# What's the distinction between “planetary formation mechanisms” and “gravitational collapse”?

As I was curious about the exact identical comment to my title here: https://astronomy.stackexchange.com/a/16135/10102

And as it seems no one asked it yet, I'm simply interested in the difference of the both, so:

What's the distinction between "planetary formation mechanisms" and "gravitational collapse"?

• @adrianmcmenamin: I'm not quite sure (probably due to lack of knowledge) if this is still asking the same. The answer was showing them up as 2 different things. While now my question reads like "are there things opposite to?" – Zaibis Jun 6 '16 at 15:06
• The answer you referred to says: "The other is that Jupiter formed by planetary formation mechanisms rather than by gravitational collapse," and "There is also debate on what to call objects above 13 Jupiter masses that formed by planetary formation mechanisms as opposed to by gravitational collapse." But your formulation asked what the "difference" between planetary formation mechanisms and gravitational collapse was. But gravitational collapse is a planetary formation mechanism... so my assumption was surely what you meant to ask is what other mechanisms beyond gravitational collapse exist? – adrianmcmenamin Jun 6 '16 at 15:10
• @adrianmcmenamin: Well when I read that answer I was thinking "Hm, isn't 'gravitational collapse' just a subset of 'planetary formation mechanisms'? But why he writes then 'one rather the other'" Then I read the comment asking for whats the distingtion of them, and instead anyone saying something like "probably a typo, as they're of the same set" and instead read "Would be a good question itself" I asked it by asuming, that I was wrong and they are different things. So while this is ANOTHER good question, you sadly completly falsified my question with that edit. – Zaibis Jun 7 '16 at 8:12

"Gravitational collapse" is not the generally used name of any planet formation model it simply refers to the collapse of a cloud of gas or collection of objects under the influence of their mutual gravity. "Planetary formation mechanisms" simply refers to mechanisms which form planets, all of which involve the mutual gravitational attraction of the planetary components at some phase.

In the context of the answer you refer to, there "gravitational collapse" refers to where a small object (like a brown dwarf) forms through the same direct collapse and fragmentation process that forms stars.

Planets on the other hand are thought to form in the disc that is inevitably created around a protostar during the star formation process. This might be through something called gas disc instability, or the more likely model at present is something known as the core accretion model.

Gravitational collapse is usually meant in the context of lots of small particles falling into a large gravity well:

A. Galaxies (Clusters, Superclusters, cosmic filaments, too, I think) start off as the gravitational collapse of the hydrogen in a large volume of space (only if the density was perfectly uniform (not possible to be perfect) would there be no collapse).

B. Portions of molecular clouds (hydrogen (and helium) gas and dust) gravitationally collapse into the protostellar disk which collapses into the protostar.

C. All stars are in the process of gravitational collapse. This collapse is slowed down (stopped) by their fusion ignition and heating, but eventually that will end and collapse will continue for whatever is left from the fusion and the supernova. White dwarfs form, and they'll be stable about a trillion of years (we think) and then become cool black dwarfs.

D. Gravitational collapse is also used to describe the start of a black hole. In this case, no forces, not heat, not electron degeneracy, can overcome the gravitational attraction. Just today I saw a report of a dark rain observation - the inflow of matter directly onto a black hole, when normally we expect an accretion disk to form and for matter to spiral in, heating up enormously in the process.

E. It's possible to form a planet via gravitational collapse, but generally larger objects (stars) are what result. This also means that while gravitational collapse might be a contributor to planetary formation, we THINK the major factors are collisions and accretion.

Here's how it goes. Hydrogen is all over the Universe. Some of it collapses into stars and galaxies. Stars form and blow up, spewing dust all over the place. Dust and gas (H and He) form into clouds because of the explosions pushing them around (a supernova is to a molecular cloud as a leaf blower is to leaves).

Gravitational collapse of very large regions of the molecular cloud ensues. Protostellar disk forms, star in center forms, and stuff in disk collides and accretes into planets (as star forms, things begin to heat up, meaning planets close to center will not have much hydrogen, while planets far away will have lots of volatile gases (Gas giants).

This picture is being questioned because so many Gas Giant sized exo-planets are being found close to their stars...stay tuned, the jury is out (we think these Hot Gas Giants got there by orbital perturbation, and didn't form "close", but its a very active question.)

• Is there a reason for all the ('s and )'s? Its really hard to read your answer this way. – Zaibis Jun 9 '16 at 7:41