What is between the Oort cloud and the closest star? (Beginner)

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.

This page was last updated June 28, 2015.

About the Author

Briony Horgan

Briony is an Assistant Professor at Purdue University, and uses orbital remote sensing of Mars and the Moon supported by laboratory and field work to investigate planetary surface processes. Her primary tool is spectroscopy, including both visible/near-infrared and mid-infrared. Briony earned her B.S. in Physics from Oregon State University in 2005 and her Ph.D. in Astronomy and Space Sciences from Cornell University in 2010. Her thesis advisor was Prof. Jim Bell (now at ASU). Her thesis was titled "Wind, water, and the sands of Mars", and focused on using spectral and morphologic characteristics of sediments in the northern lowlands of Mars to reveal past and ongoing interactions with liquid water. After her PhD, Briony became an Exploration Postdoctoral Fellow in the School of Earth and Space Exploration at Arizona State University, working primarily in the Mars Space Flight Facility with Phil Christensen. There she investigated the composition, spectral properties, and terrestrial field analogs of soils and sediments on Mars. The results of these studies will aid in constraining the habitability of ancient surface environments on Mars, and may have implications for our understanding of the early Earth. 

CV: http://purdue.academia.edu/BrionyHorgan/CurriculumVitae
Website: https://purdue.academia.edu/BrionyHorgan

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