Bacchus, as seen by Voyager 1 during the flyby of Bacchus.
Bacchus is the smallest planet in the Outer Solar System, just barely smaller than Earth and bigger than Mars. Scientists have hypothesised that it was once a moon of one of the planets nearby (Jupiter, Saturn, Caelus, Aether, or Neptune) due to its small size and lack of moons. (It is the only planet beyond Juno without moons.)