Since I found nothing during February relevant to my theory of planetary formation, I thought I should outline why I think we need an alternative. The following is a very condensed look at the giant planets, and my ebook (Planetary Formation and Biogenesis) has much more detail.
 
The standard theory for a giant is that solids come together by some unknown mechanism and form planetesimals, and these, through gravity, form larger bodies, and finally planets. It is usually assumed for giants that this takes less than a million years (My), then over a period of time that depends on the assumptions, these collide to form larger planets, until they reach about 10 times the mass of the Earth, then they start accreting gas as well. (Actually, they will accrete gas by the time they get to the size of Mars, but such early atmospheres contribute little to the mass. They then take about 10 – 15 My to get to a size where runaway gas accretion starts. So, what are the problems. I consider some of these to be:
(1) After 60 years, there is still no firm idea how the planetesimals form, therefore the distribution of them is simply an unverified assumption,
(2) Simulations agree that planetesimal collisions to reach Earth take about 30 My, so how does Neptune get there so fast, when matter density is much lower and velocities are much slower? Collision probability depends on the square of particle density, and initial particle density is proportional to r^-q, where q is usually taken as 1.5, although that too is an assumption, and it could be 2. If the average body contains n initial particles, particle density is now 1/n initial particle density.
(3) If material comes together by collision, to get things to go fast enough, relative velocities have to increase as particle size increases, so why do the bodies simply not smash to pieces, assuming they do form?
(4) The star LkCa 15 is approximately 2 My old, it is slightly smaller than the sun, and it apparently has a planet of nearly 6 times Jupiter's mass at about 16 A.U, or about three times as distant from the star as Jupiter.
 
In my opinion, (4) is critical. Accretion disks last between 1 – 10 My after primary accretion, so the LkCa 15 system is a very young one, so how did its gas giant get so big? Obviously, everything has to happen a lot faster than under standard theory. What are the possibilities? To start with, standard theory ignores chemistry, so what happens if we include it?
 
My concept is that the initial cores grow like snowballs. In the outer disk water and silicates condense to form amorphous particles that adsorb other gases (Icarus 63: 317-332) and retain them to past the melting point. As the particles fall inwards, the temperature rises, and at some point, occluded volatiles that have passed their melting point are emitted. If, however, the melting point is not reached, the volatile is retained, more or less as a solid, and fills the pores. Suppose two particles collide. If they are sufficiently below a melting point, they bounce off each other, but if the volatile can melt, the energy of collision is absorbed in melting it, in other words, kinetic energy is converted to heat and the collision is, for the moment, inelastic. Now, the liquid trapped in pores between the particles cannot escape, but it can merge, then when it cools, it solidifies, thus we have pressure-induced melt-welding of the particles. This is similar to how a snow-ball grows with pressure. If so, then we look at the ices, these are (separated into subsets that have similar melting properties) in order of decreasing melting points (temperatures in degrees K): {water (273)}, {methanol/ammonia/water eutectic and CO₂ (164-195)}, {CH₄ and Ar, (84-90)},  {CO and N₂ (63-68)}, and {neon (25)}.
 
There are, therefore, zones where ice can accrete into larger bodies, which depend on the temperatures in the disk. The surfaces of the disks usually have temperature proportional to r^-0.75, but the interior should retain heat better. If we put the index = -0.825, and assume Jupiter is the optimal place for a water-based core, then we predict the solar system as Saturn (water/ammonia/methanol) at 7.8 – 9.6 A.U. (actual, 9.5); Uranus (methane/argon) at 20-21.7 A.U. (actual, 19.7); Neptune (CO and N2) at 28 – 31 A.U. (actual 30) and possibly a planet based on neon at about 95 A.U. The satellites are based on the same compositions so we predict the Jovian system to be based only on water (the rest having volatilized); the Saturnian system to have ammonia and methanol (which can undergo chemistry to produce nitrogen and methane, which explains why Titan has an atmosphere and Ganymede does not); the Uranian system to be the slowest starter, because methane and argon are relatively minor components, but which will grow faster than Neptune once total accretion gets under way because matter density is greater, while Neptune will initially grow faster than Uranus, because nitrogen and carbon monoxide are common, but slower when gravity becomes the driving force. As far as I know, this is the only theory that requires Neptune to be bigger than Uranus to start with, and always to be denser. It also predicts that planets grow proportional to their cross-sectional area, because they grow initially in a flow of ice particles, all of which are continually renewed by the stream of gas heading starwards. By not involving collisions between equally sized objects, the rate of formation increases dramatically. Note it also predicts no life under-ice at Europa because the Europan sea will be deficient in both nitrogen and carbon. There are thin atmospheres around the major Jovian moons (Thinner than the gases in a light bulb!) but on Europa, there appear to be no nitrogenous species to 7 orders of magnitude less than the major species. What do you think?