New Kuiper Belt objects lurk farther away than we ever thought

Earth's Kuiper Belt appears to be substantially larger than we thought.

Back in 2017, NASA graphics indicated that New Horizons would be at the outer edge of the Kuiper Belt by

around 2020. That hasn't turned out to be true.

Credit: NASA

In the outer reaches of the Solar System, beyond the ice giant Neptune, lies a ring of comets and dwarf planets known as the Kuiper Belt. The closest of these objects are billions of kilometers away. There is, however, an outer limit to the Kuiper Belt. Right?

Until now, it was thought there was nothing beyond 48 AU (astronomical units) from the Sun, (one AU is slightly over 150 million km). It seemed there was little beyond that. That changed when NASA’s New Horizons team detected 11 new objects lurking from 60 to 80 AU. What was thought to be empty space turned out to be a gap between the first ring of Kuiper Belt objects and a new, second ring. Until now, it was thought that our Solar System is unusually small when compared to exosolar systems, but it evidently extends farther out than anyone imagined.

While these objects are only currently visible as pinpoints of light, and Fraser is allowing room for error until the spacecraft gets closer, what their existence could tell us about the Kuiper Belt and the possible origins of the Solar System is remarkable.

Living on the edge

The extreme distance of the new objects has put them in a class all their own. Whether they are similar to other Kuiper Belt objects in morphology and composition remains unknown since they are so faint. As New Horizons approaches them, observations are now simultaneously being made with its LORRI (Long Range Reconnaissance Imager) telescope and the Subaru Telescope, which might reveal that they actually do not belong to a different class in terms of composition.

“The reason we’re using Subaru is its Hyper Suprime-Cam, which has a really wide field of vision,” New Horizons researcher Wesley Fraser, who led the study, told Ars Technica (the results are soon to be published in the Planetary Science Journal). “The camera can go deep and wide quickly, and we stare down the pipe of LORRI, looking down that trajectory to find anything nearby.”

These objects are near the edge of the heliosphere of the Solar System, where it transitions to interstellar space. The heliosphere is formed by the outflow of charged particles, or solar wind, that creates something of a bubble around our Solar System; combined with the Sun’s magnetic field, this protects us from outside cosmic radiation.

The new objects are located where the strength of the Sun’s magnetic field starts to break down. They might even be far enough for their orbits to occasionally take them beyond the heliosphere, where they will be pummeled by intense cosmic radiation from the interstellar medium. This, combined with their solar wind exposure, might affect their composition, making it different from that of closer Kuiper Belt objects.

Even though it is impossible to know what these objects are like up close for now, how can we think of them? Fraser has an idea.

“If I had to guess, they are probably red and dark and devoid of water ice on the surface, which is quite common in the Kuiper Belt,” he said. “I think these objects will look a lot like the dwarf planet Sedna, but it’s possible they will look even more unusual.”

Many Kuiper Belt objects are a deep reddish color as a result of their organic chemicals being exposed to cosmic radiation. This breaks the hydrogen bonds in those chemicals, releasing much of the hydrogen into space and leaving behind an amorphous organic sludge that keeps getting redder the longer it is irradiated.

Fraser also predicts these objects are lacking in surface water ice because more distant Kuiper Belt objects (though not nearly as far-flung as the newly discovered ones) have not shown signs of it in observations. While water ice is common in the Kuiper Belt, he thinks these objects are probably hiding water ice underneath their red exterior.

Emerging from the dark

Investigating objects like this could change views on the origins of the Solar System and how it compares to the exosolar systems we have observed. Is our Solar System even normal?

Because the Kuiper Belt was thought to end at a distance of about 48 AU, the Solar System used to seem small compared to exosolar systems, where there are still objects floating around 150 AU from their star. The detection of objects at up to 80 AU from the Sun has put the Solar System in more of a normal range. It also seems to suggest that, since it is larger than we thought, that it also formed in a larger nebula.

“The timeline for Solar System formation is what we have to work out, and looking at the Kuiper Belt sets the stage for that very earliest moment, when gas and dust start to coalesce into macroscopic objects,” said New Horizons researcher Marc Buie. Buie discovered the object Arrokoth and led another study recently published in The Planetary Science Journal.

Arrokoth itself altered ideas about planet formation since its two lobes appear to have gently stuck together instead of crashing into each other in a violent collision, as some of our ideas had assumed. Nothing like it has ever been observed before or since.

Dust to dust

There is another potential thing that the New Horizons team is watching out for, and that is whether the new objects are binary.

About 10 to 15 percent of all known Kuiper Belt objects orbit partners in binary systems, and Fraser thinks binarity can reveal many things about the formation of planetesimals, solid objects that form in a young star system through gentle mergers with other objects that cause them to stick together. Some of these objects can become gravitationally bound to each other and form binaries.

As New Horizons travels farther, its dust counter, which sends back information about the velocity and mass of dust that hits it, shows that the amount of dust in its surroundings has not gone down. This dust comes from objects running into each other.

“It’s been finding that, as we go farther and farther out, the Solar System is getting dustier and dustier, which is exactly the opposite of what is expected at that distance,” New Horizons Principal Investigator Alan Stern told Ars Technica. "There might be a massive population of bodies colliding out there.”

NASA had previously decided that it was unlikely New Horizons would be able to pull off another Kuiper Belt object flyby like it did with Arrokoth, so the mission’s focus shifted to the heliosphere. Now that the New Horizons team has found unexpected objects this distant with the help of the Subaru Telescope, and dust keeps being detected as the spacecraft travels farther out, there might be an opportunity for another flyby. Stern is still cautious about the chances of that.

“We’re going to see how they compare to closer Kuiper Belt objects, but if we can find one we can get close to, we’ll get a chance to really compare their geology and their mode of origin,” Stern said. “But that’s a longshot because we’re running on a tenth of a tank of gas.”

The advantage of using Subaru combined with LORRI is that LORRI can be pointed sideways to see objects, or at least slightly past them, at right angles. This will be the dream team of telescopes if New Horizons can approach at least one of the new objects. If an object is behind the spacecraft, combining observations from different angles gives information about the physical surface of an object.

Using the Nancy Grace Roman Telescope could yield even more surprising observations in the future. It has a smaller mirror and a very wide field of view, Stern likens it to space binoculars, and it only has to be pointed at a target region once or twice (in comparison to hundreds of times for the James Webb Space Telescope) to search for and possibly discover objects in an extremely vast expanse of sky. Most other telescopes would have to be pointed thousands of times to do that.

“The desperate hope for all of us is that we will find more flyby targets,” Buie said. “If we could just get an object to register as a couple of pixels on LORRI, that would be incredible.”

Just a note to you on some stuff that’s going on in the background here. About a year ago, NASA decided that another KBO flyby was really unlikely, so they switched the mission focus to heliophysics (i.e., the edge of the heliosphere). Stern tried to fight that, and he has really looked to keep the focus on KBOs, which NASA now considers a “if we find one it can image, it will” situation. So I think a lot of his phrasing is in keeping with what he wants—more flybys. But it’s our job to give an accurate picture, which is that this event is unlikely.

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