Ultrawide binary objects in the Kuiper belt may not have come from the earliest solar system, research suggests

Trying to understand the makeup and evolution of the solar system’s Kuiper belt has kept researchers busy since it was hypothesized soon after the discovery of Pluto in 1930. In particular, binary pairs of objects there are useful as indicators since their existence today paints a picture of how energetic or violent the evolution of the solar system was in its early days four billion years ago.

Looking closely at the evolution of an ultrawide (in separation) binary object, researchers included more physics that reveals much about their architecture and unfolding. They found that these ultrawide binaries may not have been formed in the primordial solar system as has been thought. Their work has been published in Nature Astronomy.

“In the outer reaches of the solar system, there exists a population of binary systems so widely separated that it seemed worth looking into whether or not they could even survive 4 billion years without being [completely] separated somehow,” said Hunter M. Campbell of the University of Oklahoma in the US.

“If they formed in the early Kuiper belt, and survived this long, you could make a bunch of conclusions about how energetic or violent the evolution of the solar system was in its early days.”

The Kuiper belt is the torus-shaped region of the solar system containing planetesimals and smaller bodies left over from the formation of the solar system. It begins at about the orbit of Neptune, which averages 30 astronomical units (AUs) from the sun, and extends to about 55 AU, inclined within 10° of the Earth’s ecliptic plane.

More massive than the asteroid belt by 20 to 200 times, it consists of small remnants of the solar system’s formation—most are icy volatiles made up of molecules such as methane, ammonia and water. Within it lies the dwarf planets Pluto, Eris, Orcus and more. It’s thought that there exist more than 100,000 Kuiper belt objects over 100 km in diameter.

The Cold Classical Kuiper Belt Objects, in a subset of the Kuiper Belt, are a class of small bodies with undisturbed orbits beyond Neptune’s orbit; these objects are primitive and preserve information about the formation of the solar system. They never migrated as Neptune migrated outward from the sun during the very early solar system. This region has the most ultra-wide binaries (UWBs)—nearly one-third of objects in this region are binary, gravitationally bound to another object, and several percent of those are ultra-wide binaries (UWBs), with object diameters of roughly 100 km but separated by tens of thousands of kilometers.

“Many works in the past have examined binary evolution as driven by collisions with passing bodies,” said Campbell. “Our work examines evolution driven by gravitational perturbations.”

Despite their rarity and despite their susceptibility to disruption, today’s UWBs have been utilized to constrain their minimum distance from Neptune in the early solar system, and the approximate number of kilometers-sized trans-Neptunian objects (TNOs) in today’s Kuiper belt.

However, it has been implicitly assumed that the architecture of the UWBs, with their wide spacing, came from the early, primordial solar system. But Campbell and his team wondered, could there originally have been tightly bound binary objects that, via collisions with TNOs through the eons, lost part of their grip on one another and, though still bound, had their separation evolve to ultrawide.

However, studies have shown that the number of passing or impacting TNOs in the modern Kuiper belt is too small to significantly produce the population of UWBs.

As Neptune migrated away from the sun (from 24 to 30 AU), some of these objects dynamically diffused until they began interacting strongly with the solar system’s giant outer planets, when they were ejected from the solar system or trapped in the Oort cloud.

About 99 to 99.9% of primordial belt planetesimals are thought to be ejected from the dynamic Kuiper Belt, objects that formed much closer to the sun and migrated into their present orbits as they were pushed out by Neptune. Since an affected primordial belt object takes at least 10 million years to be removed, Campbell and his colleagues wondered whether including the many cold classical belt crossings of such disturbed TNOs might be substantially higher than inferred from modern observations, which would expose binaries to greater gravitational perturbations.

That is indeed what their simulation of the Kuiper belt’s evolution found. The UWBs seem to not be primordial, so they cannot constrain the early solar system as has been thought.

“From our findings, it seems that these perturbations are quite significant, to the point where those wide binaries probably could not have survived very long,” said Campbell.

“But the perturbations are also able to make more of those wide binaries, slowly pulling initially tighter and more stable binaries further apart until they become wide.”

By subjecting the early binaries to four billion years of flybys from these different TNO paths, they found that “the widening of tighter TNO binaries into UWB arrangements was not uncommon in our simulations.”

They calculate that TNO passages would widen as many as 10% of moderately tight binaries into UWBs over the 4 billion year span of the solar system. But the result doesn’t hold for tighter binaries.

“As more Kuiper belt binaries are discovered and we get a better sense of the wide binary population, we can narrow down constraints on the evolution of the Kuiper belt and the giant planets that drove it,” said Campbell.

As similar KB disks have been seen in other star systems, “If we know more about how ours formed, we can learn a bit more about how those others formed and make some guesses about what other planets might be lurking there that are just too hard to see otherwise.”

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