A Lonely Ice Giant: Discovering 2017 OF201 in the Outer Reaches of Our Solar System

In a recent study, Sihao Cheng and collaborators announced the discovery of a distant object named 2017 OF201, a candidate dwarf planet with some extraordinary properties. It is currently located over 90 astronomical units (AU) from the Sun—more than twice as far as Pluto—and follows a wide, eccentric orbit that stretches far into the outer solar system. With an estimated diameter of about 700 kilometers and a high likelihood of being in hydrostatic equilibrium, 2017 OF201 could qualify as a dwarf planet. Its discovery provides new insight into the largely unexplored population of icy bodies at the solar system’s edge.

Hunting in Archival Sky Surveys

Rather than using a telescope to search the skies in real time, the authors dug into the Dark Energy Camera Legacy Survey (DECaLS)—an imaging project originally meant to observe distant galaxies. Using a specialized search method adapted for the survey’s irregular sampling, the team identified 2017 OF201 in ten DECaLS images taken between 2014 and 2018. They strengthened their results by finding older detections in images from the Canada-France-Hawaii Telescope dating back to 2011. This seven-year span of observations allowed them to determine the object's orbit with impressive precision.

Measuring an Outlier’s Orbit

2017 OF201 travels on one of the most elongated known orbits of any solar system object, with a semi-major axis of 838 AU and an aphelion—the farthest point from the Sun—of around 1,600 AU. Its closest approach to the Sun, or perihelion, is still a distant 45 AU. The orbit takes approximately 24,000 years to complete. Notably, the object’s longitude of perihelion—a measure of where it comes closest to the Sun—does not match the clustering pattern seen in other extreme trans-Neptunian objects (TNOs). This makes 2017 OF201 a clear outlier and raises questions about the dynamics shaping such distant objects.

Surface Properties and Shape

From the available photometric data, the team measured 2017 OF201’s color and found it to be quite red, consistent with other icy and organic-rich objects in the outer solar system like Sedna. They did not detect significant brightness changes over time, suggesting that the object is roughly spherical and not undergoing rapid rotation or tumbling. This supports the idea that its size is large enough for its gravity to shape it into a near-round form, reinforcing its status as a likely dwarf planet.

Clues About Its Journey

To understand how 2017 OF201 ended up on such an extreme orbit, Cheng and collaborators ran simulations of its orbital evolution. These simulations suggest that it may have started closer to the Sun and been scattered outward by Neptune. Over time, gravitational influences from the Milky Way—known as the Galactic tide—could have gradually altered its orbit to raise its perihelion and lock it into a more detached trajectory. This proposed migration process implies that many more such objects could exist, hidden from our view due to their distance and faintness.

Testing the Planet X Hypothesis

The unusual orbit of 2017 OF201 puts it at odds with the hypothesized Planet X, a yet-undetected giant planet believed by some to cause orbital clustering among distant TNOs. The team tested this idea by simulating how 2017 OF201 would behave in a solar system that included Planet X as proposed by Siraj et al. (2025). The result: 2017 OF201 would likely be ejected from the solar system within 100 million years, which conflicts with its current stable orbit. This finding adds to the growing list of challenges facing the Planet X theory.

Why This Discovery Matters

2017 OF201 is not just another icy rock—it hints at an entire population of similar objects that may still be undiscovered. Because it is only detectable for a small fraction of its long orbit, its discovery implies that hundreds of similar bodies could be lurking in the dark. Collectively, they could represent a significant portion of the solar system’s mass—perhaps 1% of Earth’s mass, roughly equivalent to the Moon. Continued studies of these distant TNOs will shed light on the solar system’s formation and the mysterious forces that continue to shape it.

Source: Cheng