The characteristics of a particular planetary system depend upon the interaction of a vast array of physical and chemical processes, including ambipolar diffusion of magnetic fields, turbulence and viscosity in disks composed of gas and dust, sticking and growth of small grains, torques between growing planets and their surrounding disk, etc. Theory cannot definitively predict the frequency of planet formation nor the distribution of planetary sizes and orbits. One theory predicts that Jovian mass planets are formed from dynamic instabilities in accretion disks. A more popular theory envisions the formation of terrestrial planets that grow large enough to attract a massive gaseous envelope. Large cores should be seen in the second case, but not the first.
Theory is most useful for extrapolating from known systems.
Models based on the single example of our Solar System cannot say whether our system is typical or anomalous. However, such surveys will not be able to detect terrestrial planets. The Kepler Mission uses spacebased photometry to detect planetary transits. It offers far greater sensitivity for finding terrestrial and larger planets than ground-based techniques. By providing a statistically robust census of the sizes and orbital periods of terrestrial and larger planets orbiting a wide variety of stellar types, results from this mission will allow us to place our Solar System within the continuum of planetary systems in the Galaxy and develop theories based on empirical data .