Astronomers Are Closing In on the Kuiper Belt’s Secrets

Beyond the Neptune’s orbit is home to a vast ring of ancient relics, dynamic puzzles, and perhaps a hidden planet or two.

The Kuiper Belt, a region of frozen debris about 30 to 50 times farther from the sun than Earth – and possibly further, although no one knows – has been shrouded in mystery since it first appeared in the 1990s.

Over the past 30 years, astronomers have cataloged about 4,000 Kuiper Belt Objects (KBOs), including a handful of dwarf worlds, icy comets, and planetary remnants. But that number is expected to increase tenfold in the coming years as observations from more advanced telescopes flood in. In particular, the Vera C. Rubin Observatory in Chile will shed light on this murky region with its flagship project, the Legacy Survey of Space and Time (LSST), which began operating last year. Other next-generation observatories, such as the James Webb Space Telescope (JWST), will also help shed light on the belt.

“Beyond Neptune, we have a census of what’s in the solar system, but it’s a mosaic of surveys, and it leaves a lot of room for things that might be there that have been missed,” says Renu Malhotra, the Louise Foucar Marshall Professor of Research Science and Regents Professor of Planetary Science at the University of Arizona.

“I think that’s the big thing that Rubin is going to do: fill in the gaps in our knowledge of what’s in the solar system,” she adds. “This will greatly advance our census and our knowledge of the contents of the solar system.”

As a result, astronomers are bracing for a flood of discoveries about this new frontier, which could shed light on a host of outstanding questions. Are there new planets hidden within the belt, or lurking beyond? How far does this region extend? And are there traces of cataclysmic past encounters between worlds – both local or interstellar – imprinted in this largely pristine collection of objects from the deep past?

“I think this will become a very hot area very soon, thanks to LSST,” says Amir Siraj, a graduate student at Princeton University who studies the Kuiper belt.

The Kuiper Belt is a graveyard of planetary junk scattered far from the sun during the disorderly birth of the solar system about 4.6 billion years ago. Pluto was the first KBO ever spotted, more than half a century before the belt itself was discovered.

Since the 1990s, astronomers have discovered a handful of other dwarf planets in the belt, such as Eris and Sedna, as well as thousands of smaller objects. Although the Kuiper Belt is not completely static, it is, in essence, an intact time capsule of the early solar system that can be mined for clues to planet formation.

For example, the belt contains strange structures that could be signatures of past encounters between giant planets, including a particular group of objects, known as the “core,” located about 44 astronomical units (AU) away, where one AU is the distance between Earth and the sun (about 93 million miles).

Although the origin of this core is still unexplained, a popular hypothesis is that its constituent objects, known as cold classics, were driven by Neptune’s outward migration through the solar system more than 4 billion years ago, which could have been a rocky journey.

The idea is that “Neptune was shaken by the rest of the gas giants and did a little jump; it’s called the ‘Neptune jump’ scenario,” says Wes Fraser, an astronomer at the Dominion Astrophysical Observatory of the National Research Council of Canada, who studies the Kuiper Belt, noting that astronomer David Nesvorný came up with the idea.