Universe Today
Sometimes, good things come in threes. If astronomers are correct, a system in the distant Kuiper Belt may not be two but three worlds, offering an insight into formation in the early solar system.
The study comes out of researchers at Brigham Young University and the Space Telescope Science Institute.
The Strange World(s) of 148780 Altjira
Discovered in 2001 during Kitt Peak’s Deep Ecliptic Survey, 146780 Altjira orbits 44 Astronomical Units (AU) from the Sun, once every 294 years. Later observations by Hubble in 2006 confirmed the object as a binary system.
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The distant orbit of KBO Altjira. NASA/JPL Horizons
The main binary pair are 9,904 kilometers apart on a 140 day orbit. Both are nearly similar in size, at 246 kilometers and 221 kilometers in diameter respectively. The third hypothetical object has an orbital period of just 5.5 hours. That’s swift but not improbable: Phobos, for example, orbits Mars in just seven and a half hours.
Hubble images KBO Altjira as a binary in 2006. Credit: NASA/Hubble/STScI
The system is too distant for observers to see the hinted at third companion directly. Instead, researchers used Hubble data to tease out slight perturbations in the orbit of the two wider bodies, to infer the presence of a third.
“With objects this small and far away, the separation between the two 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,” says Maia Nelsen (Brigham Young University and lead researcher in the study) in a recent press release. “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,” says Darin Ragozzine (Brigham Young University) in the same press release.
If confirmed, the system would be what’s known as a ‘hierarchical triple,’ as the third body goes around one of the binary KBOs. Hierarchical triples are common in multiple star systems: we actually have one next door, with the close Alpha Centauri A and B pair which is orbited by Proxima.
Of course, the exact nature of the system is unclear at this point. Is it a true triple, a contact binary… or perhaps pancake-like body, as was proposed for interstellar visitor 1I/Oumuamua.
Any of these three conclusions are intriguing, and have consequences for how we understand KBO formation. Specifically, three orbiting objects as are hypothesized for the Altjira system would not have been the product of collisions, but would have instead been the result of gravitational collapse early on in the formation of the solar system, 4.5 billion years ago. Newborn stars in multiple systems are known to have been formed in this fashion in proto-solar nebulae, but how multiple KBO and asteroid systems achieved this sort of stability in miniature is still controversial.
It’s strange to think: until the discovery of 243 Ida as a pair in 1993, no binary asteroids were known of… now they almost seem to be the norm, from Didymos and its tiny moon Dimorphos, to Lucy’s recent flyby of 152830 Dinkinesh and its moonlet Selam in 2023. Strange two-lobed objects also exist, including comet 67P/Churyumov-Gerasimenko explored by ESA’s Rosetta mission, and 486958 Arrokoth, as revealed during New Horizon’s New Year’s Day 2019 flyby.
The strange, twin-lobed world of Arrokoth. Credit: NASA/New Horizons.
Clearly, close binary and contact binary asteroids and comets have something key to tell us about the early history and formation of the solar system. To date, 40 binaries are known, among the over 3,000 currently known KBOs.
In 2007, 47171 Lempo was the first confirmed triple KBO. Discovered in 1999, the system was revealed as the first multiple KBO by Hubble.
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Hubble tracks the orbit of the Lempo KBO system. Credit: NASA/STScI
Certainly, Altjira and its ilk are worthy of further scrutiny by JWST and the new generation of sky surveys in coming years. Light curve analysis and occultations of background stars can reveal the nature of a distant object, as the ‘shadow profile’ of the occulting body and any hidden moon can make itself known.
1999 TC36 (later 47171 Lempo) as resolved by the Hubble Space Telescope. Credit: NASA/STScI
Though such an opportunity is rare, we may not have to wait long. The two main companions in the Altjira system are currently nearing edge-on from our vantage point, with a mutual eclipse season over the next decade centered on 2028… this will give astronomers an opportunity to probe the system in greater detail.
Perhaps, there will never be a dedicated mission to the remote worlds of Altjira… after all, our lucky views of 486958 Arrokoth were only because New Horizons was already headed in that general direction, post Pluto encounter. Still, a whole new generation of telescopes are techniques are in the pipeline for the next decade, so who knows? We might get a good look at Altjira and its companions yet.