The TNO is potentially large enough to qualify as a dwarf planet, the same category as the much more well-known Pluto. The new object is one of the most distant visible objects in our solar system and, significantly, suggests that the empty section of space thought to exist beyond Neptune in the Kuiper Belt is not, in fact, empty at all.

Cheng made the discovery alongside colleagues Jiaxuan Li and Eritas Yang from Princeton University, using advanced computational methods to identify the object's distinctive trajectory pattern on the sky. The new object was officially announced by the International Astronomical Union's Minor Planet Center and in an arXiv pre-print.

Trans-Neptunian objects are minor planets that orbit the Sun at a greater average distance than the orbit of Neptune. The new TNO is special for two reasons: its extreme orbit and its large size.

"The object's aphelion -- the farthest point on the orbit from the Sun -- is more than 1600 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."

This extreme orbit, which takes the object approximately 25,000 years to complete, suggests a complex history of gravitational interactions. "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.

"Many extreme TNOs have orbits that appear to cluster in specific orientations, but 2017 OF₂₀₁ deviates from this," says Li. This clustering has been interpreted as indirect evidence for the existence of another planet in the solar system, Planet X or Planet Nine, which could be gravitationally shepherding these objects into their observed patterns. The existence of 2017 OF₂₀₁ as an outlier to such clustering could potentially challenge this hypothesis.

Cheng and his colleagues estimate 2017 OF₂₀₁'s diameter to be 700 km, which would make it the second largest known object in such a wide orbit. The diameter of Pluto, meanwhile, is 2,377 km. Further observations, potentially using radio telescopes, are needed to determine the object's exact size.

Cheng discovered the object as part of an ongoing research project to identify TNOs and possible new planets in the outer solar system. The object was identified by pinpointing bright spots in an astronomical image database from the Victor M. Blanco Telescope and Canada France Hawaii Telescope (CFHT), and trying to connect all possible groups of such spots that appeared to move across the sky in the way a single TNO might. This search was carried out using a computationally efficient algorithm produced by Cheng. Ultimately, they identified 2017 OF₂₀₁ in 19 different exposures, captured over 7 years.

The discovery has significant implications for our understanding of the outer solar system. The area beyond the Kuiper Belt, where the object is located, has previously been thought to be essentially empty, but the team's discovery suggests that this is not so.

"2017 OF₂₀₁ spends only 1% 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 states. "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."

The detection also demonstrates the power of open science. "All the data we used to identify and characterize this object are archival data that are available to anyone, not only professional astronomers," says Li. "This means that groundbreaking discoveries aren't limited to those with access to the world's largest telescopes. Any researcher, student, or even citizen scientist with the right tools and knowledge could have made this discovery, highlighting the value of sharing scientific resources."

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