The previous post on rogue planets mentioned that condensation was the likely means by which a large number of rogue planets could be produced. Stripping planets out of a solar system, for example by a nearby supernova, could certainly produce large numbers of them, but they would be limited to some multiple of the number of supernovas. The size of the multiplier should certainly be looked at, but the number of supernovas is not going to be so high.
Consider how condensation works. In a cool gas, just sitting still, with no rotation, a fluctuation in gas density could lead to some gravitational contraction, which would be a feedback effect to the size of the fluctuation. Contraction would continue. Why would contraction not just continue until an O class star was formed, and then keep going? Because of the stellar wind. Once a star is formed, the stellar wind starts blowing outward, carrying some of the fusion energy away from the star where it is being created. At some point, the stellar wind slows and stops the condensation. If the density of the original gas is very low, it doesn’t take much stellar wind to stop it. You get a red dwarf. If the density of the original gas is high, it takes a stronger stellar wind to stop it from being pulled inwards by the stellar mass. Thus, where density is the highest in the outer parts of a spiral galaxy, in the spiral waves, you get the formation of the brightest, largest stars, the O’s, B’s and A’s.
This example says that brown dwarfs and rogue planets would just keep attracting more mass until they accumulated enough to ignite and start their own solar wind. So, if there are no other mechanisms which would limit the amount of gas which contracts, there would be few brown dwarfs and rogue planets.
Angular momentum is one suspect. As a blob of gas contracts around some point of maximum density, it possesses some angular momentum relative to that point. The momentum can couple between different sections of the gas, but it cannot disappear. So, something has to still contain it. If there is not much, it can be accumulated into the rotation of the blob itself, and whatever the blob turns into, such as a star, a brown dwarf or a rogue planet. More angular momentum and some disc forms to hold it, such as a galactic disk, on the huge scale, or as a planetary disk, on the stellar scale. A disc could form around a rogue planet that was condensing as well. This usual amounts of angular momentum can be accommodated without putting a large amount of the mass into the disk.
A lack of gas could limit the amount of gas which condenses to form something. This could happen if the gas were already occupied falling into another nearby blob. So, the initial conditions of the blob make a difference. If there are large fluctuations in gas density, each of them will start pulling in nearby gas to build up their own mass, but with a high density of them, none of the blobs will get enough to become a star, and they will all have to be content with becoming rogue planets. What would lead to fluctuations in gas density? If the gas was all alone in the universe, it would achieve a fairly constant universal density, but if there is any gravitational pull from previously formed objects, there could be fluctuations. There is a question of scale here. If the scale of the blob contains enough mass to become a red dwarf, then that is what it would become. If the scale of the blobs is smaller than that, in other words, more fluctuations per cubic light year, then they can only become rogue planets.
In galaxies, there are large irregularities visible. Look at any disk galaxy, spiral or barred or whatever, and there are large-scale fluctuations in the light output. This corresponds to the largest stars, but these fluctuations can extend downward in scale. Our galaxy is surrounded by dwarf galaxies, as well as a few large ones, and they are all in motion. The gravitational, or tidal, pull of each of these affects the gas in a galactic disk. Possibly the spiral waves come from excitation by other large objects, which might explain why they are so irregular in many galaxies.
These excitations do not directly cause small scale fluctuations in gas density. Instead, they cause large scale motions, of the size of the orbit of the object causing the fluctuations, or perhaps down to the size of the object itself if it comes close to the disk. There are many galactic clusters interacting with the disk as well, and the same size of directly caused fluctuations would result. But, it is well known that turbulence extends to lower scale motions. In other words, the original size of the disturbance leads to flows in the gas, which gradually become smaller and smaller in size. How long this takes is questionable, but the decrease in scale is inexorable until the fluctuations driving it die away. However, the orbiting objects, dwarf galaxies and galactic clusters, are not going away. They just keep inducing fluctuations. So, over time, perhaps shorter than the rotation time of the galaxy, small scale fluctuations keep happening, and this means small objects might form.
There is likely a large difference in the response of the central bulge of a disk galaxy and the response of the disk itself. The central bulge is a ball of stars, and when an orbiting dwarf galaxy pulls on it, the nearer stars will respond, like to a tidal forcing, but the inner stars less so. If there is larger internal motion in the central bulge, this will tend to damp out the effects. So, for the formation of rogue planets, it would seem that the disk is a more likely place for their formation.
One other point about rogue planets is that they are like red dwarfs in that their lifetime is longer than the age of the universe. If they were made, in any way, they would just keep accumulating their numbers, unlike hotter stars which burn up and blow up and then disappear from the counts. There does not seem to be any mechanisms for rogue planets to be destroyed or to destroy themselves. Perhaps if one was near a supernova when it detonated, it would be vaporized. This cannot amount to a large reduction. This also means that if, during the early days of the galaxy, when it was just forming, if there were lots of rogue planets made, they would still be around. So, to estimate how many rogue planets there might be, it is even necessary to consider early formation.
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