The 148780 Altjira system, located approximately 3.7 billion miles from the Sun, appears to consist of three gravitationally bound objects. This discovery supports the hypothesis that some Kuiper Belt objects (KBOs) formed through direct gravitational collapse rather than through collisions.
"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 study lead author Maia Nelsen, a physics and astronomy graduate from Brigham Young University in Provo, Utah.
KBOs, first identified in 1992, are remnants from the early solar system. Over 3,000 have been cataloged, with estimates suggesting that hundreds of thousands more, each over 10 miles in diameter, remain undiscovered. The largest known KBO is Pluto.
The Hubble observations indicate that the two primary objects in the Altjira system orbit approximately 4,700 miles (7,600 kilometers) apart. However, precise tracking of their movements suggests that what appears to be a single inner body is actually two objects so close together that they cannot be distinguished at this 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," explained Nelsen.
Researchers have spent 17 years observing Altjira with Hubble and Keck, tracking the motion of the outermost object. Their findings indicate that the inner object is not singular but either a very elongated body or two distinct objects in close orbit.
"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, a co-author of the study from Brigham Young University. "A triple system was the best fit when we put the Hubble data into different modeling scenarios."
Previously, only about 40 binary KBOs had been identified. With the addition of two probable three-body systems, scientists believe these triples may not be anomalies but rather part of a larger, yet-undiscovered population. However, confirming this requires ongoing observations.
The Kuiper Belt remains largely unexplored, with NASA's New Horizons spacecraft providing the only close-up views of its objects. The spacecraft flew past Pluto in 2015 and the smaller Arrokoth in 2019, revealing the latter to be a "contact binary" where two objects have merged or are in direct contact. Scientists believe Altjira, which is 10 times larger than Arrokoth at approximately 124 miles (200 kilometers) across, may share a similar formation history.
While no missions are currently planned to visit Altjira, its current "eclipsing season" provides a valuable opportunity for further study. Over the next decade, its outer body will pass in front of the central body, allowing researchers to analyze the system in greater detail.
"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 noted.
NASA's James Webb Space Telescope will contribute additional observations in its upcoming Cycle 3 study to determine whether Altjira's components exhibit similar characteristics.
Research Report:Beyond Point Masses. IV. Trans-Neptunian Object Altjira Is Likely a Hierarchical Triple Discovered through Non-Keplerian Motion
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