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Also,does the absence of the accumulation of anything large such as rogue planets by the solar system suggest the Milky Way has been orderly and stable for billions of years?

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    $\begingroup$ Our solar system has been orderly and stable, our existence proves this. Dust and gas wouldn't "accumulate" as there is nothing to slow it down and keep it in the solar system. Dust and rocks that enter the solar system from the interstella medium fall towards the sun, speeding up, and then sail away, unless they hit something $\endgroup$
    – James K
    Oct 28, 2023 at 13:12
  • $\begingroup$ @James K The stability of our solar system over billions of years as it circles the galactic centre suggests that there are no large accumulations of randomly moving cold dark bodies . $\endgroup$
    – user52681
    Oct 28, 2023 at 13:25
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    $\begingroup$ You assume that the solar system moves through stuff. Yet everything orbits the galactic center at about the same speed, thus it won't accumulate stuff like you do when driving a Cabrio, but more like you overtaking or being overtaken by other runners at a marathon past half distance: something which happens rarely. And even then James's logic applies $\endgroup$ Oct 28, 2023 at 14:55
  • $\begingroup$ @planetmaker I was assuming that there could be some matter moving randomly around in the Galaxy over the last 4 billion years for example stars orbiting the central black hole can be ejected from its vicinity.I think the stability of the solar system shows that most matter moving too fast or too slow to orbit the Galaxy moved to the centre of or was ejected from the Galaxy early on in its history. $\endgroup$
    – user52681
    Oct 28, 2023 at 20:37
  • $\begingroup$ Or that the Galaxy formed initially in a very orderly manner. $\endgroup$
    – user52681
    Oct 28, 2023 at 20:48

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How does objects in space accumulate other objects? If we ignore direct, inelastic collisions, the way to capture a passing object is to have an interaction where it loses enough momentum to get gravitationally bound. If it doesn't, it will just sail past. For slow objects tidal interactions might do it, but for fast ones you typically need a third object to pick up the momentum in some kind of three-body interaction.

However, the probability of this being enough goes down rapidly with higher relative velocity. Typical interplanetary velocities are about 10 times the typical interplanetary velocities: it takes a very precise angle and moment for some interloping dust grain or planet to get captured. Old calculations suggest this is very rare (about one comet per 60 million years), but modern multi-planet simulations suggest about 1200 captured (asteroid sized) objects per million years.

Note that since these interactions are time-reversible the solar system also likely loses objects over time.

Another thing to note is that there are many more tiny dust grains than interstellar asteroids or comets, and they in turn outnumber rogue planets. So while we may capture some dust and asteroids, the number of rogue planets captured (or just messing up orbits) is very low even under 20 galactic rotations.

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  • $\begingroup$ How many rogue black holes or stars are there ? Would these have left the Galaxy a long time ago? $\endgroup$
    – user52681
    Oct 29, 2023 at 13:08
  • $\begingroup$ An article about stars that escape from their birth place phys.org/news/2023-10-astronomers-infant-star.html Whether or not they can become rogue stars that could affect our solar system isn't discussed. $\endgroup$
    – user52681
    Oct 29, 2023 at 13:22

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