Reading - remainder of Chapter 5.
How can we use the 2 facts - (i) this cosmic abundance table and (ii) the temperature gradient in the cooling and condensing solar nebula - to explain "Why do we have just 2 types of planets (small, rocky terrestrial planets and giant, gas jovian planets) rather than a spectrum of different sizes and densities?"
The answer coming from 2 main pieces of information - the cosmic abundance table and the decreasing temperature with distance from the Sun. As in this Astro 101 diagram of the situation - you can see that the refractory materials condense close in but there are very low abundance of these rocks and metals (which comprise low abundance elements at the bottom of the table). But when you combine the (non-inert ) more abundant oxygen, nitrogen and carbon with the most abundant hydrogen you get water, ammonia and methane - the "ices". Beyond the frost-line you get snow/ice/frost which accretes (snow-balls) to make huge icy planetesimals - the cores of the giant planets - which have sufficient gravity to pull in hydrogen, the most abundant element - and become bloated, fat giant planets. Inside the snow line there is only the less-abundance rock and metal - making small terrestrial planets.
This simple story can be elaborated into a more sophisticated story using proper chemistry. See pages 107-9 of the text.
BUT - these chemical equilibrium models have problems - they assume equilibrium, they do not allow for the hydrodynamics and radiative processes that are probably happening.
Ignoring such problems for now, we let the gas condense and then accrete. But there are also problems with the accretional models since we really do not know how to make the particles stick together.
Finally, assuming that we "fudge" the issues of micro-physics (how the particles glomp together) and just model that they do so somehow, we can make planets from planetesimals - different solar systems for different initial conditions put into the dynamical models - see Figure 5-10 on page 116.
How many of the original list of features of the solar system have been explained by solar system formation models? (e.g. see Table on page 102).
Realistically, in modelling the solar system we are far from understanding how things work. It is a large, complex problem and people work on little pieces of it. There is much work to be done! Here is a list of the types of models that are being worked on - there successes and failures.
Gravity of star + planetesimals -> DYNAMICAL MODELS - these solve "many body problems" using major number crunching to follow the equations of motions of multiple objects and their gravitational interactions with each other. At best allows simple accretion (using a "sticking probability" to approximate the physics of collisions).
Collisions -> MICRO-PHYSICS MODELS - studies of what happens when objects of different scales collide - molecular forces, surface tension, cohesion, micro-gravity, electrostatic forces, etc.
Cloud/gas (+dust) -> HYDRODYNAMIC MODELS - these use fluid equations to model the motions in the disk - but ignore accretion to solid and enlarging objects - as well as chemistry and radiation (usually)
+Magnetic fields -> MAGNETOHYDRODYNAMIC MODELS
Radiation - scattering, absorption, excitation, ionization, cooling -> RADIATIVE TRANSER MODELS - combined with hydrodynamic models these are useful for sorting out the cloud dynamics and energy.
Chemical reactions in gas-solid-liquid(?) phases, equilibrium? -> PHYSICO-CHEMICAL MODELS - but how well mixed was the cloud? Can you do chemistry without radiative transfer and hydrodynamics and microphysics?
Combinations of these models are beginning to be made - but limited by (a) human brain power to formulate the problems, (b) computational power, (c) human brain power to digest the output of models (interested in grad school anyone?)
Moreover, we only have detailed information about ONE solar system - ours. The ~100 solar systems we have found to date around other stars are very different to ours. Figure 5-10 shows early models of different solar systems - produced by varying the about of dust in the solar nebula, for example.
So, we have gone through solar system formation theories from collapse of the molecular cloud to formation of planets - and while such theories are moderately successful in explaining our solar system (and will, no doubt, be expanded to the new solar systems being discovered) - there is much work to be done in explaining the various components which are barely beyond the "a miracle happens here" stage of understanding.
