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Facts about how the solar system came into being from a disc of dust.

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an artists view of our solar system.
solar system.

The Accretion theory happens as a cloud of gaseous material and dust contracts under the extreme forces of gravity. Spinning mass forms a disc, probably with a bulge at the centre where a warm protostar undertakes a gestation period. And then eventually the central region of this locality collapses under the hostile force of gravity, and allows the centre to continuously heat, as the ambient gases continue to gather toward its core. From then on, the protostar dispenses and radiates much of its heat and ejects matter outward from its polar regions, where the disc itself offers little restriction to this process. And during this period, a lot of the protostar`s dust and debris is removed toward the newly forming solar system`s periphery.

From there, fusion commences at the star`s core, and the star begins its active nuclear life. But it should be remembered by the reader, only stars that are 6 percent or more than the mass of the Sun can attain temperature and pressure in the core which is required to initiate fusion. The disc at this stage either disappears entirely - of forms embryonic planets.

Planetoids develop: when matter swirling around an emerging star forms small pellets which collide and make larger bodies, we just called `Planetoids`. They coalesce at that point to form large Planets with tracks of mostly empty space between them. And in the inner system, light gases are blown away by the star`s radiation to leave large rocky planets, and moons behind.

No one knows: how many stars might actually have Planets orbiting them. First generation stars which form from Hydrogen and helium only, might have Planets around them, but these might only be Gas giants like Jupiter devoid of a rocky core. For Earth like Planets to build, the star must be a second or third generation with a Hydrogen and helium cloud laced atmosphere with heavier elements. And since our Sun if reasonably typical, it seems highly unlikely other stars will not have similar solar systems to our own. But even with today`s modern telescopes, we do not have enough observational power to see directly whether this is the case or not! However, our near neighbour, Barnard`s star wobbles as it moves across the sky. Calculations currently show that the wobble of Barnard`s star could be caused by the gravitational effects of....

two Jupiter sized planets.

And so it seems: a reasonable explanation, except for the fact, our own star does not produce enough gravitational influence to have created Uranus and Neptune on the peripheral wall of our own solar system. Therefore, we either need to expand the accretion theory and produce more gravitational forces, or look around for an entirely new model of how solar systems are created. I have done both. I have expanded gravitational fields at intermittent periods in the stars history, but simultaneously postulated new stella activity, to hone and adapt the evolutionary cycle of life.

The Solar System consists of the Sun; the nine planets, more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

The inner Solar System contains the Sun, Mercury,  Venus,  Earth and Mars:

The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The Planets of the outer Solar System are Jupiter,  Saturn,  Uranus. Uranus  Neptune and Pluto:

The first thing to notice is that the Solar System is mostly empty space. The Planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits.

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 (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. The above diagrams show the relative sizes of the orbits of the nine Planets from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.

(The above diagrams show correct positions for October 1996 as generated by the excellent planetarium program Starry Night; there are also many other similar programs available, some free.)


The above composite shows the nine planets with approximately correct relative sizes (see another similar composite and a comparison of the terrestrial planets or Appendix 2 for more).

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. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away.

Not shown in the above illustrations are the 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 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 (the big bodies orbiting the Sun), their satellites (a.k.a. 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 many small moons that are probably started out as asteroids and were only later captured by a planet;
  • 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".

Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; our present understanding is insufficient to establish clear categories. In the pages that follow, I will use the conventional categorizations.

The nine bodies officially categorized as Planets are often further 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.
    • jovian 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 atmospheres, rings and lots of satellites.
    • Pluto.
  • by size:
    • small planets: Mercury, Venus, Earth, 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 (not to be confused with minor planets which is the official term for asteroids).
    • The giant Planets are sometimes also referred to as gas giants.
  • by position relative to the Sun:
    • inner planets: Mercury, Venus, Earth and Mars.
    • outer planets: 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:
    • inferior planets: Mercury and Venus.
      • closer to the Sun than Earth.
      • The inferior Planets show phases like the Moon's when viewed from Earth.
    • Earth.
    • superior planets: Mars thru Pluto.
      • farther from the Sun than Earth.
      • The superior Planets always appear full or nearly so.
  • by history:
    • classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
      • known since prehistorical times
      • visible to the unaided eye
    • modern planets: Uranus, Neptune, Pluto.
      • discovered in modern times
      • visible only with optical aid
    • Earth.

More General Overview

The Big Questions

  • What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear.
  • How common are planetary systems around other stars? There is now good evidence of Jupiter-sized objects orbiting several nearby stars. What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other.
  • Is there life elsewhere in the solar system? If not, why is Earth special?
  • Is there life beyond the solar system? Intelligent life?
  • Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common?

Answers to these questions, even partial ones, would be of enormous value. Answers to the lesser questions on the pages that follow may help answer some of these big ones.

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