At the edge of our solar system lies the Kuiper Belt, a ring-shaped region populated by small, icy objects such as asteroids, comets, and dwarf planets. Right now, NASA’s New Horizons spacecraft is speeding through the Kuiper Belt on its way out of the solar system.
The Kuiper Belt extends from 30 to 50 astronomical units (AU) from the Sun. After its illuminating flybys of Pluto and Arrokoth, astronomers thought the New Horizons journey through the Kuiper Belt would be relatively uneventful and more close encounters would be nearly impossible.
“Beyond about 55 AU, there just isn’t anything,” explained Wesley Fraser, an astronomer at the National Research Council of Canada in Vancouver, B.C., and a New Horizons team member. “We call that the Kuiper cliff.”
But New Horizons still has a small amount of fuel left, about 5 kilograms of its initial 77 kilograms of hydrazine, so if another suitable target could be found, the spacecraft might be able to do one more flyby, a last hurrah for the nearly 20-year-old mission.
A new discovery might reinvigorate the search for a new target. Fraser and his colleagues used Japan’s Subaru Telescope in Hawaii to scour the distant solar system, and though they didn’t find anything in the spacecraft’s path, they did find a strange cluster of 11 objects at about 70–80 AU from the Sun.
The discovery, published in the Planetary Science Journal, suggests the Kuiper cliff isn’t a sharp cutoff and may even hint at a ring of more distant objects beyond the Kuiper Belt. “That was unexpected,” Fraser said.
Shift and Stack
The objects were found between 2020 and 2022 using a technique called shift and stack. Essentially, the researchers took repeated snapshots of the outer solar system in the hope that photons from small, faint objects might slowly start to emerge.
“We get a decent measure of their inclination and their distance, and that’s about it. We don’t really know anything more about them.”
“It’s a trick to add all of your exposures together,” said Megan Schwamb, a planetary scientist at Queen’s University Belfast in Northern Ireland who was not involved in the study. Shift and stack is “computationally intensive,” she added, because astronomers need to predict in which direction objects might be orbiting to collect exposures in the same direction.
The views of objects captured with this technique are fleeting, and so the information on offer is limited. The team doesn’t know the exact orbits of the objects, their chemistry, or their sizes, although their dimness suggests they are no more than several hundred kilometers across. “We get a decent measure of their inclination and their distance, and that’s about it,” Fraser said. “We don’t really know anything more about them.”
Resonance
Nevertheless, the mere existence of the objects might suggest there is a sudden uptick in the population of the Kuiper Belt around this region, potentially even another ring, or belt, of hundreds of thousands of objects encompassing the solar system. “The 11 objects we found imply a 4-times-larger number of objects at that sort of distance than we would have expected,” Fraser said.
A separate survey of the outer solar system called DEEP (DECam Ecliptic Exploration Project) has not seen such evidence, however. “They don’t find the buildup of objects,” Fraser said. That discrepancy could suggest the 11 objects are a statistical fluke or perhaps not real objects at all.
“The easiest out is either they’re wrong or we’re wrong,” said Fraser, and Schwamb added that “we need more observations to really tell” whether there is another belt.
There might be another possibility. Fraser suggested that the objects could be groups of asteroids and comets that orbit in the outer solar system at a particular resonance. That is, they move closer to and farther away from the Sun in tandem with the gravitational pull of the planet Neptune. “Like a snowplow, these resonances will gobble up objects and trap a bunch of them,” Fraser said.
“If they find a target, I’ll be ecstatic.”
One way to test this idea will be with the Vera C. Rubin Observatory in Chile, set to begin a 10-year survey of the sky next year. This survey will include observations of the outer solar system, and it could turn up many new objects—perhaps in resonances as Fraser suggested—or even the existence of another ring beyond the Kuiper Belt.
While that telescope is not intended to hunt for extremely dim objects like those seen by Fraser and his colleagues, it might be able to spot objects the size of dwarf planets that might be hiding in this distant region of the solar system. “We would be able to see those,” Schwamb said.
New Horizons is currently 60 AU from the Sun, headed toward this potentially crowded neighborhood. An ideal outcome of the Rubin survey, Fraser said, would be to find a new target within the spacecraft’s path, although it’s an outcome he called a “pipe dream.”
“If they find a target, I’ll be ecstatic,” Schwamb said.
—Jonathan O’Callaghan (@astro_jonny), Science Writer
