Hubble Uncovers a Hidden Trio That Could Rewrite Kuiper Belt History

This artist’s concept depicts one of the possible scenarios for the 148780 Altjira system in the solar system’s Kuiper Belt. It is likely a hierarchical triple formation, in which two very close companions are orbited by a third member at a greater distance. Credit: NASA, ESA, Joseph Olmsted (STScI)

A newly identified Kuiper Belt trio, possibly only the second of its kind, could provide strong evidence that these distant objects didn’t form from collisions but instead took shape through gravitational collapse—just like stars.

The universe is home to many gravitationally bound three-body systems, from triple star systems to planets with two moons, like Mars. Now, new research suggests that the Kuiper Belt — our solar system’s distant region of icy objects—may also host such systems. Scientists have identified a second potential triple system in the Kuiper Belt, where what was previously thought to be a binary may actually contain a hidden third member. This third object is so close to its companion that it can only be detected through its influence on the system’s orbital motion.

If confirmed, this discovery would increase the likelihood that many more hidden triple systems exist in the Kuiper Belt. More importantly, it would support the streaming instability hypothesis, a theory that suggests Kuiper Belt objects (KBOs) didn’t form through collisions but instead emerged as triple systems via gravitational collapse.

Hubble Space Telescope Orbiting Over Earth — Illustration of the Hubble Space Telescope over the Earth. Credit: ESA/Hubble (M. Kornmesser & L. L. Christensen)

Hubble Space Telescope Finds Kuiper Belt Duo May Be Trio

Predicting the motion of three gravitationally bound objects has puzzled mathematicians for centuries, a challenge recently brought into the spotlight by the novel and TV series 3 Body Problem. But researchers have now identified what appears to be a stable trio of icy space rocks in the Kuiper Belt, a distant region of the solar system. This discovery was made using data from NASA’s Hubble Space Telescope and Hawaii’s W. M. Keck Observatory.

A Rare Trio in the Kuiper Belt?

If confirmed, the 148780 Altjira system would be only the second known three-body system in this region, suggesting that more could exist. This finding would support a specific theory about the history of our solar system and how Kuiper Belt objects (KBOs) formed.

“The universe is filled with a range of three-body systems, including the closest stars to Earth, the Alpha Centauri star system, and we’re finding that the Kuiper Belt may be no exception,” said the study’s lead author Maia Nelsen, a physics and astronomy graduate of Brigham Young University in Provo, Utah.

Kuiper Belt Objects: Relics of the Early Solar System

First discovered in 1992, KBOs are ancient, icy remnants from the early solar system, located beyond Neptune’s orbit. So far, over 3,000 have been cataloged, but scientists estimate there may be hundreds of thousands more, each over 10 miles wide. The largest known KBO is the dwarf planet Pluto.

The Hubble finding is crucial support for a KBO formation theory, in which three small rocky bodies would not be the result of collision in a busy Kuiper Belt, but instead form as a trio directly from the gravitational collapse of matter in the disk of material surrounding the newly formed Sun, around 4.5 billion years ago. It’s well known that stars form by gravitational collapse of gas, commonly as pairs or triples, but that idea that cosmic objects like those in the Kuiper Belt form in a similar way is still under investigation.

Altjira: A Hidden Trio in the Outer Solar System

The Altjira system is located in the outer reaches of the solar system, 3.7 billion miles away, or 44 times the distance between Earth and the Sun. Hubble images show two KBOs located about 4,700 miles (7,600 kilometers) apart. However, researchers say that repeated observations of the objects’ unique co-orbital motion indicate the inner object is actually two bodies that are so close together they can’t be distinguished at such a great distance.

“With objects this small and far away, the separation between the two inner members of the system is a fraction of a pixel on Hubble’s camera, so you have to use non-imaging methods to discover that it’s a triple,” said Nelsen.

This takes time and patience, Nelsen explained. Scientists have gathered a 17-year observational baseline of data from Hubble and the Keck Observatory, watching the orbit of the Altjira system’s outer object.

“Over time, we saw the orientation of the outer object’s orbit change, indicating that the inner object was either very elongated or actually two separate objects,” said Darin Ragozzine, also of Brigham Young University, a co-author of the Altjira study.

“A triple system was the best fit when we put the Hubble data into different modeling scenarios,” said Nelsen. “Other possibilities are that the inner object is a contact binary, where two separate bodies become so close they touch each other, or something that actually is oddly flat, like a pancake.”

A Growing Population of Three-Body Systems?

Currently, there are about 40 identified binary objects in the Kuiper Belt. Now, with two of these systems likely triples, the researchers say it is more likely they are looking not at an oddball, but instead a population of three-body systems, formed by the same circumstances. However, building up that evidence takes time and repeated observations.

The only Kuiper Belt objects that have been explored in detail are Pluto and the smaller object Arrokoth, which NASA’s New Horizons mission visited in 2015 and 2019, respectively. New Horizons showed that Arrokoth is a contact binary, which for KBOs means that two objects that have moved closer and closer to one another are now touching and/or have merged, often resulting in a peanut shape. Ragozzine describes Altjira as a “cousin” of Arrokoth, a member of the same group of Kuiper Belt objects. They estimate Altjira is 10 times larger than Arrokoth, however, at 124 miles (200 kilometers) wide.

An Eclipsing Event Offers New Clues

While there is no mission planned to fly by Altjira to get Arrokoth-level detail, Nelsen said there is a different upcoming opportunity for further study of the intriguing system.

“Altjira has entered an eclipsing season, where the outer body passes in front of the central body. This will last for the next ten years, giving scientists a great opportunity to learn more about it,” Nelsen said. NASA’s James Webb Space Telescope is also joining in on the study of Altjira as it will check if the components look the same in its upcoming Cycle 3 observations.

The Hubble study is published in The Planetary Science Journal.

Reference: “Beyond Point Masses. IV. Trans-Neptunian Object Altjira Is Likely a Hierarchical Triple Discovered through Non-Keplerian Motion” by Maia A Nelsen, Darin Ragozzine, Benjamin C. N. Proudfoot, William G. Giforos and Will Grundy, 4 March 2025, The Planetary Science Journal.
DOI: 10.3847/PSJ/ad864d

The Hubble Space Telescope (HST) has been revolutionizing our understanding of the universe for over three decades. Launched in 1990, Hubble is a joint mission between NASA and the European Space Agency (ESA), providing some of the most detailed and stunning images of deep space ever captured. Managed by NASA’s Goddard Space Flight Center in Maryland, with support from Lockheed Martin Space in Colorado, its mission operations continue to push the boundaries of astronomical research. Scientific operations are conducted by the Space Telescope Science Institute (STScI) in Baltimore, which is operated by the Association of Universities for Research in Astronomy (AURA). From uncovering the expansion rate of the universe to detecting exoplanets and distant galaxies, Hubble remains one of the most important observatories in history.