Beyond the unknown: The coming Kuiper belt revolution

If space is the final frontier, then it’s easy to imagine the Kuiper belt as the wilderness beyond the last ghost town. Out in the far reaches of the Solar System, there are up to millions of unidentified objects, probably a dozen undiscovered Pluto-size worlds, and — maybe — a long-lost ninth planet. Rather than a barren wasteland, the Kuiper belt is the Solar System’s great unknown. Only one dedicated mission has ever ventured there, and we know of more worlds around other stars than we do beyond the orbit of Neptune. 

But change is coming. In the near future, a flood of Kuiper belt discoveries is set to revolutionize our census of the Solar System and help unveil its ancient past. Though these worlds may live in our own backyard, what we learn from them could transform how we think about the entire Cosmos.

The outer wilds 

“The Solar System does not end at Pluto,” said Jim Bell, board member of The Planetary Society and professor in the School of Earth and Space Exploration at Arizona State University. 

As Bell explained, many scientists consider some denizens of the Kuiper belt to be just as compelling as the more famous worlds that live closer to the Sun. The belt, which is really more of a donut-shaped region past the orbit of Neptune, contains thousands of known members. These are called Kuiper belt objects (KBOs), though that term is also often applied to other scattered icy objects farther away. 

The largest KBOs are surprisingly complex, and some are downright strange. There’s Haumea, an oblong dwarf planet with moons and rings that rotates once every four hours — faster than any other object of its size in the Solar System. There’s 307261 Máni, which might have a mountain taller than Olympus Mons despite being nearly 10 times smaller than Mars. And some Kuiper belt worlds are predicted to freeze out their entire atmospheres and then regain them as their orbits take them closer to or farther from the Sun. 

Of all these, Pluto might be the crown jewel. NASA’s New Horizons spacecraft flew by the dwarf planet in 2015, unveiling a surprisingly active world. Pluto hosts jagged mountains, sweeping dunes, and the largest glacier in the Solar System. Ice volcanoes may rise from its surface, and an ocean of liquid water might lurk below.

Recent findings hint that the Kuiper belt is actually full of such surprises. In 2024, researchers using the NASA/ESA/CSA James Webb Space Telescope found that two members of the Kuiper belt, Eris and Makemake, may also show signs of geologic activity. That could mean there are other feature-rich KBOs out there, possibly even with subsurface oceans of their own. 

The suggestion has been met with some skepticism by the scientific community, though. “Everybody wants things to be geologically active,” said Mike Brown, professor of planetary astronomy at the California Institute of Technology. “They might be, but I think the evidence is pretty sparse.” 

Brown argued that we would need a mission to visit these KBOs to know for sure one way or another. Even if the worlds do host subsurface oceans, he added, they might not make for compelling habitats if their water does not interact with a rocky ocean floor. 

“I just don’t buy this idea that the magical phrase ‘ocean world’ suddenly means that you’re at this special place where unicorns swim around with the whales or something,” Brown added. 

With no new missions to the Kuiper belt currently planned, scientists don’t expect this debate to end anytime soon.

Cold cases and clues

But whether or not these worlds harbor oceans, they also offer something else. The same things that make the Kuiper belt hard to explore — that it’s sparse, dark, and distant — also make it a trove for Solar System archaeology. Compared to worlds closer to the Sun, many KBOs have remained virtually unchanged since their birth. 

“They hold little pictures of what the early Solar System was like, and you can read those stories,” said Brown. 

When New Horizons flew by Arrokoth, for example, it discovered the KBO had a snowmanlike shape. This showed that Arrokoth may have formed gently from smaller components rather than through violent collisions. This lent support to the idea that the building blocks of worlds can form as loose piles of “pebbles,” which has helped astronomers predict what planets are like throughout the galaxy. 

There is another chapter of our cosmic history that KBOs can shine light on too. As it turns out, the creation of the Kuiper belt itself was one of the most cataclysmic episodes in the story of the Solar System. Exploring its worlds gives us a glimpse into this massive upheaval that left its imprint everywhere, from the asteroid belt to Neptune.

Solar System shuffle

It all began within the first 100 million years or so after Earth was born. At that time, there was no Kuiper belt. Instead, a much more massive disk of debris circled the Sun, possibly hosting up to thousands of Pluto-size worlds. 

