  The recent discovery of a number of circumbinary planets lends a new tool to
astrophysicists seeking to understand how and where planet formation takes
place. Of the increasingly numerous circumbinary systems, Kepler-16 is arguably
the most dynamically interesting: it consists of a planet on an almost
perfectly circular orbit (e = 0.0069) around a moderately eccentric binary (e =
0.16). We present high-resolution 3D smoothed-particle hydrodynamics
simulations of a Kepler-16 analogue embedded in a circumbinary disc, and show
that the planet's eccentricity is damped by its interaction with the
protoplanetary disc. We use this to place a lower limit on the gas surface
density in the real disc through which Kepler-16b migrated of \Sigma_min ~ 10 g
cm^-2. This suggests that Kepler-16b, and other circumbinary planets, formed
and migrated in relatively massive discs. We argue that secular evolution of
circumbinary discs requires that these planets likely formed early on in the
lifetime of the disc and migrated inwards before the disc lost a significant
amount of its original mass.
