  The nature and origin of free-floating planets (FFPs) are still largely
unconstrained because of a lack of large homogeneous samples to enable a
statistical analysis of their properties. So far, most FFPs have been
discovered using indirect methods; microlensing surveys have proved
particularly successful to detect these objects down to a few Earth masses.
However, the ephemeral nature of microlensing events prevents any follow-up
observations and individual characterization. Several studies have identified
FFPs in young stellar clusters and the Galactic field but their samples are
small or heterogeneous in age and origin. Here we report the discovery of
between 70 and 170 FFPs (depending on the assumed age) in the region
encompassing Upper Scorpius and Ophiuchus, the closest young OB association to
the Sun. We found an excess of FFPs by a factor of up to seven compared with
core-collapse model predictions, demonstrating that other formation mechanisms
may be at work. We estimate that ejection from planetary systems might have a
contribution comparable to that of core-collapse in the formation of FFPs.
Therefore, ejections due to dynamical instabilities in giant exoplanet systems
must be frequent within the first 10 Myr of a system's life.
