A new official member of the Solar System

 New minor planet of the Solar system just announced, in an extreme orbit   

The Solar System has just gotten a new official member. Currently, with the name of 2017 OF201, this is a trans-Neptunian object (TNO). This means that it orbits the Sun further away than Neptune. Actually, its orbit is so big that it takes about 25,000 years to complete. The object is big enough to be a new dwarf planet and challenge the Planet 9 hypothesis. On 21 May, 2025, the International Astronomical Union’s Minor Planet Centre added a new object to their list: 2017 OF201. This is the largest new object found in our solar system since the minor planet designated 2014 UZ224, which the IAU recognized in 2016. Astronomers found the new object has a diameter somewhere between 290 and 510 miles (470 and 820 km's) across. At its closest, 2017 OF201 comes within 45 astronomical units (1 AU equals the Earth-sun distance) of the sun. At its farthest, it can extend as far as 838 AU from the sun. Currently, the new solar system body is about 90.6 AU distant. Sihao Cheng of the Institute for Advanced Study in New Jersey led the team which made the discovery. They made 10 detections in old survey data from between 2014 and 2018. They were able to put together a tentative orbit for the object and wrote: It became rather clear that they correspond to a single moving object that has an extremely wide and eccentric orbit. 

The discovery of the object was officially announced on 21 May, 2025, by the International Astronomical Union’s Minor Planet Centre. It is big enough to be a dwarf planet. It's distance would make it a trans-Neptunian object. These objects orbit the sun at a distance greater than Neptune, which is 30 AU from the sun. There are many trans-Neptunian objects in our solar system. Pluto is one. So are Eris, Haumea, Makemake and more. In fact, there are some 3,000 known trans-Neptunian objects and about 900 of those have official numbered designations. “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,” discovery team leader Sihao Cheng said. “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.” 2017 OF201 gets so far from the sun that it also qualifies as an extreme trans-Neptunian object. These objects have their longest distance from the sun of at least 150–250 AU. So with a distance that extends more than 800 AU, 2017 OF201 certainly qualifies. 1 % is a small number, and it indicates just how big this TNO’s orbit is. With a diameter a third of Pluto's, it is incredible that it has been spotted.

They also found it was relatively bright compared to other trans-Neptunian objects. After knowing the object’s orbit, they were able to look for it in other survey images and found nine more with the object in exactly the positions they expected. In the end they found data across seven years of the object’s orbit. The object is moving away from us. The paper said: The last time 2017 OF201 passed close to us was in November of 1930, and it will come back again in about 25,000 years. “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,” explained 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.” 

While 2017 OF201 is probably bigger than the dwarf planet Ceres in the asteroid belt, it’s not what astronomers have been looking for to solve the mystery of Planet 9. Notably, the longitude of perihelion of 2017 OF201 lies well outside the clustering observed in extreme trans-Neptunian objects, which has been proposed as dynamical evidence for a distant, undetected planet. The discovery of this object will certainly cause issues in the hunt for Planet 9, a hypothetical planet larger than Earth, expected to exist far beyond the orbit of Pluto. The idea of a planet has been suggested because the orbit of many other TNOs seems to be organized in a specific way. “Many extreme TNOs have orbits that appear to cluster in specific orientations, but 2017 OF201 deviates from this,” explained co-discoverer Jiaxuan Li from Princeton University.

Certainly a spanner in the works for the Planet 9 hypothesis. If more objects like 2017 OF201 are found, it might all come crashing down, and it might not be too difficult. The observations that made this discovery possible came from archival data. More TNOs and dwarf planets might be hiding in them. “All the data we used to identify and characterize this object are archival data that are available to anyone, not only professional astronomers,” explained Li. “This means that ground breaking 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.” The astronomers found 19 detections on data from the Canada France Hawaii Telescope and DECam. These telescopes made the observations between August 2011 and October 2018. In a nutshell, The International Astronomical Union’s Minor Planet Centre announced a new minor planet on May 21, 2025, which is fairly large and has a huge, irregular orbit around the sun.