On May 21, the International Astronomical Union’s Minor Planet Center announced the discovery of 2017 OF201, which, because of its distance beyond Neptune, is classified as a trans-Neptunian object (TNO).
The science team that found the new body was led by Sihao Cheng, Martin A., and Helen Chooljian of the Institute for Advanced Study’s School of Natural Sciences. Cheng made the discovery with Jiaxuan Li and Eritas Yang from Princeton University. They used sophisticated methods to identify the object’s orbit. In fact, they identified 2017 OF201 in 19 different images, captured over seven years. The new TNO is special for two reasons: its extreme orbit and its large size.
Characteristics
“The object’s aphelion — the farthest point on the orbit from the Sun — is more than 1,600 times that of the Earth’s orbit,” explains Cheng. “Meanwhile, its perihelion — the closest point on its orbit to the Sun — is 44.5 times that of the Earth’s orbit, similar to Pluto’s orbit.” Pluto’s orbital extremes are 29½ times the Earth-Sun distance at its closest and 49⅓ times that distance at its farthest.
The orbit of 2017 OF201 means that it takes it approximately 25,000 years to travel just once around the Sun. “It must have experienced close encounters with a giant planet, causing it to be ejected to a wide orbit,” says Yang. “There may have been more than one step in its migration. It’s possible that this object was first ejected to the Oort cloud, the most distant region in our solar system, which is home to many comets, and then sent back,” Cheng adds.
The size of 2017 OF201 — the team estimates its diameter to be 435 miles (700 km) —also sets it apart. If confirmed, that size would make it the second largest known TNO. As a comparison, Pluto’s diameter is 1,477 miles (2,377 km).
Bad news for Planet Nine?
“Many extreme TNOs have orbits that appear to cluster in specific orientations, but 2017 OF201 deviates from this,” says Li. Some researchers have interpreted this clustering as indirect evidence for the existence of Planet Nine, whose gravity could be shepherding these objects. The existence of 2017 OF201 as an outlier to such clustering could challenge this hypothesis.
Whatever that result turns out to be, the discovery will help us better understand the outer solar system. Many scientists think the area beyond the Kuiper Belt, where 2017 OF201 is located, is essentially empty, but the team’s discovery suggests that this is not the case.
“2017 OF201 spends only 1 percent of its orbital time close enough to us to be detectable. The presence of this single object suggests that there could be another hundred or so other objects with similar orbit and size; they are just too far away to be detectable now,” Cheng says. “Even though advances in telescopes have enabled us to explore distant parts of the universe, there is still a great deal to discover about our own solar system.”
