NASA’s Hubble Space Telescope has discovered that a system of asteroids in the distant Kuiper Belt may be triplets, not twins as previously suspected.
If so, the stable trio of icy space rocks would be just the second example of three gravitationally bound space rocks found in the Kuiper Belt, the doughnut-shaped region of icy bodies that lurks out beyond the orbit of Neptune.
The discovery could also challenge our understanding of how Kuiper Belt objects (KBOs) form.
If it is confirmed as a triplet, the system — designated 148780 Altjira — could offer scientists a chance to improve their models of how three gravitationally bound bodies move through space together.
This puzzle, known as the “three-body problem,” has been a challenge since Isaac Newton published his work “Principia” in 1687.
“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,” team leader Maia Nelsen, a physics and astronomy graduate of Brigham Young University in Provo, Utah, said in a statement.
Related: What is the Kuiper Belt?
Three’s company in the Kuiper Belt
Astronomers Dave Jewitt and Jane Luu discovered the first icy body in the Kuiper Belt, known as 1992 QB1, in 1992. Since then, a further 3,000 KBOs have been cataloged.
Astronomers estimate that several hundred thousand more KBOs measuring over 10 miles (16 kilometers) in diameter could lurk in this icy donut, which begins around 2.8 billion miles (4.5 billion km) from the sun.
The Kuiper Belt is thought to stretch as far as 4.6 billion miles (7.4 billion km) from the sun, which is around 50 times the distance between Earth and our star. The Altjira system sits in the middle of the Kuiper Belt, at around 3.7 billion miles (6.0 billion km) from the sun, or about 40 times the distance between Earth and the sun.
The Hubble Space Telescope images initially seemed to show that the Altjira system was composed of two KBOs located about 4,700 miles (7,600 km) apart.
When the team conducted repeated observations of the Altjira system object’s unique co-orbital motion, however, they found the inner object is actually two bodies. These KBOs are so close together that they can’t be distinguished individually from such a great distance away.
“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,” Nelsen said.
It took 17 years of data from Hubble and the Keck Observatory in Hawaii to observe orbital changes in the Altjira system and make this determination. The data was added to various modeling scenarios, with the most likely explanation being a triple-body system.
“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,” Nelsen added.
Of 40 multiple-body systems identified in the Kuiper Belt, this is just the second identified composed of more than two objects. The researchers think these aren’t outliers and there are more multi-asteroid systems out there in the outer reaches of the solar system, waiting to be discovered,
The Hubble observations of the Altjira system, which suggest it has a third occupant, support a theory of KBO creation that involves the direct gravitational collapse of matter in the disk of material surrounding the infant sun around 4.5 billion years ago.
This direct collapse pathway is similar to the formation process of stars, albeit it on a vastly smaller scale. Star formation from dense patches of gas and dust can also result in two and three-body systems.
The alternative KBO creation theory, which sees these icy space rocks created from collisions between larger bodies, would not create a three-body arrangement as the Altjira system appears to be.
The Altjira system joins the dwarf planet Pluto and the “space snowman” Arrokoth, a contact binary composed of two touching space rocks, as the most heavily studied bodies in the Kuiper Belt.
NASA’s New Horizons probe flew by Pluto in 2015 and Arrokoth in 2019. There’s no visit to Altjira in the works, but the researchers behind the new study hope that detailed remote observations of the system will be possible in the future.
Particularly exciting will be forthcoming observations of Altjira conducted by the James Webb Space Telescope during its third year of operations.
“Altjira has entered an eclipsing season, where the outer body passes in front of the central body,” Nelsen said. “This will last for the next 10 years, giving scientists a great opportunity to learn more about it.” \
The new study was published on Tuesday (March 4) in The Planetary Science Journal.
