Forming Gias-giant planets isn’t easy. The current dominant paradigm is Core Accretion, which requires 10 Earth mass planets to form, then capture gas from the protoplanetary nebula to grow to gas giant size. Making such large objects isn’t overly difficult given a steady supply of planetary embryos – objects roughly Ceres in size. Just how do such objects form in the first place?
That’s not too hard either, requiring a supply of km size clumps of dust and ice. But then there’s a difficulty. In between sand grain and kilometer size, objects interact strongly with the protoplanetary gas nebula – they spiral into the Sun because of a ‘headwind’ effect. How can the leap from sand grain to comet be scaled?
One option is to make planets from ‘the top down’ via gravitational instability – dense patches of gas and dust directly becoming gravitationally bound objects. Surprisingly this is easier to do for Jupiter-sized objects and larger and is basically how stars form. Smaller planets aren’t so easily made, but nothing is stopping a gaseous protoplanet from losing most of its mass to become a “down-sized” Neptune or even Earth-sized planet.
Work by Aaron Boley and Sergei Nayakshin led them both to propose this option separately in 2010…
Clumps in the Outer Disk by Disk Instability: Why They are Initially Gas Giants and the Legacy of Disruption (Aaron C. Boley et.al.)
A New View on Planet Formation (Sergei Nayakshin)
…both emphasizing the role of tidal forces in the down-sizing of the gaseous protoplanets. A wide variety of planets can result then from Gas-giants, depending on the amount of gas loss, and the pace of the migration process which draws them in from out past the orbit of Pluto to their Phoenix-like rebirth as a terrestrial planet or Hot-Jovian.
So, I wonder, what are the implications for Earth, as a planet, if it was born from the heavy element core of a Gas-giant? Just how much pressure did the proto-Earth experience and what effect might that have had on geological history? A speculative theory, proposed by J. Marvin Herndon is that the whole Earth has expanded over geo-time from a compressed state, thus producing the great fault-lines we call the mid-ocean ridges, and the plunging of oceanic crust into the mantle in what we call subduction zones. Plate-tectonics or Whole-Earth decompression? I’ll save my discussion for Part II.