wrote:

"A team of astronomers has identified a novel
new kind of galactic wanderer - lone, Jupiter-sized
planets expelled from forming solar systems
and drifting in the empty void between the stars.

The researchers, led by Takahiro Sumi of Japan's
Osaka University, spotted 10 such free-floating
"orphan planets" in data from a 2006-7
microlensing survey of our galaxy's centre, which
searched for the tell-tale sign of transiting bodies'
gravitational fields distorting light from distant
stars.

Team member David Bennett, of the University of
Notre Dame in Indiana, explained that this first
sighting in a small portion of the Milky Way
points to enormous numbers of orphans. He
said: "Our survey is like a population census.
We sampled a portion of the galaxy, and based
on these data, can estimate overall numbers in
the galaxy.""

See:
http://www.theregister.co.uk/2011/05/19/orphan_planets/

These are objects smaller than brown dwarves. It makes sense that there
would be a nearly continuous trend of objects from the largest class O
stars all the way down to comets a kilometer or two across throughout
interstallar space.

The question becomes one of estimated densities by size. How many
Trans-Neptunian Objects in the Kuiper belt are how big? How large is
the Oort cloud and how much does it overlap with the clouds of other
stars? How many planet sized objects are alone out there? How many
large objects have entire dark moon systems out there? The list of
questions only gets easy to answer once the objects are big enough to
acheive ignition and shine from their own fusion. Even with M class
stars they are too dim to be accurately surveyed beyond 20-30 light
years.

My usual model for an interstellar civilization is to develop fusion
power and fusion drives then move out into the Oort cloud and lose
interest in the denser stellar material deep in a star's gravity well.
With enough large objects out there it's a very large resource on which
to base a fusion powered interstellar civilization. Neutrino emisions
would be the way to detect such distant civilizations I suspect.