The cold outer reaches of the solar system are home to a plethora of small worlds, many of which have moons of their own. For a few, the moon is massive enough to make the pair into a binary; the Pluto-Charon system is the most famous of those. And a small icy body named Lempo is a trinary: three objects of comparable mass in mutual orbit.
Now astronomers have identified another possible trinary object, a very distant world known as Altjira (al-TCHEE-ruh), named to honor the creator deity in Arrernte Australian cosmology. Observers discovered Altjira in 2001 and its as yet unnamed moon in 2006. With data accumulated over the past 2 decades, researchers determined the path of the moon doesn’t match what would be expected if it were orbiting a spherical (or mostly so) world.
“Our modeling tells how squished the central object is.”
“When we take images [of Altjira], we just see two objects, and we can track those two objects gravitationally,” said Benjamin Proudfoot, an astronomer at the Florida Space Institute who led the observational part of the project. “We can see that the orbit precesses, and precession is caused by the nonspherical shape of one of the objects. Our modeling tells how squished the central object is.”
In fact, the analysis showed the central object was too squished to be one single body: It is almost certainly two, making Altjira the second known trinary beyond Neptune. And that’s a big deal.
“One of the primary goals of planetary science is kind of understanding how things formed,” Proudfoot said, noting that these trans-Neptunian objects (TNOs) are remnants of the earliest days of our solar system. One major theory explaining the formation of the solar system, known as streaming instability, predicts the formation of a number of trinaries.
“Finding triples like Altjira is really important for telling us how we got here,” Proudfoot explained. “Although this icy debris is in the outer solar system, this was the first step into forming the planets that we have today.”
Proudfoot and his colleagues published their results in The Planetary Science Journal.
One Lump or Two?
Binary systems are common throughout the known cosmos: two objects of comparable mass orbiting each other, such as Pluto and Charon or even Earth and the Moon.
A similar type of system exists where three bodies of comparable mass mutually orbit each other: a hierarchical triple. Lempo, Hiisi, and Paha (named for figures in Finnish mythology) are such a trinary: Lempo and Hiisi form a close pair, with Paha orbiting both in a wider path. Altjira is 44 times farther from the Sun than Earth is; if it is a hierarchical triple, it is too far away, and its inner pairing is too tight for even the most powerful telescopes to resolve. Upcoming observations with NASA’s James Webb Space Telescope (JWST) are unlikely to provide direct evidence, though they will help the indirect case.
Without direct observation, indirect measurements showed Altjira’s companion has a precessing orbit, meaning it traces a sort of spirograph pattern rather than a circle or ellipse. That indicates Altjira must actually be two objects, either in mutual orbit like Lempo and Hiisi or stuck together, like the TNO Arrokoth.
To model the shape and nature of Altjira, Proudfoot and his coauthors—including Maia Nelson, who at the time was an undergraduate student at Brigham Young University in Utah—used detailed motion of the object’s companion and worked backward.
“The most likely thing is [Altjira] is a triple system,” Proudfoot said. “Slightly less likely, but not unreasonable, would be something like Arrokoth with a moon.”
The existence of hierarchical triples helps reveal how the solar system formed from its primordial protoplanetary disk of dust and gas. According to the streaming instability theory, a sort of gravitational drag slowed the larger molecules in that disk, allowing them to clump together into larger aggregates. Some of those objects grouped into binaries, and others grouped into hierarchical triples. But the question remains how stable these trinaries would be over billions of years because many things can separate the outermost member of the triad, including the simple passage of time.
“We don’t have money to send spacecraft to all the objects we think are interesting.”
“A trinary configuration like Lempo favors the streaming instability theory,” said Flavia Luane Rommel, an astronomer at the University of Central Florida who has previously worked with Proudfoot but was not part of the Altjira study. One trinary could be a special case, she noted, but the confirmed identity of two triples means there are likely more that have yet to be detected—lending strong support to the streaming instability theory.
More direct evidence of Altjira’s nature requires further observations.
Although JWST will provide some data, “we don’t have money to send spacecraft to all the objects we think are interesting,” Rommel said.
To study such objects, astronomers in her area of research often use stellar occultations: a sort of miniature eclipse in which the object they want to study passes in front of a star. If Altjira is a trinary, an occultation would result in one eclipse for each object blocking the star. “The thing is, stellar occultations are not in our control, they don’t happen when we want them to,” she said.
Proudfoot noted that Altjira’s unnamed moon could provide data similar to those supplied by stellar occultations, however, because the moon’s orbit takes it between Earth and Altjira’s mysterious inner body, an event called a mutual planetary occultation or, simply, a mutual event.
Watching a mutual event’s subtle fluctuations in reflected light could reveal whether the object is a pair or a lumpy single object like Arrokoth. Mutual events are how scientists measured the properties of the binary asteroids Didymos and Dimorphos before sending the Double Asteroid Redirection Test (DART) mission.
“That is imminently doable,” Proudfoot said. “I am working on getting telescope time right now to observe a mutual event in October of this year.”
—Matthew R. Francis (@BowlerHatScience.org), Science Writer
Citation: Francis, M. R. (2025), Distant icy twins might actually be triplets,
Eos, 106, https://doi.org/10.1029/2025EO250149. Published on 18 April 2025.
Text © 2025. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
