2017 OF201: A newly discovered object on the edge of the Solar System, with an orbit that doesn’t quite fit current models. This distant icy world is helping astronomers test one of the most debated ideas in planetary science: the possible existence of a hidden giant planet far beyond Neptune. What can this single object reveal about the structure of our Solar System—and the case for Planet X? Join us in today’s Astrobito to find out more!
Paper details:
Title: Discovery of a dwarf planet candidate in an extremely wide orbit: 2017 OF201
Authors: Sihao Cheng, Jiaxuan Li, and Eritas Yang
First-author institution: Institute for Advanced Study, Princeton, NJ 08540, USA
Status: Acceso abierto en arXiv
Over the last few decades, astronomers have discovered thousands of small icy bodies beyond Neptune’s orbit, offering new insights into the structure and history of the outer Solar System. One of the most recent additions to this growing catalogue is 2017 OF201, a large trans-Neptunian object (TNO) with an unusually wide and very elongated elliptical orbit.
This object is not only notable for its size and distance, but also for how it fits—or doesn’t fit—into current models of orbital dynamics in the outer Solar System. In particular, it raises new questions about the existence of Planet X, a hypothesised distant planet proposed to explain certain patterns seen in the orbits of extreme TNOs.
2017 OF201 was discovered using archival data from the Dark Energy Camera Legacy Survey (DECaLS); a project originally designed to constrain the properties of dark energy and measure the expansion of the universe. By analysing the survey’s images, researchers identified 19 observations of 2017 OF201 across a seven-year period, allowing for precise calculations of its orbit and physical properties.
The orbit of 2017 OF201 is unusual: it has a semi-major axis of 838 AU (that’s its average distance from the Sun) and a perihelion distance of 44.9 AU (its closest distance to the Sun), placing it well beyond the classical Kuiper Belt. This orbit suggests that 2017 OF201 is located at the intersection of the scattered disk and the inner Oort Cloud. Its orbital inclination of 16.2° is relatively low compared to many other distant TNOs, and the object is currently located around 90 AU from the Sun.
2017 OF201 orbit. Figure taken from https://en.wikipedia.org/wiki/2017_OF201
With an estimated diameter of around 700 kilometres and a low variability in brightness, 2017 OF201 is likely spherical and large enough to qualify as a dwarf planet. Its surface appears relatively red in colour—similar to objects like Sedna—suggesting that it may be composed of complex organic materials that have been modified by space weathering.
To understand how objects like 2017 OF201 end up on such distant and detached orbits, astronomers model their long-term dynamical evolution. One plausible scenario begins with gravitational scattering by Neptune. As Neptune migrated outward in the early Solar System, it gravitationally disrupted other objects, scattering them into more distant orbits with high eccentricity. Once an object reaches a large enough distance from the Sun, other forces—such as the Galactic tide (the gravitational influence of the Milky Way)—become important.
Simulations show that over billions of years, these galactic forces can gradually modify an object’s orbit. In particular, they can raise the object’s perihelion, effectively detaching it from Neptune’s influence. This process is consistent with the orbit we observe for 2017 OF201 today. Its current position and trajectory indicate that it is no longer strongly affected by the giant planets and instead evolves mostly under the influence of long-term gravitational forces.
In 2016, a group of researchers proposed that an undiscovered planet—sometimes referred to as Planet Nine or Planet X—could explain why many extreme TNOs share similar orbital orientations. Specifically, several of these objects have longitudes of perihelion that are clustered around 60°, a feature that is statistically unlikely to occur by chance. The gravitational influence of a distant, massive planet could maintain this clustering by preventing the objects from drifting apart over time. In other words, this theory suggests that a hidden giant planet might be gravitationally organising the orbit of these TNOs, keeping them clustered instead of letting them drift randomly.
However, 2017 OF201 does not follow this trend. Its longitude of perihelion is 306°, far from the clustered region of extreme TNOs. This makes it an outlier and provides a new test case for the Planet X hypothesis.
The orbit of 2017 OF201 does not match those of other TNOs, whose orbits appear relatively clustered—presumably due to the gravitational influence of a possible Planet X. Figure adapted from Figure 2 of the original paper.
To explore whether 2017 OF201 could still be consistent with the presence of such a planet, the researchers ran detailed simulations including the gravitational effects of Neptune and Uranus, the influence of the Galactic tide, and, in some scenarios, a modelled Planet X with parameters based on previous proposals (mass ~4 Earth masses, semi-major axis ~296 AU, eccentricity ~0.34). The results were clear: without Planet X, 2017 OF201 remained in a stable orbit for over a billion years. But when Planet X was included, the simulations showed that gravitational interactions would eventually destabilise its orbit, often leading to the object’s ejection from the Solar System within about 100 million years.
Without Planet X, 2017 OF201’s orbit stays stable for over 1 billion years; with Planet X, it’s typically ejected within 100 million years. Figure adapted from Figure 4 inthe original paper.
This suggests that 2017 OF201’s orbit doesn’t fit the pattern that Planet X was proposed to explain. So, this object complicates the Planet X story—but doesn't close the book on it. While not conclusive, it adds another constraint to an already complex picture and highlights the need for more data before any definitive conclusions can be drawn.
2017 OF201 is not just interesting because of its orbit. Its detection also points to the possibility of a much larger population of similar objects. Given that it was visible for only about 0.5% of its orbital period, the probability of discovering it at this particular time was very low. This strongly implies that many more such bodies likely exist, simply too distant or faint to be seen with current telescopes.
If confirmed, this hidden population could account for a significant amount of mass in the outer Solar System—potentially as much as 1% of Earth’s mass, comparable to the estimated total mass of the classical Kuiper Belt. Future observations, particularly from upcoming wide-field surveys such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), may detect more of these distant objects and help build a more complete picture.
The discovery of 2017 OF201 provides valuable information about the structure, dynamics, and history of the Solar System’s outermost regions. It supports models where distant TNOs evolve through the combined influence of Neptune and the Galactic tide. At the same time, it introduces challenges for some versions of the Planet X hypothesis by presenting a case that doesn’t fit the expected orbital clustering.
As more distant objects are discovered and their orbits are measured with increasing precision, our understanding of the outer Solar System will continue to evolve. Whether or not Planet X exists, objects like 2017 OF201 are essential for testing our models of planetary formation, migration, and long-term orbital dynamics. These icy, distant worlds may be small, but they carry important clues about the early history of the Solar System—and they remind us just how much there is still to learn.