17th century, 9 new planets had been discovered and the Copernicus’s heliocentric theory was widely accepted. The total number of known planets had more than doubled to 17. In 18th century, only 5 new planets (not counting comets) were discovered, all by William Herschel, bringing the total to 22. The number of planets in the solar system increased dramatically in the 19th century with the discovery of the asteroids (464 of which were known at by 1899) but only 9 more “major” planets were discovered. The number of major planets rose to 31 (almost double the 17th century). In the first three quarters of the 20th century, 13 more major planets (and thousands of comets and asteroids) were discovered bringing the total upto 43. In the space age 27 more small moons were discovered by the two voyager spacecraft. In CCD Age dozens more small moons have been discovered in recent years with have ground based telescope and CCD cameras. There are well over 100 now known.
Overview: The solar system consists of the sun; the nine planets, more than 130 satellites of the planets, a large number of small bodies like the comets and asteroids, and the interplanetary satellites that have not yet been discovered.
The inner solar system contains the Sun, Mercury, Venus, Earth and Mars. The main asteroid belt lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and Pluto. The first thing to notice is that the solar system is mostly empty space. The planets are very small as compared to the space between them. The orbits of the planets are eclipses with the Sun at one focus, though all except Mercury and Pluto are very close to circular. The orbits of the planets are all more or less in the same plane, called the ecliptic and defined by the plane of the Earth’s orbit. The ecliptic is inclined only 7 degrees from the plane of the Sun’s equator. Pluto’s orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. They all orbit in the same direction, counter-clockwise looking down from above the Sun’s north pole, including Venus, Uranus and Pluto also rotate in that same sense.
One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. The model Earth would be about 1.3 cm in diameter. The Moon could be about 30 cm from Earth. The Sun would be 1.5 meters in diameter and 150 meters from the Earth. Jupiter would be 15 cm in diameter and 5 blocks away from the Sun. Saturn would be 10 blocks away; Uranus and Neptune 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be ever 40000 km away.
There are numerous smaller bodies that also inhabit the solar system, the satellites of the planets, the large number of asteroids orbiting the Sun, mostly between; Mars and Jupiter but also elsewhere, the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.
Traditionally, the solar system has been divided into planets, their satellites, asteroids and comets. Unfortunately, the solar system has been found to be more complicated than this would look. There are several moons larger than Pluto and too larger than Mercury. There are many small moons that are probably started out as asteroids and Were only later captured by a planets, comets sometimes fizzle out and become indistinguishable from asteroids, the Kuiper Belt objects and others like Chiron don’t fit this scheme well and some even want to consider Pluto as part of this class, The Earth/Moon and Pluto/ Charon systems are sometimes considered “double planets.”
The planets are divided into the inner planets and the outer planets. The inner planets are Mercury, Venus, Earth and Mars. The Earth is the largest of the inner plants and densest of all plants. All the inner planets are dense rocky bodies and are collectively called terrestrial plants (Earth-like). They appear to chiefly consist of iron and rock.
The nine bodies officially categorized as planets are often further classified in several ways : By composition : as planets are (further in) composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites, By size : Small Planets : Mercury, Venus, Earth, Mars and Plutos. 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.
The outer planets, Jupiter, Saturn, Uranus and Neptune are very big (sometimes called Giant Planets), with large satellite families. They are composed mostly of hydrogen, helium, ammonia and methane. These planets are called Jovian, after love, the Greek name for Jupiter, because they resemble Jupiter in many aspects. The outermost planet Pluto is in a class by itself. It is supposed to be dense planet like the inner planets, although it is the farthest of the outer planets.
Mercury and Pluto are sometimes referred to as lesser planets. The giant planet are sometimes also referred to as gas giant planet. By position relative to the Sun : inner planets : Mercury, Venus, Earth and Mars; outer plants : Jupiter, Saturn, Uranus, Neptune and Pluto. The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system, by position relative to Earth : interior planets : Mercury and Venus, closer to the sun than Earth.
