The basic premise in the understanding of our origins, and the properties of all the planets we have studied this term, is that natural forces created and shaped the Solar System. And that there is a continuity to that process, i.e. it is not a sequence of random events.
Any model or theory for the formation of the Solar System must have a set of explanations for large-scale and small-scale properties.
The above is not to ignore the fact that a great deal of evolution occurred in the Solar System after it formed (see below). For example, the origin secondary atmospheres of the terrestrial worlds underwent a large amount of chemical processing (Venus was baked, Mars was frozen, Earth developed life). There was also orbital evolution as well, rings were formed, moons captured, tidal locking between worlds (e.g. Pluto and Charon). So the Solar System is not a static system, it is dynamic.
How does one test a hypothesis?
To answer scientific questions requires the formulation of a hypothesis. The hypothesis is tested against the facts to look for contradictions that rule out or require modification to the hypothesis. Note that the process of hypothesis formulation and then theory building is a lengthy, career dependent operation. So the sociology of science requires that a hypothesis be tested and confirmed by many scientists since the creator of the hypothesis has a strong psychological attachment to his work.
One of the earliest theories for the formation of the planets was called the encounter hypothesis. In this scenario, a rogue star passes close to the Sun about 5 billion years ago. Material, in the form of hot gas, is tidally stripped from the Sun and the rogue star. This material fragments into smaller lumps which form the planets. This hypothesis has the advantage of explaining why the planets all revolve in the same direction (from the encounter geometry) and also provides an explanation for why the inner worlds are denser than the outer worlds.
A second theory is called the nebular hypothesis. In this theory, the whole Solar System starts as a large cloud of gas that contracts under self-gravity. Conservation of angular momentum requires that a rotating disk form with a large concentration at the center (the proto-Sun). Within the disk, planets form.
The current working model for the formation of the Solar System is called the protoplanet hypothesis. It incorporates many of the components of the nebular hypothesis, but adds some new aspects from modern knowledge of fluids and states of matter.
Notice also that the lighter compounds are vaporized in the inner Solar System. So where did all the outgassing material come from? The answer is comets that fall from the outer Solar System after the planets form.
Meanwhile in the outer Solar System:
Any leftover large bodies were captured as moons or ejected by gravity assist into the Oort cloud. The start of thermonuclear fusion in the Sun's core created enough luminosity so that the remaining hydrogen and helium gas in the solar disk was removed by radiation pressure.
The only remaining problem is the distribution of angular momentum. The current explanation for the fact that most of the angular momentum is in the outer planets is that, by some mechanism, the Sun has lost angular momentum. The mechanism of choice is magnetic braking.
The protoplanet hypothesis explains most of the features of the Solar System; however, the outer solar system is still strange, especially the properties of Pluto/Charon. One explanation is that the Solar System was not born in the configuration that we see today. That the planets in the outer Solar System migrated to their present positions.
Migration requires some interaction between the planet and a fairly large body or the gravitational forces are too weak. Early in the formation of the Solar System, there were lots of Moon-sized to Mars-sized bodies, especially in the outer SS. A large planetesimal that crosses near Neptune will lose some energy, fall down near Jupiter, gain energy to be ejected into the Oort Cloud.
This will have the effect of decreasing the size of Jupiter's orbit, and expanding the size of Saturn, Uranus and Neptunes' orbits. As Neptune moves outward, it will beginning to perturb the orbits of the trans-Neptunian objects (large ice covered astroids of which Pluto/Charon are a member). This pushes Pluto/Charon into a highly eccentric, inclined 3:2 resonant orbit that it occupies today.
All the leftover planetesimals near Neptunes orbit are pushed into a torus shaped region called the Kuiper belt. Smaller planetesimals are thrown farther out into the Oort cloud.
Support for the protoplanet hypothesis has been found by the detection of disk material around of stars, such as Beta Pictoris and by Hubble images of the Orion Nebula.
There are now numerous verified solar type stars that have Jupiter size planets in orbit around them.
All these object have been detected using Doppler measurement of the velocity of the host star to look for periodically variations that are the signature of orbital motion. Note that there doesn't appear to be a correlation between the distance from the host star of the planet and its mass.
A more direct measure occurs is a planet transits across the front of a star, as happened with HD 209458, a distant planet passing in front of its star, providing direct and independent confirmation of the existence of extrasolar planets.
Astronomers predicted the planet would cross the face of the star if the planet's orbital plane were lucky enough to carry it between Earth and the star. Until now, none of the 18 other extrasolar planets discovered has had its orbital plane oriented edge-on to Earth so that the planet could be seen to transit the star, nor have any of the other planets discovered by other researchers. However, on Nov. 7, 1999 an automatic telescope observed a 1.7 percent dip in the star's brightness.
With the orbital plane of the planet known, the astronomers for the first time could determine precisely the mass of the planet and, from the size of the planet measured during transit, its density. Interestingly, while the planet's mass is only 63 percent of Jupiter's mass, its radius is 60 percent bigger than that of Jupiter. This fits with theories that predict a bloated planet when, as here, the planet is very close to the star.