Little is known about the existence of extrasolar planets around ultracool dwarfs .
Furthermore , binary stars with Sun-like primaries and very low-mass binaries composed of ultracool dwarfs show differences in the distributions of mass ratio and orbital separation that can be indicative of distinct formation mechanisms .
Using FORS2/VLT optical imaging for high precision astrometry we are searching for planets and substellar objects around ultracool dwarfs to investigate their multiplicity properties for very low companion masses .
Here we report astrometric measurements with an accuracy of two tenths of a milli-arcsecond over two years that reveal orbital motion of the nearby L1.5 dwarf DENIS-P J082303.1-491201 located at 20.77 \pm 0.08 pc caused by an unseen companion that revolves about its host on an eccentric orbit in 246.4 \pm 1.4 days .
We estimate the L1.5 dwarf to have 7.5 \pm 0.7 % of the Sun ’ s mass that implies a companion mass of 28 \pm 2 Jupiter masses .
This new system has the smallest mass ratio ( 0.36 \pm 0.02 ) of known very low-mass binaries with characterised orbits .
With this discovery we demonstrate 200 micro-arcsecond astrometry over an arc-minute field and over several years that is sufficient to discover sub-Jupiter mass planets around ultracool dwarfs .
We also show that the achieved parallax accuracy of < 0.4 % makes it possible to remove distance as a dominant source of uncertainty in the modelling of ultracool dwarfs .