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.