3 days ago
Search for 'Planet Nine' Yields Unexpected Discovery
Astronomers have long been on the hunt for "Planet Nine," a hypothetical planet that lies within our solar system just beyond Neptune. Scientific evidence has alternately pointed to and against the existence of such a planet, but the search is still on, stoked by the 2006 demotion of Pluto to "dwarf planet." This time, scouring the cosmos may have yielded a concrete result—it's just not the kind astronomers were hoping for.
A small team of researchers at the Institute for Advanced Study and Princeton University spent half a year sifting through data from the Victor M. Blanco Telescope's Dark Energy Camera Legacy Survey (DECaLS). Gathered in 2019 by the telescope's Dark Energy Camera, or DECam, the archive consists of wild-field optical imaging data from the green, red, and z bands. Though these filters make it possible to search for distant space objects via photometric redshift, no one (to the researchers' knowledge) had looked for Planet Nine in the DECaLS dataset before.
According to a preprint paper that has not yet undergone peer review, the researchers found a dwarf planet candidate they've since dubbed 2017 OF201. With an estimated diameter of approximately 700 kilometers (435 miles), the object is just big enough to classify as a dwarf planet. The International Astronomical Union's Minor Planet Center announced the finding this month.
This illustration shows just how wide 2017 OF201's orbit is, compared with the orbits of solar system planets. Credit: Jiaxuan Li, Sihao Cheng
But the team is lucky they found 2017 OF201 at all: Only 0.5% of its wide, elliptical orbit comes close enough to Earth for Blanco to detect. At its farthest point from the Sun (aphelion), the object is more than 1,600 times farther away than Earth, making a complete orbit 25,000 years long.
"This limited visibility window strongly suggests that a substantial population of similar objects—with large sizes, wide orbits, and high eccentricities—should exist but be difficult to detect due to their extremely large distance," the paper reads.
The orbit of 2017 OF201 is also strange because it appears to contradict a common hypothesis about Planet Nine. The hypothetical planet's gravity is thought to shepherd trans-Neptunian objects into a cluster of sorts, but 2017 OF201 resists such clustering. What this means within the broader search for Planet Nine will likely be determined by Chile's Vera Rubin Observatory, which is expected to go online later this year.
