The Solar System The Solar System consists of the Sun, the nine planets and their satellites; the comets, asteroids, meteoroids, and interplanetary dust and gas. It is composed of two systems, the inner solar system and the outer solar system. The inner solar system contains the Sun, Mercury, Venus, Earth and Mars. The outer solar system contains Jupiter, Saturn, Uranus, Neptune and Pluto. The inner planets are relatively small and made primarily of rock and iron.
The asteroids orbit the sun in a belt beyond the orbit of Mars, tumbling and sometimes colliding with one another. Made mostly of rock and iron, the asteroids may be the remnants of a planet that never formed. The outer planets, with the exception of Pluto, are much larger and made mainly of hydrogen, helium, and ice. Many astronomers believe that Pluto was and interstellar wanderer that was captured by the Suns gravity and was not an original part of the solar system. The orbits of the planets are ellipses with the sun at one focus, though all except mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane that is called the ecliptic. The ecliptic is inclined only seven degrees from the plane of the ecliptic with and inclination of seventeen degrees.
Again with the exception of Pluto, the planets all orbit the sun in almost the same plane. The average distance of the earth to the sun is used as a standard for measuring distances in the solar system and is called an astronomical unit (AU). One AU is about 93 million miles or 150 million kilometers. Mercury the planet closest to the sun is at about 0.387 AU. Pluto is the outermost planet, and it is 39.44 AU from the sun. The heilopause is the boundary between the solar system and interstellar space, and it is about 100 AU from the sun.
The comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more. The Sun is a regular star of intermediate size and luminosity. It is one of more than 100 billion stars in our galaxy. The Sun is by far the largest object in our solar system. The Sun is personified in many mythologies, the Greeks called it Helios and the Romans called it Sol. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Suns hot, dense interior.
The Sun is, at present, about 75% hydrogen and 25% helium by mass; everything else amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The Suns outer layers exhibit different rotation, at the equator the surface rotates once every 25.4 days: near the poles its as much as 36 days. This weird behavior is caused by the fact that the Sun is not a solid body like the Earth. The different rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.
The Suns core conditions are extreme. The pressure is 250 billion and the temperature is 15.6 million Kelvin. At the center of the core the Suns density is more than 150 times that of water. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. For the Suns entire steadiness, it is an extremely active star.
On its surface dark sunspots bounded by intense magnetic fields come and go in 11-year cycles. Sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth; and a continuous stream of protons, electrons and ions leave the Sun and move out through the solar system, spiraling with the Suns rotation. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil. The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core.
It will continue to radiate peacefully for another 5 billion years or so. But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards will result in the total destruction of the Earth and probably the creation of a nebula. Today there are nine major planets in the solar system. They are currently known as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
The planet that is closest to the Sun is Mercury. It is about 36 million-miles from the Sun and its period of revolution is 88 days. Mercury is surprisingly dense, apparently because it has an unusually iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Mercury passes through phases similar to those of the moon as it completes each revolution around the Sun.
It has such a thin atmosphere that in a single day it reaches temperatures of up to 750*F. At night, it gets as cold as -300*F. This planet can only be seen for a short time before or after sunset. Mercury is the second smallest planet in the solar system, having a diameter of about 3,000 miles. Its mean density can compare to the earth. Its small mass and proximity to the Sun prevent it from having an appreciable atmosphere. The surface of Mercury is lots like that of the moon.
Venus is the second planet from the Sun. It is often called the Evening Star or Morning Star, and it is brighter than any object in the sky except the Sun and the moon. Venus can really never be seen much longer than 3 hrs. before or after sunrise. Venus revolves around the Sun at a distance of about 67 million miles.
Venus is often referred to as the sister planet of the Earth because it is only slightly smaller in size and mass. Venus is covered with a thick blanket of clouds that hides its surface from view. The thick atmosphere is composed mainly of carbon dioxide, with a slight amount of water vapor and some nitrogen and their elements. The high surface temperature is assumed to result partly from the greenhouse effect because it is blocked out by the top layer. Venus rotates on its axis in a retrograde direction with a period of about 243 days.
As a result of the Greenhouse effect Venus is the hottest of any planet about 477*C. Venus lies between the orbit of the Sun and Earth, so Venus passes through phases like the moon, varying from a large bright crescent (when it is close) to a silvery disk (when it is far away). Venus comes closer to the Earth than any other planet. The surface of Venus is thought to be erratic and stormy, but radio waves indicate the possibilities of two long mountain ranges. Scientists have estimated that the surface of Venus is only about 800 million years old.
Earth is the fifth largest planet and the only planet definitely known to support life. Due to gravitational forces the earth is molded into a sphe …