What is between the Oort cloud and the closest star?

What is between the Oort cloud and the closest star? It seems strange to think that such a HUGE area of space is not filled with a vast amount of planets/comets and all sort of things beyond our imagination or at least something other than nothing. But do we know anything, or is it simply to dark to see it?

Once you get beyond the Oort Cloud, there really isn't much mass to speak of. The interstellar volume is largely occupied by the appropriately named Interstellar Medium, or ISM. The ISM is really just the leftover or ejected material from planetary nebulae, stellar winds, and supernovae, 99% gas and 1% dust and other particles. The ISM has a density of between one thousand and one million particles per cubic meter, which is pretty thin by solar system standards (better than the best vacuum on Earth!).

So the question remains: why isn't there more stuff in interstellar space? This is mostly because stars are pretty good at keeping the matter from the planetary nebulae they formed from in their immediate vicinity, just through gravity. Larger objects, especially, are very unlikely to be ejected or to wander away from the solar system, as it would take a huge amount of energy to escape the sun's influence. Even the comets of the Oort cloud are pretty well gravitationally bound to long but inescapable orbits.

But that doesn't mean there isn't anything out there! You always hear a few theories about "rogue planets" that were ejected from their systems by supernovae, or comets on hyperbolic trajectories that escape the solar system. However, these are all pretty unlikely situations, and even if rogue objects do exist, they take up an inconceivably small fraction of interstellar space.

September 2007, Briony Horgan (more by Briony Horgan) (Like this Answer)

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