Layout of the Solar System

Distances within the Solar System are measured most often in AU, or astronomical units. 1 AU is the distance between the Earth and the Sun, or roughly 150 million kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumfrences of planets, the terameter (Tm, one billion kilometers) is sometimes used.

Despite the fact that many diagrams, for practicality's sake, represent the solar system as having each planet the same distance apart, in actuality the planets are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus's distance from the Sun is roughly double that of Mercury, Earth's distance is roughly double that of Venus's, Mars's double that of Earth's and so on. This is expressed in the Titius-Bode rule, (or Bode's Law}, which predicts the semi-major axes of the planets in AU in a mathematical formula:

where k=0,1,2,4,8,16,32,64,128

By this formula, we would expect Mercury to be 0.4 AU from the Sun, and Uranus to be 19.6 AU from the Sun. Their orbits are, respectively, 0.387 AU and 19.19 AU.

Famously, a "gap" in the Titius Bode rule for the region between Mars and Jupiter led many in the 19th century to suspect that a small planet must reside within it. Investigation of this gap led to the discovery of the asteroid belt.

The only exception to this rule is Neptune, which is substantially closer to the Sun than predicted, though Pluto lies at the point where Neptune would be if it followed the law.

The objects within the Solar System are collected in several discrete regions. The four terrestrial planets, characterised by their dense, rocky makeup, (Mercury, Venus, Earth, and Mars), along with the rocky asteroid belt, form the inner solar system. Beyond this lie the four gas giant planets (Jupiter, Saturn, Uranus and Neptune). The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored. The known objects in this region are composed mainly of ice. This area contains Pluto and the Kuiper Belt, Sedna, and possibly several other formations. Very far out, at around 50,000 AU to 100,000 AU the Oort Cloud is believed to lie.

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Origin and evolution of the solar system
The solar system is believed to have formed from the Solar nebula, the collapsing cloud of gas and dust which gave birth to the Sun. As it underwent gravitational collapse, the Solar Nebula would have collapsed into a disk, with the protosun accreting at the centre. As the protosun heated up, volatile substances were driven away from the central regions of the nebula—hence the formation of rocky planets closer to the sun and gas giants further out.

For many years, our own system was the only planetary system known, and so theories only had to explain one system to be plausible. The discovery in recent years of many extrasolar planets has uncovered systems very different to our own, and theories of planetary system formation have had to be revised accordingly. In particular, many external systems contain a hot Jupiter—a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in our system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system.

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Galactic orbit of the solar system
The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is rather large and bright. (The vast majority of stars are red dwarfs; our Sun is placed near the middle of the Hertzsprung-Russell diagram, but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common, although we can observe only those few other red dwarfs that are very near our Sun in space.)

Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s.

The solar system appears to have a very unusual orbit. It is both extremely close to being 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 supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

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Edge of the solar system
The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity.

The charted regions of our solar system exist within the heliosphere; a highly tenuous "atmosphere" of solar wind (charged particles eminating from the Sun) that expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a cosmic windsock; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.

But even at this point, we could not be said to have left the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, the great mass of comets which surrounds our solar system like a shell and extends from 50,000 to 100,000 AU (nearly a light year) beyond the Sun.

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