2013 SY99

Lore & Background

2013 SY99 was discovered on 29 September 2013 by the Outer Solar System Origins Survey using the Canada–France–Hawaii Telescope at Mauna Kea Observatory. 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 2013 SY99 is 19,700 years.

As of April 2019, its perihelion distance of q=50.029±0.056 AU and semi-major axis a=690±22 AU make it a possible sednoid according to the most common definition (q>50 AU, a>150 AU), and it is listed as a sednoid by some. However, it is usually considered 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 50 AU only since October 2018. It 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, and it will come to perihelion around 2055 when it will be 50 AU from the Sun.

Reader's Guide

2013 SY99 holds significance in the study of the outer Solar System as one of the most distant known objects with a high perihelion. Its discovery provides additional evidence for the existence of Planet Nine according to astronomers Mike Brown and Konstantin Batygin, though Michele Bannister, one of the astronomers who reported the discovery, disputes this due to the orientation of the orbit. This disagreement highlights the ongoing debate about the influence of a possible ninth planet on the orbits of extreme trans-Neptunian objects. The object's large semi-major axis and high perihelion place it among a rare class of bodies that may have been perturbed by a distant massive planet or by past stellar encounters. Its classification as either a sednoid or an extreme TNO remains unsettled, reflecting the complexity of defining such distant populations. The object's moderately red color and estimated diameter of about 250 km provide clues about its surface composition and formation history. Its upcoming perihelion in 2055 will offer a rare opportunity for detailed observation, as it will then be at its closest approach to the Sun in its nearly 20,000-year orbit.

Did You Know?

• It has the fourth largest semi-major axis for an orbit with perihelion beyond 38 AU.
• Its barycentric orbital period is nearly 20,000 years.
• It will come to perihelion around 2055, at a distance of 50 AU from the Sun.

Discovery Through the Outer Solar System Origins Survey

2013 SY99 was identified as part of the Outer Solar System Origins Survey, a program dedicated to cataloging extreme trans-Neptunian objects. This survey contributed to a growing roster of distant bodies, and by early 2016 the cumulative count of known ETNOs—objects with perihelia beyond 30 AU and semi-major axes exceeding 250 AU—had reached sixteen. 2013 SY99 sits within this small but scientifically rich population. The survey's findings placed it alongside other notable discoveries such as 2015 KG163, 2015 RX245, and 2015 GT50, each with distinct orbital orientations relative to the proposed Planet Nine. The survey's systematic approach helped build the statistical picture that researchers like Trujillo and Sheppard later used to argue for gravitational anomalies in the outer Solar System.

Orbital Profile and Classification

As an extreme trans-Neptunian object, 2013 SY99 orbits the Sun in the outermost reaches of the Solar System, well beyond Neptune's 30 AU boundary. Its semi-major axis places it firmly in the ETNO category, requiring a value of at least 150 to 250 AU. One distinguishing feature noted in the literature is its relatively low orbital inclination compared to many of its peers in the ETNO population. This contrasts with outliers like 2015 BP519, nicknamed Caju, which exhibits both the highest inclination and the farthest nodal distance among known ETNOs. Because of its great distance from the Sun, 2013 SY99 is much less susceptible to gravitational perturbations from the known giant planets than typical trans-Neptunian objects. Like most ETNOs, it is likely a relatively small body, though it can appear comparatively bright when near perihelion in its long elliptical orbit.

Scientific Attention and Public Engagement

2013 SY99 attracted attention in the scientific community beyond its initial survey catalog entry. Michele Bannister presented it during a March 2016 lecture hosted by the SETI Institute, bringing the object to a broader audience interested in outer Solar System dynamics. The object was subsequently discussed again at an October 2016 American Astronomical Society conference, indicating sustained research interest in its orbital properties. These presentations occurred during a period of intense activity in ETNO research, as the total known population was growing and the Planet Nine hypothesis was gaining traction. The fact that a single object was featured in both a public outreach lecture and a major professional conference underscores how 2013 SY99 became a useful reference point in discussions about the statistical anomalies—particularly the nodal asymmetry—observed across the ETNO population.

Place in the ETNO Population and the Planet Nine Question

2013 SY99 belongs to a small group of extreme trans-Neptunian objects whose collective orbital patterns have fueled debate about a possible ninth planet. The known ETNOs display a highly statistically significant asymmetry in the distribution of ascending versus descending nodal distances, a pattern that might indicate a response to external gravitational perturbations. Some researchers propose that a hypothetical Planet Nine could be shepherding these distant bodies into similar orbital configurations. 2013 SY99's lower inclination distinguishes it from more extreme outliers, yet it remains part of the broader population that is largely free from the gravitational influence of Neptune and the other giant planets. Whether its orbit reflects the imprint of an unseen massive body or simply the natural dynamics of the outer Solar System remains an open question in planetary science.