  We use the Robo-AO survey of Kepler planetary candidate host stars, the
largest adaptive optics survey yet performed, to measure the recovery rate of
close stellar binaries in Gaia DR2. We find that Gaia recovers binaries down to
1" at magnitude contrasts as large as 6; closer systems are not resolved,
regardless of secondary brightness. Gaia DR2 binary detection does not have a
strong dependence on the orientation of the stellar pairs. We find 177 nearby
stars to Kepler planetary candidate host stars in Gaia DR2 that were not
detected in the Robo-AO survey, almost all of which are faint (G>20); the
remainder were largely targets observed by Robo-AO in poor conditions. If the
primary star is the host, the impact on the radii estimates of planet
candidates in these systems is likely minimal; many of these faint stars,
however, could be faint eclipsing binaries that are the source of a false
positive planetary transit signal. With Robo-AO and Gaia combined, we find that
18.7% of Kepler planet candidate hosts have nearby stars within 4". We also
find 36 nearby stars in Gaia DR2 around 35 planetary candidate host stars
detected with K2. The nearby star fraction rate for K2 planetary candidates is
significantly lower than that for the primary Kepler mission. The binary
recovery rate of Gaia will improve initial radius estimates of future TESS
planet candidates significantly, however ground-based high-resolution follow-up
observations are still needed for precise characterization and confirmation.
The sensitivity of Gaia to closely separated binaries is expected to improve in
later data releases.
