RE: Oh no! Another alien probe
December 14, 2017 at 9:27 am
(This post was last modified: December 14, 2017 at 9:46 am by Anomalocaris.)
That is right.
Think of it this way. Gas and dust Cloud experience temporary local over-densities due to random motion, passage of pressure wave, and other causes. If the mass of overdensities are not enough, natural pressure of the gas and dust will cause over-density to simply disperse, and nothing will happen. For something to happen, Some thing has to not only hold overdensity together, but add to it.
The only way this can initiate is if some large scale event, like try he shockwave from a supernova, create overdensity in large volume of space, so that total mass of the over density is enough right at the beginning to create its own gravity strong enough to arrest the natural tendency to disperse under its own pressure. When this happens, the overdensity become self perpetuating, and will start to pull in more gas from surrounding, get bigger and bigger, until eventually the region runs out of more gas, or the overdensity comes so massive it ignites and becomes a star, and photo pressure from the light it emits arrest further in fall of surrounding material.
The threshold mass for an initial overdensity in interstellar gas and dust to be able to arrest its own dispersal with its own gravity is calculated to be several Jupiter masses. No initial overdensity the mass of a mere asteroid in interstellar medium is going to coalesce into an asteroid rather than disperse. Unless some initial overdensity is over several Jupiter masses, it would disperse and leave no remnant or residual object.
Only way for smaller objects to accrete is if some large nearby object can excert so much gravity that it pulls in material from around itself, concentrate them into an accretion disk or cloud, and confine them with its own gravity. If this happens the density of dust and gas around the large object goes up to many orders of magnitude above interstellar medium. Then the vastly increased chance of collision between small particles as well as reduced relative velocities between particles due to cohesive motion in the accretion disk lets particles stick and not disperse. This is how small objects can accrete.
So the upshot is in interstellar space, objects more than several times the mass of Jupiter can form by themselves and then gain further mass. Objects less than several Jupiter masses can't form by themselves , and require An accretion disks or accretion cloud confined by the gravity of nearby stars or protostars.
This is why we think any interstellar asteroid or comet we find almost certainly formed as an circumstellar asteroid or comet around another star, and then somehow got loose.
.
Think of it this way. Gas and dust Cloud experience temporary local over-densities due to random motion, passage of pressure wave, and other causes. If the mass of overdensities are not enough, natural pressure of the gas and dust will cause over-density to simply disperse, and nothing will happen. For something to happen, Some thing has to not only hold overdensity together, but add to it.
The only way this can initiate is if some large scale event, like try he shockwave from a supernova, create overdensity in large volume of space, so that total mass of the over density is enough right at the beginning to create its own gravity strong enough to arrest the natural tendency to disperse under its own pressure. When this happens, the overdensity become self perpetuating, and will start to pull in more gas from surrounding, get bigger and bigger, until eventually the region runs out of more gas, or the overdensity comes so massive it ignites and becomes a star, and photo pressure from the light it emits arrest further in fall of surrounding material.
The threshold mass for an initial overdensity in interstellar gas and dust to be able to arrest its own dispersal with its own gravity is calculated to be several Jupiter masses. No initial overdensity the mass of a mere asteroid in interstellar medium is going to coalesce into an asteroid rather than disperse. Unless some initial overdensity is over several Jupiter masses, it would disperse and leave no remnant or residual object.
Only way for smaller objects to accrete is if some large nearby object can excert so much gravity that it pulls in material from around itself, concentrate them into an accretion disk or cloud, and confine them with its own gravity. If this happens the density of dust and gas around the large object goes up to many orders of magnitude above interstellar medium. Then the vastly increased chance of collision between small particles as well as reduced relative velocities between particles due to cohesive motion in the accretion disk lets particles stick and not disperse. This is how small objects can accrete.
So the upshot is in interstellar space, objects more than several times the mass of Jupiter can form by themselves and then gain further mass. Objects less than several Jupiter masses can't form by themselves , and require An accretion disks or accretion cloud confined by the gravity of nearby stars or protostars.
This is why we think any interstellar asteroid or comet we find almost certainly formed as an circumstellar asteroid or comet around another star, and then somehow got loose.
.
