  Ultra-cool dwarfs are very low-mass stars or brown dwarfs and because of
their faintness they are difficult targets for radial velocity and transit
planet searches. High-precision astrometry is one way to efficiently discover
planets around these objects. We are conducting a planet search survey of 20
M8-L2 using ground-based imaging astrometry with FORS2 at VLT. The realised
accuracy of 100 micro-arcseconds allows us to set stringent constraints on the
presence of planets, to discover astrometric binaries, and to measure
parallaxes with an unprecedented precision of 0.1 %. The obtained detection
limits firmly establish that giant planets are rare around UCDs at all
separations. The astrometric performance of our programme is comparable to what
is expected from Gaia observations of single faint objects and we discuss
potential synergies for planet searches around ultracool dwarfs. We estimate
that Gaia will be able to characterise ~100 astrometric binaries with an
ultracool primary.
