Our solar system consists of a number of different types of objects:

One star - the Sun

Four terrestrial planets - Mercury, Venus, Earth, Mars

Four jovian planets - Jupiter, Saturn, Uranus, Neptune

Moons, asteroids, comets, Kuiper Belt objects, and Pluto

The arrangement is far from random:

Planets fall in 2 main classes:

The solar system originally began as a large cloud of gas and dust that collapsed due to the mutual gravitational pull of each piece of matter on every other piece. Unless the cloud was absolutely stationary at the start, as it collapsed it would naturally flatten out into a disk-like structure. [Figure 11.2]

The majority of this material formed a protostar which eventually settled down to become a normal main sequence star - our Sun. In the rest of the disk the temperature was cooler, so that solid grains (silicates and some organics) could survive intact, and coagulate into larger bodies - planetesimals. Some of these planetesimals had enough mass that they could stick together gravitationally, and in a few cases terrestrial planets were formed. Far enough out in the disk it was cool enough for ice to be stable, and the bodies formed there could grow to even greater sizes, such as Uranus and Neptune. In a couple cases, the masses were so large that gases such as H and He were drawn in and held, allowing very massive planets such as Jupiter and Saturn to form.

The dividing line where ice could form marks the boundary between where terrestrial and jovian planets could form - about 5 AU - the "snow line". Inside this limit it was too hot for ice to form, and only the "refractory" materials could be used to make planets - terrestrial planets. Outside this limit, jovian planets can form. And since ice is potentially the most abundant solid in the universe, it can make a big difference to the mass of the protoplanet trying to form. If Jupiter and Saturn grew so large, what happened to Uranus and Neptune? They probably suffered from not having enough stuff nearby. The total density of material was lower where they formed.

Comets are mixtures of the same solid materials that went into forming the jovian planets: ice, rock, organics. They are simply the material that was left over from the disk that did not get incorporated into planets. Most of them were probably initially near and beyond the orbit of Saturn. However, those between the orbits of Saturn (maybe even Jupiter) and Neptune were tossed about by the gravity of these planets. Many fell inward toward the Sun, while others were tossed outward.

Face-on view of the young outer solar system. The cometary objects near Jupiter were scattered out of the solar system, or inward toward the Sun. Those between Saturn and Uranus are believed to have been tossed outward to populate the Oort Cloud. Those beyond Neptune were not significantly affected, and remain today as Kuiper Belt Objects

It is thought that these now occupy a large, diffuse spherical region called the Oort Cloud. Jan Oort was suggested the existence of this cloud back in the early 1950's, based on the orbits of known comets. Those just beyond the orbit of Neptune were not affected, and remain close to their original orbits. This second region, the Kuiper Belt, is now known to contain many dozens of objects. Pluto is in fact a Kuiper Belt Object (KBO) - the first and (so far) largest one known. Its mass and size is smaller than many of the moons in the solar system, and it is only slightly larger than Ceres, the largest asteroid.

The orientation of the Oort Cloud, Kuiper Belt, and the plane of planetary orbits in the solar system today

There has been some discussion lately about whether Pluto should be called a planet or not. My own opinion is that calling Pluto a planet does not make it a planet any more than calling a duck a goose makes it a goose. Some prefer to call it a planet for historical reasons. If it had been discovered today, it would not have been called a planet.

Even to this day, some comets are gravitationally perturbed into elliptical orbits that bring them into the inner solar system, where their ices sublime (go directly from the solid to gaseous state), carrying the dust particles away as well. The gas and dust flow out from the solid nucleus into a large fuzzy ball, the coma, and then are drawn back away from the Sun by the solar wind (for the gas) and the pressure of sunlight (for the dust) into one or more tails.

We have sampled some of the cometary material directly. In 1986, Soviet and European spacecraft flew through the coma of Halley's Comet. Camera's on the spacecraft shoed a surface that was very dark, like fresh asphalt. This is probably due to organic materials mixed in with the ice. Special instruments (mass spectrometers) sampled the dust in the coma, and indicated that it was composed mostly of silicates and CHON. Large macromolecules containing C, possibly polymers, were detected. Spectra of comets also showed the presence of things like H, OH, O, CN, CH, and NH₂, C₂, and C₃.

The more recent arrivals of the bright comets Hyakutake and Hale-Bopp came at a time when major advances in instrumentation capable of millimeter-wavelength spectroscopy had been made. Ground-based telescopes detected CO, CH₃OH (methanol), HCN (hydrogen cyanide), H₂O, H₂S, CS, H₂CO (formaldehyde), CH₃CN, HNC, etc.

Comets are chock-full of organic molecules!

When Jupiter grew to its enormous size, its gravity made forming another terrestrial planet just inside the snow line impossible. The debris kept getting all jumbled up. The result is that between the orbits of Mars and Jupiter, there is no planet, just a bunch of junk we call the asteroids. Reflectance spectra of most of the asteroids are similar to rock/organic mixes.

Impacts and collisions can fragment the asteroids, and the fragments reach the Earth as meteorites. Meteorites come in a variety of types: iron, stony, and stony-iron (a mix). A small fraction of the stony meteorites, the carbonaceous chondrites, are very interesting in that they are rich in carbon including organic materials, and water.

These are little bits (micron-sized) of comets and asteroids that hit the Earth's atmosphere. When collected and analyzed in the laboratory, they are found to consist of silicates, metals, organic and inorganic carbon compounds.

We detect molecules in the giant molecular clouds. Some are long chains, others small rings. Glycine, an amino acid, may have been detected.

Polycyclic Aromatic Hydrocarbons (PAHs) are found all over the galaxy: interstellar space, star-forming regions, etc. In these distant regions, they are detected through characteristic emission lines at 3.3 (the C-H bond stretching vibration), 7.7, 8.6, and 11.3 microns. CH₃, when attached to a PAH-like C-ring, produces a spectral feature at 3.4 microns that is seen in interstellar space.

Amino Acids - 74 detected so far - non-terrestial in origin:

Because this meteoritic material is constantly falling on the Earth, these may provide an alternate source of hydrocarbons for the origin of life. Comets may also be responsible for delivering much of the water in the oceans. If so, the organics must have come along for the ride, and may be responsible for most of the carbon on the Earth!

Interestingly, some of the organic materials from carbonaceous chondrites, when added to water, naturally form membrane-enclosed vesicles similar to the membranes of living cells. It is quite possible that many (maybe all?) of the raw materials needed for life came from space, and were the result of naturally-occurring chemical reactions that are commonplace in the universe. This has profound implications for the possibility of life elsewhere in the universe.