2013 SY99

, also known by its OSSOS survey designation uo3L91, is a trans-Neptunian object discovered on September 29, 2013 by the Outer Solar System Origins Survey using the Canada–France–Hawaii Telescope at Mauna Kea Observatory. This object orbits the Sun between 50 and 1300 AU, and has a barycentric orbital period of nearly 20,000 years. It has the fourth largest semi-major axis for an orbit with perihelion beyond 38 AU. has one of highest perihelia of any known extreme trans-Neptunian object, behind sednoids including Sedna (76 AU), (80 AU), and Leleākūhonua (65 AU).

Discovery
According to astronomers Mike Brown and Konstantin Batygin, the discovery of provides additional evidence for the existence of Planet Nine, but Michele Bannister (see ), one of the astronomers who reported the discovery of this object, disputes this due to the orientation of the orbit.

Its existence was announced in 2016, but the observations were kept private until 2017. It was listed at the Minor Planet Center and the JPL Small-Body Database on 6 April 2017 with a three-year observation arc and an epoch 2017 heliocentric orbital period of 17,500 years. Barycentric orbital solutions, however, are more stable for objects on multi-thousand year orbits, and the barycentric period for is 19,700 years.

As of April 2019, its perihelion distance of q=$0.927$ and semi-major axis a=$19,700 yr$ make a possible sednoid, according to the most common definition of the term (q>$18,142.4 yr$, a>$4.228 °$). It is listed as a sednoid by some. However, is usually considered to be an extreme trans-Neptunian object and not a sednoid, due to its high eccentricity which makes the heliocentric orbit unstable. In the heliocentric reference frame, the perihelion is currently rising, and the nominal orbit has a perihelion distance above $29.493 °$ only since October 2018.

is estimated to be about 250 km in diameter and moderately red in color. In 2052 it will be roughly 20.3 AU from Neptune. It will come to perihelion (closest approach to the Sun) around 2055 when it will be 50 AU from the Sun.