Then, as the giant planets settled into their current positions and Neptune migrated outward, gravitational changes tore this disk apart. Some pieces careened inward and smashed into other worlds. Others were captured into stable orbits as asteroids and comets, while one remnant likely became Neptune’s moon Triton. Much of the rest flew out of the Solar System entirely, turning into interstellar objects — perhaps some other star system’s 3I/ATLAS or ‘Oumuamua, one day.

Only a tiny fraction of the disk survived in the outermost Solar System. That debris formed the Kuiper belt plus a number of farther-off scattered worlds. 

This drama, known as the Nice model (after the city in France), connects the Kuiper belt’s faraway worlds to others throughout the Solar System. Thanks to those pieces of the old disk, some moons of the giant planets still host massive craters today. Certain satellites may have even been disrupted by the collisions and then re-formed multiple times. 

Without the Kuiper belt, the orbits of the giant planets would be different. We wouldn’t have the same Oort cloud. Captured objects like those that help make up the two groups of asteroids that share Jupiter’s orbit (called the Trojans) wouldn’t exist. 

“All of these things are somehow keyed into what was going on in the earliest Kuiper belt,” said Bill Bottke, senior research scientist at Southwest Research Institute. “You can’t have one without the other.”

Lost worlds

The Nice model has become the leading framework for understanding the Kuiper belt’s origins. When scientists study the current orbits of KBOs for clues to this past, though, they find that the Nice model can’t explain everything they see. Something else shaped the Kuiper belt. We don’t yet know what. 

One proposal is that the Solar System hosts another undiscovered planet: a “Planet Nine” several times more massive than Earth. This world might have formed among several others, the rest of which were later scattered or thrown out of the Solar System. Debate over Planet Nine has raged for years, with evidence sometimes seeming to point one way, then another. 

“Both sides are really smart, so I don’t know exactly who is going to win this,” Bottke said. “The stakes are high.” 

Regardless of whether Planet Nine is real, Bottke thinks the Kuiper belt’s history also hints at a lost planet of a different kind. According to Bottke, it’s much easier to explain the captured KBOs we see today if, as the Kuiper belt formed, the Solar System ejected a major planet. 

“I really do think our system once had five giant planets,” Bottke said. “I think we’ve lost a gas giant.” 

There are other possibilities on top of these. The early Solar System might have been affected by the gravity of a nearby star or a large interstellar object. Or it could have ejected two giant planets instead of one. 

Confirming any one of these ideas would utterly shift humanity’s sense of its own past. But for now, they remain just that — ideas. It will take new ongoing efforts to test each alternate history.

The great unknown

Last year, atop a remote mountain in Chile, the Vera C. Rubin Observatory began a decade-long survey of the night sky. Equipped with the largest digital camera in the world, Rubin is expected to discover about 35,000 new KBOs and increase the known population of the Kuiper belt nearly tenfold. Most of these discoveries will come in the first year or two of data. 

“It won’t be slow and steady — it’s going to be a firehose,” said Bottke.

The results are expected to place strong limits on the Nice model, including whether an interstellar object helped rearrange the ancient Solar System. Within its first year, Rubin could also solve the mystery of Planet Nine for good. 

“We will have either found Planet Nine or we will have found definitive evidence that it does or does not exist,” Brown said. “One of those three things is definitely happening.” 

More Kuiper belt discoveries may be on the horizon too. NASA’s Lucy spacecraft, launched in 2021, is currently on its way to explore several of Jupiter’s Trojan asteroids. By studying whether these worlds are actually captured remnants of the old debris disk, Lucy will help us understand how the Solar System rearranged over time. And if we’re lucky, New Horizons could fly by another KBO before it ends its mission. That depends on (among other things) whether Rubin discovers an appropriate target in time. 

While each of these efforts would help investigate the Kuiper belt, Rubin is likely to be the most transformative. Bell said Rubin could mark a revolution for KBOs similar to what NASA’s Kepler mission did for planets around other stars. Just like with exoplanets in the pre-Kepler era, right now we are limited to a “tiny snapshot” of the population of the Kuiper belt, he explained. That snapshot might be representative, or the most easy-to-spot KBOs might be nothing like the others. 

The Kuiper belt already hosts oblong worlds, seasonal atmospheres, and giant glaciers. There is no telling what unexpected wonders remain to be discovered. 

“The diversity of worlds is going to expand dramatically,” said Bell, “and there will be surprises.”