Astrophysicists say they’ve identified an object beyond the orbit of Neptune that’s likely to qualify as a dwarf planet, alongside other trans-Neptunian objects including Pluto, the erstwhile “ninth planet.”
The discovery of 2017 OF201 touches upon another ninth-planet controversy: namely, whether there’s a large planet nicknamed Planet 9 or Planet X lurking somewhere on the edges of the solar system.
2017 OF201 was discovered as part of a research project aimed at identifying trans-Neptunian objects in the outer solar system. A trio of researchers at the Institute for Advanced Study and Princeton University used advanced computational methods to analyze seven years’ worth of images captured by the Victor M. Blanco Telescope and the Canada France Hawaii Telescope.
The algorithm identified 19 images that pinpointed 2017 OF201’s movement through the night sky. That led to a determination of the object’s orbit and size. This week, the International Astronomical Union’s Minor Planet Center published an electronic circular detailing the mini-world’s orbital parameters, and the Princeton researchers submitted a pre-print study for review.
The object is thought to be about 700 kilometers (435 miles) in diameter. Further observations, perhaps using radio telescopes, will be needed to firm up that estimate. But if the measurements are correct, 2017 OF201 could qualify as a dwarf planet.
So far, the International Astronomical Union has given formal designation to five dwarf planets in our solar system: Pluto, Ceres, Eris, Makemake and Haumea. Such worlds are massive enough to crush themselves into a ball (or, in Haumea’s case, something like a football), but not big enough to be gravitationally dominant in their orbits.
The smallest of those five dwarf planets, Ceres, has a diameter of roughly 940 kilometers (585 miles).
Other solar system objects — for instance, Quaoar, Sedna, Orcus and Gonggong — are generally thought to be dwarf planets as well, even though the IAU hasn’t yet added them to its list. One candidate, the asteroid Hygiea, has a diameter of only 430 kilometers (267 miles). Now there’s yet another candidate for the IAU’s list.
A composite image shows the five dwarf planets recognized by the International Astronomical Union, plus an image of 2017 OF201. (Credit: NASA / JPL-Caltech / Cheng et al.)
2017 OF201’s extremely eccentric orbit is also notable.
“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,” study lead author Sihao Cheng, an astrophysicist at the Institute for Advanced Study, said today in a news release. “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.”
2017 OF201 takes 25,000 years to make a complete orbit — and Princeton astrophysicist Eritas Yang said the orbit’s extreme eccentricity suggests a complex history of gravitational interactions with other solar system bodies.
“It must have experienced close encounters with a giant planet, causing it to be ejected to a wide orbit,” said Yang, a co-author of the study.
Cheng said there may have been more than one step in 2017 OF201’s migration to its oddball orbit. “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 said.
2017 OF201 has an extremely eccentric orbit compared to the orbits of Neptune and Pluto. (Credit: Jiaxuan Li, Sihao Cheng)
The characteristics of the object’s orbit challenge one of the claims relating to the hypothetical Planet X or Planet 9. Although many trans-Neptunian objects appear to cluster in specific orientations, “2017 OF201 deviates from this,” said Princeton’s Jiaxuan Li, another author of the study.
The apparent clustering of trans-Neptunian objects has been cited as indirect evidence for the existence of Planet 9, which was thought to be gravitationally shepherding those objects into clusters.
“The existence of 2017 OF201 may be difficult to reconcile with this particular instantiation of the Planet X hypothesis,” the study’s authors wrote. “While not definitive, 2017 OF201 provides an additional constraint that complements other challenges to the Planet X scenario, such as observational selection effects and the statistical robustness of the observed clustering.”
Even if the clustering hypothesis is a dead end, that wouldn’t necessarily rule out the existence of Planet 9. Last month, a different team of astronomers said they picked up potential hints of just such a planet in archival astronomical imagery. Like 2017 OF201, the object they observed is in a location that wouldn’t be consistent with the clustering hypothesis, thus adding another twist to the controversy.
The next phase in the search for Planet 9 is due to begin this year when the Vera C. Rubin Observatory begins science operations. The Rubin Observatory is thought to have enough sensitivity and a wide enough field of view to pick up definitive signs of Planet 9 — if it exists.
Correction: This report has been revised to make clear that the Institute for Advanced Study and Princeton University are separate institutions.
