Three bodies? No problem!
The “three-body problem” has traditionally referred to the devilishly tricky challenge of working out the trajectories of three objects orbiting each other in space. The concept has inspired a sci-fi trilogy about an alien invasion, plus a Netflix series based on the novels.
In the books and in the TV show, the alien invaders are coming from the Alpha Centauri star system — where three stars are gravitationally bound to each other just a little more than 4 light-years away from us. But we don’t have to look that far away to find a three-body system.
Back in 2020, astronomers reported the detection of a trio of celestial objects in the Kuiper Belt, the broad ring of icy material at our solar system’s edge — and now scientists analyzing data from the Hubble Space Telescope and the W.M. Keck Observatory say they may have come across the Kuiper Belt’s second three-body system.
A report about the system, known as Altjira, was published today in The Planetary Science Journal.
“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,” study lead author Maia Nelsen, a physics and astronomy graduate of Brigham Young University, said in a NASA news release.
Thousands of icy worlds — including Pluto and other dwarf planets — orbit the sun in the Kuiper Belt. The Altjira system is 3.7 billion miles away, which is hundreds of millions of miles farther out than Pluto.
Hubble spotted two objects that are 4,700 miles apart. However, researchers say repeated observations of the objects’ complex motions around each other, collected over the course of 17 years, led them to conclude that the inner object is probably a pair of icy worlds.
“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.
Hubble’s readings were supplemented by data from the Keck Observatory. “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 study co-author Darin Ragozzine, an astronomer at BYU.
Nelsen said a three-body system is the best fit for the Hubble data, based on computer modeling. “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,” she said.
NASA’s New Horizons spacecraft captured images of just such a contact binary, dubbed Arrokoth, in 2019 as a follow-up to the probe’s up-close look at Pluto and its moons. If it turns out that the inner object is actually a contact binary rather than two separate bodies, it would be about 124 miles wide, or 10 times larger than Arrokoth.
There won’t be a flyby of Altjira in the foreseeable future, but researchers are planning to continue studying the three (or maybe two?) icy mini-worlds from afar.
“Altjira has entered an eclipsing season, where the outer body passes in front of the central body. This will last for the next 10 years, giving scientists a great opportunity to learn more about it,” Nelsen said.
NASA’s James Webb Space Telescope is due to track Altjira during a future round of observations. Such studies could shed new light on the evolution of our solar system’s cold, dark rim. And maybe they’ll shed light on the three-body problem as well.
In addition to Nelsen and Ragozzine, the authors of the study in The Planetary Science Journal — “Beyond Point Masses. IV. Trans-Neptunian Object Altjira Is Likely a Hierarchical Triple Discovered through Non-Keplerian Motion” — include Benjamin C.N. Proudfoot, William G. Giforos and Will Grundy.
