  The co-planarity of solar-system planets led Kant to suggest that they formed
from an accretion disk, and the discovery of hundreds of such disks around
young stars as well as hundreds of co-planar planetary systems by the Kepler
satellite demonstrate that this formation mechanism is extremely widespread.
Many moons in the solar system, such as the Galilean moons of Jupiter, also
formed out of the accretion disks that coalesced into the giant planets. We
report here the discovery of an intermediate system OGLE-2013-BLG-0723LB/Bb
composed of a Venus-mass planet orbiting a brown dwarf, which may be viewed
either as a scaled down version of a planet plus star or as a scaled up version
of a moon plus planet orbiting a star. The latter analogy can be further
extended since they orbit in the potential of a larger, stellar body. For
ice-rock companions formed in the outer parts of accretion disks, like Uranus
and Callisto, the scaled masses and separations of the three types of systems
are similar, leading us to suggest that formation processes of companions
within accretion disks around stars, brown dwarfs, and planets are similar.
