On Wed, Feb 24, 2016 at 07:16:53AM +1100, Greg Chalik wrote:
> But Bruce, how many exo-system Plutos could one expect to be out
> there?

Quite a lot, actually.  That hard part would be finding them.

However, far more common would be smaller bodies less than a kilometre
in diameter.  For refuelling purposes even a few dozen metres is more
than enough for many starships.  Almost all of them will have large
fractions of water ice, because it is one of the most common compounds
in the universe.  Most of what isn't water ice will be methane ice,
which is also a source of hydrogen (the most common element in the
universe).

Still, space is *big*.  Having ten thousand times the mass of Earth's
oceans per cubic parsec isn't very helpful when the nearest chunk of
sufficient size is a near invisible boulder a hundred million km away.
That's probably a fairly typical separation of such bodies in
interstellar space.

- Tim