Are there any plausible 'alternative' methods of planetary formation?
The selected answer on this question provides an extremely good, and detailed, description of what is generally accepted to be the main (only?) way a terrestrial planet can form, i.e. through accretion of the gaseous disk around a forming star.
My question is: Assuming that some discovery is made that concludes that this accretion process is false (this 'hand-waved' discovery is for the sake of argument, and it need not have any other bearing on our understanding of physics beyond this unique aspect of planet formation, though it can have other repercussions on known physics if it helps generate an answer), no planets, ever, anywhere in our known universe, were created using this accretion method, what is (are?) the next most likely method(s?) of planetary creation?
Best answers will use a minimum of change to known physics whenever possible, and those changes need not be described or explained. I'm interested mainly in identifying the process itself, rather than why one process doesn't work and a new one does, I just want to know what the new process is. In other words, hand-waving known physics is allowed, but should be avoided as much as possible.
Answers do not need to consider habitability of planets, except to the extent that we know of at least 1 (Earth) in the universe that ended up habitable due to this planetary formation process. Likewise, answers should not be limited to a process that obviously excludes the possible formation of any planet that we know to have formed in reality (don't describe a process that can only account for gas giants, but cannot produce terrestrials, or vice versa).
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1 answer
Gravitational instabilities
There's actually a second idea for planet formation that's been around for some time (see Kuiper 1951). It requires the nebula hypothesis, like accretion, but it's a top-down process, not a bottom-up one, involving the fragmentation of accretion disks around young stars to form giant planets.
Protoplanetary disks can be unstable under specific conditions. In certain regions of overdensities inside a disk, clouds of gas may fragment into gaseous protoplanets, soon to evolve to become gas giants. The timescales of formation via this method are much shorter timescales than those required by the accretion hypothesis by an order of magnitude or so - tens or hundreds of thousands of years,
Fragmentation would likely happen farther out in a protoplanetary disk (Boss 1997, and volatiles like ice would form part of the core. The hotter inner portions have been pretty firmly ruled out as potential sites. Current views hold that fragmentation is certainly a possibility, and may even account for the formation of planets with large semi-major axes (including, I believe, the system around HR 8799, but it is unlikely that it contributes much to the general exoplanet population. Nonetheless, it requires no changes to the laws of physics.
Carbon planets and binary accretion
Another set of methods involve interactions between a star and a companion object, where the star accretes much of the companion's outer layers, leaving a remnant that may appear to be a gas giant or a carbon planet, depending on the precise mechanism and the companion's original form:
- EF Eridani B may be a star that had much of its outer layers siphoned off by its companion star, leaving an object that might be about as massive as a massive giant planet, but isn't a brown dwarf.
- PSR J1719-1438 b, the "diamond planet", may be a very low-mass white dwarf that was mostly accreted by its companion (see Bailes et al. 2011), leaving a dense object that would appear to be a carbon planet - terrestrial, but far from habitable.
These processes can form either terrestrial or giant planets - an advantage the instability model doesn't have. Another upside is that we have solid candidates for "planets" that formed this way. However, these pathways aren't conducive to forming habitable worlds (although I noticed you didn't require habitability).
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