In our solar system, there are two basic types of major planets:

	Terrestrial planets. These are composed of rock, with a metallic core.Volatile substances, if any, only exist in comparatively small amounts at the surface. Terrestrial planets of our sun are Mercury, Venus, Earth and Mars. Of the larger moons, the Earth's moon, Io and Europa (which has only a comparatively thin ice crust above an ocean heated by tidal friction) fall into this class.
	Jovian planets, a.k.a. gas giants. Besides having a small (perhaps Earth-sized) rock/metal core, these are composed mainly of hydrogen and other volatiles. A jovian planet has no solid surface, but rather a deep atmosphere which becomes denser and denser towards the planet's core until it turns into a supercritical, liquid-like state. Between this massive layer of liquid hydrogen and the rocky core lies a layer composed of water and other hydrogen compounds. The jovian planets of our solar system can be divided into two distinct types:
	Type I: Jupiter, Saturn. The pressure within these is so high that the inner part of their hydrogen bodies has turned into a metallic state.
	Type II: Uranus, Neptune. These are smaller and contain a much higher fraction of water and other hydrogen compounds. The hydrogen layer is much thinner; the pressure does not suffice to create metallic hydrogen.

Pluto falls in neither of these categories. Recent observations seem to mandate the conclusion that Pluto is just a very large comet rather than a planet. (The assumption that Pluto is a former satellite of Neptune has been dropped.)

It is clearly noticeable that, while terrestrial planets are confined to the inner part of the solar system, the jovian planets are to be found in the outer parts. But is this necessarily so, or is this merely coincidental? The truth probably lies somewhere in between -- it might be possible that jovian planets form in the inner area of a planetary system, but it is less likely. The `seed' of a jovian planet is always a large solid body, massive enough to grab large amounts of hydrogen and other gases from the solar nebula.

Outside the 170-kelvin (the evaporation point of water ice in vacuum) limit (in our solar system, this is at about 3 A.U. from the sun) there is just much more material available to build such an object (there is much more water vapour in the solar nebula than silicates or metals!), thus, out there it is more likely that a juvenile planet gains enough mass to grow into a gas giant.

In the inner solar system, the initial solid body must be entirely built from rocks and metals. Then it can start attracting water vapor, carbon dioxide, nitrogen and other heavier volatiles first, and then hydrogen and helium. Besides, higher temperatures and solar wind constitute further difficulties. The gases are just harder to hold. This makes the formation of a jovian planet on a close orbit less likely. But if there are such objects in other solar systems, they probably contain a higher proportion of metals, silicates and heavy volatiles (water, nitrogen, etc.) than Jupiter. They will probably differ enough from Jupiter and Saturn to justify being classified as a third type besides the Jupiter/Saturn and Uranus/Neptune types. (The reason why, in our solar system, the smaller type II giants occur farther outward than the big type I planets is probably because there was less material available out there -- the solar nebula doubtlessly was thinner at its edges. The temperature differences might also have played a role.)

The next question is: how big can a jovian planet be, before igniting itself and becoming a star? It is assumed that the smallest stars have about 8% of the Sun's mass. Smaller objects don't reach the core temperature necessary to ignite nuclear fusion. They might gain enough heat from contraction to emit a faint reddish glow, but such a brown dwarf is not really a star. On a related issue, there are probably no planets which exceed Jupiter's radius by very much. Huge gas giants are simply denser, but not that much thicker. Jupiter isn't that much bigger than Saturn (142000 km as compared to 120000 km), though it has more than three times its mass, but it is denser. A planet three times as massive as Jupiter might have a diameter of 150000 or perhaps 160000 km, but probably not more. The smallest red dwarf stars aren't much larger, either.

The asteroid belt is considered to consist of left-overs from the formation of the solar system; it is assumed that Jupiter precluded the formation of a planet here. (The old, popular theory which states that the asteroids are the remnants of a destroyed planet, is now dead meat.) An asteroid belt between the terrestrial planets in the inner and the giants in the outer part of the system might well be a typical case.

Thus we get the gross picture: terrestrial planets (and only occasionally a jovian with terrestrial moons) in the inner solar system, and jovian planets with icy satellites in the outer solar system, with an asteroid belt bounding the two realms from each other, and a huge cloud of comets around.

The aforementioned reasons why jovian planets are less likely to form inside the 170-kelvin limit might also be the reason why massive stars rotate faster. Being brighter and hotter, they push their 170-kelvin limit towards the fringes of their solar nebula (and heat up the inner areas so much that not even iron and silicates condense), such that only a few smaller planets form (if any). Without planets, the star retains its original rotation momentum, while smaller stars share most of it with their planets and therefore rotate slowly.