The chemical composition of Pluto and Triton suggests they originated in the same region of the outer solar system before the latter was pulled into Neptune’s orbit
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| Triton (the largest moon of Neptune) and Pluto may be long-lost siblings (Credit: NASA/JPL/Johns Hopkins university Applied Physics Laboratory/SRI) |
Neptune’s largest moon, Triton, and the dwarf planet Pluto may have shared a common origin before being separated in the early solar system, an analysis of their composition suggests.
Triton and Pluto have both been visited once by spacecraft, the former by NASA’s passing Voyager 2 in 1989 and the latter by NASA’s New Horizons probe in 2015. Both are icy bodies smaller than Earth’s moon with similar densities that appear to have hosted subsurface oceans at some point in the past.
Triton, which is about 50 per cent more massive than Pluto, is particularly unusual in that it orbits in the opposite direction to its planet’s rotation, the only large moon to do so in the solar system. This has led to suggestions it may have originated in the Kuiper belt, a region of icy objects like Pluto that exist beyond Neptune, because moons that formed with Neptune should orbit in the same direction that it spins.
Kathleen Mandt at NASA’s Goddard Space Flight Center in Maryland and her colleagues have gone further by suggesting that Triton and Pluto formed very close to each other before the solar system settled down. “They probably formed in the same region, which wouldn’t be where the Kuiper belt is now – it would have been either closer or farther away,” says Mandt.
Studying prior observations of the two bodies, the team found that both have a large amount of nitrogen and trace amounts of methane and carbon monoxide, which could have accumulated in the outer regions of the young nebula that surrounded our sun 4.5 billion years ago.
This suggests they both formed in cold outer regions of the early solar system, which were rich in nitrogen. But they also have low amounts of water, similar to some known icy comets such as comet C/2016 R2 (PanSTARRS) that may have come from a similar location.
“They had to have formed beyond the water-ice line,” says Mandt, referring to the distance from the sun where water would freeze into ice or snow, which is where Jupiter is now. She says the two bodies would have formed within 1 to 5 astronomical units (AU), or sun-Earth distances, of each other based on their shared compositions and dynamical modelling of the solar system, and no more than 30 AU from the sun.
For some reason, Triton was then ejected from this region and ensnared by Neptune. One possibility is that the giant planets moved closer to the sun early in the first 100 million years or so of the solar system, which may have disrupted the orbits of some bodies like Triton.
While the idea that Triton and Pluto were siblings has been suggested before, Paul Schenk at the Lunar and Planetary Institute in Texas says pinning down their origins could give us a greater insight into how our solar system came to be. “It’s an indication there was more than one Pluto,” he says. “It’s probable there were other bodies of similar size and composition that were lost.”
Truly understanding the origins of Triton and Pluto would require us to revisit the two bodies, something that is unlikely to happen any time soon given the vast travel times involved.
Carly Howett at the University of Oxford was the deputy lead on a proposed NASA mission to Triton, called Trident, that was overlooked for development in 2021. “Triton has the biggest area of a solid body in the solar system that we’ve never explored,” she says. “If they didn’t form in the same region of space, why are they so similar?”
Reference:
arXiv DOI: 10.48550/arXiv.2406.03815
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