(687170) 2011 QF99

(687170) 2011 QF₉₉ is a minor planet from the outer Solar System and the first known Uranus trojan to be discovered. It measures approximately 60 kilometers (37 miles) in diameter, assuming an albedo of 0.05.^[3][4] It was first observed 29 August 2011 during a deep survey of trans-Neptunian objects conducted with the Canada–France–Hawaii Telescope, but its identification as Uranian trojan was not announced until 2013.^[3][5]

(687170) 2011 QF₉₉

Animation of 2011 QF99 relative to Sun and Uranus 1600-2500   2011 QF99 ·    Uranus  ·   Sun
Discovery
Discovered by	M. Alexandersen[1]
Discovery site	Mauna Kea Obs.
Discovery date	29 August 2011[2] (first observation only)
Designations
MPC designation	2011 QF99
Minor planet category	Uranus trojan[3] centaur[2] · distant[1]
Orbital characteristics[2]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 3
Observation arc	3.97 yr (1,449 days)
Aphelion	22.422 AU
Perihelion	15.659 AU
Semi-major axis	19.040 AU
Eccentricity	0.1776
Orbital period (sidereal)	83.08 yr (30,346 days)
Mean anomaly	283.84°
Mean motion	0° 0m 42.84s / day
Inclination	10.833°
Longitude of ascending node	222.52°
Argument of perihelion	288.25°
Physical characteristics
Mean diameter	60 km (calculated)[3]
Geometric albedo	0.05 (assumed)[3]
Absolute magnitude (H)	9.6 (R-band)[3] 9.7[2]

2011 QF₉₉ temporarily orbits near Uranus's L₄ Lagrangian point (leading Uranus). It will continue to librate around L₄ for at least 70,000 years and will remain a Uranus co-orbital for up to three million years. 2011 QF₉₉ is thus a temporary Uranus trojan—a centaur captured some time ago.^[3][6]

Uranus trojans are generally expected to be unstable and none of them are thought to be of primordial origin. A simulation led to the conclusion that at any given time, 0.4% of the centaurs in the scattered population within 34 AU would be Uranus co-orbitals, of which 64% (0.256% of all centaurs) would be in horseshoe orbits, 10% (0.04%) would be quasi-satellites, and 26% (0.104%) would be trojans (evenly split between the L₄ and L₅ groups).^[3] A second Uranian Trojan, 2014 YX49, was announced in 2017.^[7]