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The Kuiper Belt is a band on the outer edge of the Solar System made up of space’s odds and ends: comets, dwarf planets, and other miscellaneous chunks of rock and ice.
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Astronomers just spotted what they think might be a third object in a Kuiper Belt system known as Altjira, which would make it a triple system instead of a binary.
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If confirmed, it would mean Altjira is subject to a conundrum commonly known as the three-body problem.
Floating around at the edge of the Solar System are leftovers from its formation. This is the Kuiper Belt, which is composed of chunks of rock and ice like comets, dwarf planets, and random objects that failed to become planets. And this already-weird frontier of space billions of miles from the Sun just got weirder.
Anyone who’s binged 3 Body Problem on Netflix might recognize that the sci-fi series (based on a novel series by Cixin Liu) was inspired by an actual conundrum in space. Triple systems of planets, stars, and other objects are all over the universe, ut because of how complicated the physics of these systems can be, experts don’t actually know how exactly three bodies orbit each other. This problem has now come up again after scientists—using NASA’s Hubble Space Telescope and the W. M. Keck Observatory—observed that the Kuiper Belt system Altjira could be a triple system.
If Altjira really is a triple system, it would be the second one to surface in the Kuiper Belt (the first one being a system known as Lempo). It first appeared as a binary, but further observations revealed something peculiar. On Hubble’s camera, a space between the inner objects of a proposed triple system 3.7 billion miles away would only show up as a fraction of a pixel. Researchers trying to figure out whether this system has two or three objects needed to look to its outer orbit for answers.
“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,” researcher Darrin Ragozzine from Brigham Young University said in a NASA press release.
The team—led by astronomer Maia Nelsen, also from BYU—found that Altjira could give further backing to a theory about how Kuiper Belt objects formed, which sees a triple system as the result of gravitational collapse. Just like stars form from clouds of gas collapsing in on themselves, binary systems can emerge from clouds of dust and debris with significant angular momentum. Some of this angular momentum is transferred to each of the objects in a nascent binary, which is how they start to orbit each other.
What ends up forming might not always be a binary, however. It is thought that there are sometimes more than two objects in the cloud, and they receive so much angular momentum that at least one pair ends up being a secondary binary within the original system. This results is an outer and inner binary that make up a triple system. Lempo is the only confirmed example of this in the Solar System, but Altjira might be hiding an unresolved inner binary—which could, if we’re lucky, turn out to be a contact binary much like another Kuiper Belt object known as Arrokoth.
The bizarre shape of Arrokoth challenged theories about how planets formed, with its double lobes suggesting that the objects that formed it slowly merged instead of smashing into each other. Analysis of observations by NASA’s New Horizons spacecraft suggested that Arrokoth was a contact binary that resulted from two space rocks orbiting each other closer and closer until they finally stuck together. While Arrokoth has moved out beyond an observable distance, Altjira belongs to the same population of Kuiper Belt objects and could offer answers that Arrokoth is now to far away to provide.
“By going beyond [assumptions], we open a new window into the shapes and spins of the components, including the “inner” binaries,” the researchers said in a study recently published in the Planetary Science Journal.
Because there is no mission currently set to observe Altjira up close, and no existing telescope can resolve it entirely, the alleged triple system cannot be imaged in as much detail as Arrokoth once was. Therefore, which of its many hypothetical orbits will prove to be its actual orbit remains unknown. The most likely shape for it seems to be a triple system, but that is still debatable.
That said, all hope is not lost. Altjira is now in an eclipsing season, during which its outer body will pass in front of its inner body (or bodies). This might shed some light on the mystery.
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