Scientists Discover New Dwarf Planet in Solar System, Far Beyond Pluto
It's called 2017 OF201, a rock that appears to be some 700 kilometers (435 miles) across, large enough to qualify as a dwarf planet. What makes it even more interesting is its orbit, which implies that there is no giant Planet Nine, somewhere out there in the dark outer wilds of the Solar System.
"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," says astrophysicist Sihao Cheng of Princeton University. "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."
Cheng and his colleagues have been undertaking a campaign to find and study trans-Neptunian objects (TNOs), chunks of rock and ice that orbit the Sun out beyond the orbit of Neptune at about 30 astronomical units (where one astronomical unit is the distance between Earth and the Sun). Finding these objects is hard – they're very small and, that far from the Sun, very cold, and reflect very little light.
In recent years, more powerful instruments have emerged that are better at peering into the Kuiper Belt and beyond to identify individual objects there. The most distant object detected to date is FarFarOut, a rock about 400 kilometers across, found at a distance of 132 astronomical units.
Cheng and his colleagues found 2017 OF201 in archival data collected by the Dark Energy Camera Legacy Survey (DECaLS) and the Canada France Hawaii Telescope (CFHT). Between 2011 and 2018, DECaLS and CFHT managed to observe 2017 OF201 a total of 19 times – data that allowed the team to characterize the object and its orbit with a high degree of certainty.
2017 OF201 was initially spotted at a distance of 90.5 astronomical units, more than twice Pluto's orbital distance of about 40 astronomical units. Its orbit is an extreme ellipse, bringing it in as close as 44 astronomical units and carrying it out as far as 1,600 astronomical units – into the inner Oort Cloud, the cloud of rocks and ice that surrounds the Solar System at its very outer limits.
We don't know how this orbit, which takes 25,000 years to complete, came into existence. It's possible that 2017 OF201 had a gravitational interaction with something large that knocked it for a literal loop, or that the evolution of its orbit was a multi-step process.
What is clear, however, is that it's a very different orbit from the grouped orbits of previously discovered TNOs that some astronomers thought were diagnostic of a large, unseen planet in the outer Solar System.
In fact, the team even conducted simulations of 2017 OF201's orbit in the Solar System, both with and without a Planet Nine. They found that, without Planet Nine, 2017 OF201 can have a stable, long-term orbit, as it does today. With Planet Nine, however, gravitational interactions with Neptune boot 2017 OF201 clean out of the Solar System within 100 million years.
It's one of the strongest pieces of evidence yet against the existence of Planet Nine; but it also implies that there are a lot more objects like it that we haven't found in the Kuiper Belt, and beyond.
"2017 OF201 spends only one percent 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," Cheng says.
"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 has been officially announced by the International Astronomical Union, and described in a paper available on preprint website arXiv.
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