Astronomers may have just identified a rare example of a "three-body problem" hiding in plain sight beyond the solar system's most distant planet. If the observation can be confirmed, it suggests that many more cosmic triplets could be hiding in the outer reaches of our cosmic neighborhood, researchers say.
Back in 2001, astronomers discovered what they thought was a binary system made up of two large bodies orbiting each other approximately 3.7 billion miles (6 billion kilometers) from Earth in the Kuiper Belt — a ring of asteroids, comets and dwarf planets, including Pluto, that lies beyond the orbit of Neptune.
The two icy rocks, collectively named 148780 Altjira, are separated by around 4,700 miles (7,600 km), or roughly one-fiftieth the distance between Earth and the moon. (The system is named after the creation deity of the Aboriginal Arrernte people from Australia.)
But in a new study published March 4 in The Planetary Science Journal, researchers propose that the inner body in the Altjira system is actually a pair of smaller objects circling extremely close to one another, making this a triple system.
The study team came to this conclusion after combining images from the Hubble Space Telescope with 17 years' worth of data collected by the W. M. Keck Observatory on Hawaii's Mauna Kea volcano. This revealed subtle shifts in the trajectory of the outer body, suggesting that it is being gravitationally tugged on by two objects instead of one. However, the system is too far away to get visual confirmation of the separation of the inner bodies.
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"A triple system was the best fit [when comparing different modeling scenarios]," study lead author Maia Nelsen, an astronomer at Brigham Young University in Utah, said in a NASA statement. However, it is also possible that the inner body could be a "contact binary" — two objects that touch each other and act as a single entity — or "something that actually is oddly flat, like a pancake," she added.
Over the next 10 years, the Altjira system will be in its "eclipsing season," where the outer body will be frequently positioned between the inner body and the sun, which could allow for more detailed observations of its orbital trajectory, the researchers wrote.
During this time, the James Webb Space Telescope is also scheduled to survey the Altjira system, which could conclusively settle the debate with its unmatched image-resolving powers, according to NASA.
The three-body problem
When three objects with similar mass orbit one another, the mathematics involved in calculating their trajectories becomes extremely challenging and leaves very little room for error. That means the slightest change to one of the objects' trajectories can throw the whole system out of balance.
"The puzzle of predicting how three gravitationally bound bodies move in space has challenged mathematicians for centuries," NASA representatives wrote in the statement. This is often referred to as the three-body problem, which has also inspired a popular science fiction novel and a recent TV series of the same name.
Several types of triple systems have been found throughout the cosmos, including triple-star systems like our nearest stellar neighbors — the Alpha Centauri system — and "Tatooine" exoplanets with two suns, providing several solutions to this problem. However, there is no single generally accepted solution to working out the orbital mechanics of a triple system, so the "problem" is often considered unsolved.
If confirmed as a triple system, Altjira will be the second of its kind discovered in the Kuiper Belt; 47171 Lempo, which has a nearly identical configuration as the one proposed for Altjira, was also previously classified as a binary system.
There are around 40 other known binary systems in the Kuiper Belt, some of which could be unrecognized triple systems. However, there are likely many more three-body systems there.
So far, scientists have found around 3,000 Kuiper Belt objects, but they estimate that there could be "several hundred thousand more" smaller objects there, each wider than 10 miles (16 km) across, researchers wrote. Therefore, this part of the solar system could be a great place to hunt for many more of these systems.
"The universe is filled with a range of three-body systems," Nelson said. "And we're finding that the Kuiper Belt may be no exception."
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