  Trojan objects share a planet's orbit, never straying far from the triangular
Lagrangian points, 60 degrees ahead of (L4) or behind (L5) the planet. We
report the detection of a Uranian Trojan; in our numerical integrations, 2011
QF99 oscillates around the Uranian L4 Lagrange point for >70 kyr and remains
co-orbital for ~1 Myr before becoming a Centaur. We constructed a Centaur
model, supplied from the transneptunian region, to estimate temporary
co-orbital capture frequency and duration (to factor of two accuracy), finding
that at any time 0.4% and 2.8% of the population will be Uranian and Neptunian
co-orbitals, respectively. The co-orbital fraction (~2.4%) among Centaurs in
the IAU Minor Planet Centre database is thus as expected under transneptunian
supply.
