In the article Nomads of the Galaxy (or this video), it is said that there are possibly 100000x more "free planets" (not orbiting a particular star) than stars. If the size distribution of "free objects" is similar to size distribution of asteroids (some power law - much more small object than large objects), then it could be expected that there are a considerable amount of smaller objects which migrate between the sphere of influence of one star to another.

In such a case it would be better to describe the situation as star system is immersed in some continuum (like gas) of free objects with some non-negligible background density, rather than to speak about discrete Oort clouds. Is there any model or other work which addresses this?

(I'm interested in the connection to the possibility of interstellar travel)

Below is a graphical illustration of my question. Which is more realistic, the upper or the lower chart? mass distribution vs. distance

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    $\begingroup$ Our neighboring stars are moving pretty fast with regard to one another. See my answer at astronomy.stackexchange.com/questions/3601/… A comet drifting out of Alpha Centauri's Oort cloud (for example) would not drift into our Oort cloud. If the velocities of neighboring instellar comets is like that of neighboring stars, interstellar space is somewhat turbulent. $\endgroup$
    – HopDavid
    Feb 10, 2015 at 15:43

1 Answer 1


Interstellar comets are icy bodies traveling in the areas between stars. They're just like the comets in the Oort Cloud, with the exception that they've been ejected from whatever stellar system they formed in. Wikipedia cites a good paper that analyzes the distribution of these comets, The Demographics of Long-Period Comets by Paul J. Francis. Francis gives an estimate of a number density of $4.5 \times 10^{-4} \text{ comets/AU}^3$. Francis used data from the Lincoln Near-Earth Asteroid Research survey (LINEAR).

The Oort Cloud, on the other hand, has trillions of objects within a sphere of about 50,000 AU in radius, which, at a lower bound of one trillion objects, gives a number density or about $1.91 \times 10^{-3} \text{ comets/AU}$ - and that's a conservative estimate.

So even though the density of interstellar comets is much lower than the density of comets in the Oort Cloud, the environment is more "continuous" rather than "discrete."

  • $\begingroup$ Thank you very much, this basically answer the question. So density of interstellar comets is just 5-times lower than density of comets in Oort cloud. But still I would prefer some model which estimates e.g. an analytic shape of radial distribution function from first principles (from some theory of star formation), rather than estimate of density inside and outside a sphere of rather arbitrarily choosen radius. $\endgroup$ Feb 10, 2015 at 8:37
  • $\begingroup$ @ProkopHapala I don't think there's a model that can do that, especially because - as HopDavid said - stars are constantly changing their positions relative to one another. $\endgroup$
    – HDE 226868
    Feb 11, 2015 at 22:32

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