Introduction to our Solar System

Table of Contents

• Introduction
  • Major Concepts
  • Forward
  • The Planets, Moons, Asteroids, and Comets
  • Composition Of The Solar System
  • Interplanetary Space
  • The Terrestrial Planets
  • The Jovian Planets
  • Sun and Planet Summary

Major Concepts

1. A planet is a body, not large enough to generate nuclear fusion reactions, that orbit a star. Planets are largely composed of Metals, Silicates, Ices, and Gases.
2. The solar system contains nine planets and 24 other planetary bodies (moons and asteriods) with diameters greater than 350 km. Some icy comets at the edge of solar system may also fall into this category.
3. The small inner planets (Mercury, Venus, Earth, & Mars) are composed mostly of silicate rocks and metals; the outer planets (Jupiter, Saturn, Uranus, & Neptune) are much larger and consist mainly gaseous Hydrogen, Helium and Ice. The outer planets have many moons; some icy, some dry & barren, & some are very irregular. Some of these moons may very well sustain some sort of life. Pluto, the smallest & outermost planet is very similar to some of these moons. It had been suggested that pluto used to be, several million years ago, a moon of Neptune that got pulled out of orbit by a passing comet.
4. The asteriods, which are fragments of once larger bodies, otbit the sun between the orbits of Mars & Jupiter.
5. Comets are small icy bodies, formed in the outermost parts of our solar system. Generly their orbits are very large, and it may take several million years for a comet to make a comlete orbit. However ther orbit can easly be changed by passing to close to a planet. This can shorten their orbit dramaticly, such is the case with Halley's Comet.
6. The surface features, compositions, and internal structures of planetary bodies are history records of the events that have shaped them over the years. Each world has a story to tell us about it's development.

Forward

Imagine that we are crusing far out in intersteller space, our ship flying amoung millions of stars strewn along one curving arm of a sprial galaxy. We are looking carefully at each star. We pick out an averige star, which just happens to be our sun. We discover that it not only has planets, but many other bodies circling around it. Each planet is a large nonluminous that shines by reflected sunlight. Countless smaller examples of such bodies (smaller than 1,000 km, or 620 miles, across) are traditionally divided into three categories that we will discuss later, namely: Comets, Asteroids, and tiny Meteoroids. We will set out and explore these bodies one by one. This is the goal of these web pages, to help individules or orginazations educate themselves on our solar system.

At first when approaching our sun, at say a distance of serveral light years, we see no planets at all, because the sun very bright and much more massive than any of them. The planets are lost in the lumonus glare of the sun. If we had a radio reciever its just possible that we might be able to pick up some sort of signal at radio, TV, and other wavelengths. These signals might not necessarily reveal intelligent life in the system: they might be network TV broadcasts, or just random snaps, crackles, and pops of electrical activity such as lightning in the atmospheresof Jupiter, Venus or Earth. But they at least suggest the presence of planets.

We can also search for planets at this distance by looking for their gravitational effects. As they orbit around the sun, the more massive planets actually tug at the sun, moving it slightly to and fro. Very sinsitive instruments can pick up this "wiggle" in the sun's position. The major wiggle is due to our largest planet Jupiter. Jupiter has more mass, hence gravity, than all the other planets put together. So at first we notice only Jupiters effect and conclude that th sun is essentially a double system. The largest object in our solar system - the sun - is said to have 1 solar mass. The second largest object - Jupiter - has about 0.001 solar mass. The rest of the planets put together have only 0.0004 solar mass. It's very easy to overlook our earth when it's put to scale.

Our solar system consistes of one star, a family of nine planets, at least 58 moons, thousands of asteroids, and billions of comets and meteoroids. In terms of mass, however, the solar system consists of very little else than the Sun itself. The ball of gas makes up 99.87% percent of the mass of the solar system. Most of the remaining 0.13% resides in Jupiter. Thus, most of the solar system is empty space. From the nearest star; using technology currently available on Earth, nothing could be seen of our solar system except the sun, and it would only appear as a small yellow star, which are very common in our galaxy.

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of more than 100 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.

The Planets, Moons, Asteroids, and Comets

- Planets

Planets are objects that orbit (or revolve) around a central luminous star. Our Sun is a medium-sized star, which like other stars generates energy by nuclear fusion at high temperatures. It is only one about 100 billion stars that form a slowly rotating sprial galaxy. Planets are commenly solid, but liquids and gases are also important. Planets are too small for nuclear fusion reactions to have initiated within their interiors

Composition Of The Solar System

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.

• Sun: 99.875%
• Planets: 0.135%
• Comets: 0.01% ?
• Satellites: 0.00005%
• Minor Planets: 0.0000002% ?
• Meteoroids: 0.0000001% ?
• Interplanetary Medium: 0.0000001% ?

Interplanetary Space

Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of "space" comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons -- plasma -- which stream from the Sun, called the solar wind.

The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 kilometers (250 miles) per second in the vicinity of Earth's orbit. The point at which the solar wind meets the interstellar medium, which is the "solar" wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence perhaps 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and solar system, is referred to as the heliosphere.

The solar magnetic field extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields.

The Terrestrial Planets

The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The following diagram shows the approximate distance of the terrestrial planets to the Sun.

The Jovian Planets

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. The following diagram shows the approximate distance of the Jovian planets to the Sun.

Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA's Cosmic Background Explorer's (COBE) Diffuse Infrared Background Experiment (DIRBE). This never-before-seen view shows the Milky Way from an edge-on perspective with the galactic north pole at the top, the south pole at the bottom and the galactic center at the center. The picture combines images obtained at several near-infrared wavelengths. Stars within our galaxy are the dominant source of light at these wavelengths. Even though our solar system is part of the Milky Way, the view looks distant because most of the light comes from the population of stars that are closer to the galactic center than our own Sun. (Courtesy NASA)

Andromeda Galaxy, M31
The Andromeda Galaxy, M31, is located 2.3 million light years away, making it the nearest major galaxy to our own Milky Way. M31 dominates the small group of galaxies (of which our own Milky Way is a member), and can be seen with the naked eye as a spindle-shaped "cloud" the width of the full Moon. Like the Milky Way, M31 is a giant spiral-shaped disk of stars, with a bulbous central hub of older stars. M31 has long been known to have a bright and extremely dense grouping of a few million stars clustered at the very center of its spherical hub.

Sun and Planets
This image shows the Sun and nine planets approximately to scale. The order of these bodies are: Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto

Terrestrial Planets
This image shows the terrestrial planets Mercury, Venus, Earth and Mars approximately to scale. The terrestrial planets are compact, rocky, Earth-like planets.

Jovian Planets
This image shows the Jovian planets Jupiter, Saturn, Uranus and Neptune approximately to scale. The Jovian planets are named because of their gigantic Jupiter-like appearance.

The following table lists statistical information for the Sun and planets:

* The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.

Return to Our Solar System

Robert R. Jones II
Copyright © 1996 by Horizon Energy Corporation. All rights reserved.
Revised: 03-15-97.