Composition and Structure of the Solar System
The principal component of the solar system is the Sun, a comparatively small star which nevertheless contains 99.86 percent of the mass of the solar system and dominates it gravitationally. In addition, because of its large mass, the interior of the Sun achieves the density necessary to sustain nuclear fusion, releasing enormous amounts of energy, some of which is radiated into space in the form of electromagnetic radiation. Most radiation is visible light. The sun also emits a flow of charged particles called the solar wind. The solar wind interacts strongly with planets that have magnetospheres, and pushes dust and gas out of the solar system. The remaining small amount of mass outside the Sun includes the nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), and their moons and planetary ring systems. In addition, there are asteroids, comets, trans-Neptunian objects and Kuiper Belt objects, a theorized Oort cloud, and interplanetary dust? and gas. The total solid surface area of bodies in the solar system is 1700 million km2 [1].
Origin and Evolution of the Solar System
The solar system as a whole is currently believed to have formed through the normal process of star formation which produced the Sun itself, rather than through some special process (such as a stellar near-collision) as was once thought. The process is believed to have begun with the gravitational collapse of a cloud of interstellar gas and dust to produce what is called the solar nebula.
Orbit of the solar system in the galaxy
The solar system is part of the Milky Way galaxy, a spiral galaxy of approximately 200 billion stars, of which our sun is fairly typical. Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center.
The solar system appears to have a very unusual orbit. It is both extremely circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovas in spiral arms would sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be uniquely free to form large animal life on its surface.
Discovery and Exploration of the Solar System
Because of the geocentric perspective from which humans viewed the solar system, its nature and structure were long misperceived. The apparent motions of solar system objects as viewed from a moving Earth were believed to be their actual motions about a stationary Earth. In addition, many solar system objects and phenomena are not directly sensible by humans without technical aids. Thus both conceptual and technological advances were required in order for the solar system to be correctly understood.
The first and most fundamental of these advances was the Copernican Revolution, which adopted a heliocentric model for the motions of the planets. Indeed, the term "solar system" itself derives from this perspective. But the most important consequences of this new perception came not from the central position of the Sun, but from the orbital position of the Earth, which suggested that the Earth was itself a planet, and the planets other Earths. This was the first indication of the true nature of the planets. Also, the lack of perceptible stellar parallax despite the Earth's orbital motion indicated the extreme remoteness of the fixed stars, which prompted the speculation that they could be objects similar to the Sun, perhaps with planets of their own.
The Solar System and Other Planetary Systems
Until recently, the solar system was the only known example of a planetary system, although it was widely believed that other comparable systems did exist. A number of such systems have now been detected, although the information available about them is very limited. The technique employed involves the detection through the Doppler effect of periodic variations in the motion of parent stars which is attributed to the presence of planets. This allows the mass and orbital characteristics of the unseen planets to be determined. Unfortunately the sensitivity of these techniques currently does not permit the detection of planets of mass and orbit comparable to the Earth.
Major Planets of the Solar System
All characterstics measured relative to Earth
| Planet | Equatorial diameter |
Mass | Orbital Radius |
|---|---|---|---|
| Mercury | 0.382 | 0.06 | 0.38 |
| Venus | 0.949 | 0.82 | 0.72 |
| Earth | 1.00 | 1.00 | 1.00 |
| Mars | 0.53 | 0.11 | 1.52 |
| Jupiter | 11.2 | 318 | 5.20 |
| Saturn | 9.41 | 95 | 9.54 |
| Uranus | 3.98 | 14.6 | 19.22 |
| Neptune | 3.81 | 17.2 | 30.06 |
| Pluto* | 0.24 | 0.0017 | 39.5 |
*Pluto traditionally has been considered a planet. However now that we have learned more about the variety of bodies in the Solar System we can see that its composition and orbit indicate that it has much more in common with Kuiper Belt objects than it has with the rest of the planets. Therefore some astronomers no longer class Pluto as a planet.
Solar System Objects
Sun
It was suggested that the Sun may be part of a [double star system]?, with a distant companion named Nemesis, a death star. Nemesis was proposed to explain some regularities of the great extinctions of life on Earth. The theory says that Nemesis creates periodical pertubations in the asteroids and comets of the solar system causing a shower of large bodies and some of them hit Earth causing destruction of life. After this theory was stated, a search was undertaken for such a dim, distant companion, and found no visible star within one light-year of the Sun.
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