'Primordial objects' may have been captured during planetary realignment.
A reshuffle of the planets in the early Solar System may explain how ancient ice balls ended up in the asteroid belt.
This new hypothesis, based on a popular theory of Solar System formation known as the 'Nice model', upends a previously held view that the asteroid belt is the remnant of a protoplanetary disc that surrounded the Sun early in its evolution.
"If the Nice model is true, then the old ideas about the asteroid belt must be wrong," says Harold Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. Levison and his colleagues published their new explanation of the belt's origins in this week's issue of Nature1.
The asteroid belt is a roughly 180-million-kilometre-wide stretch of space between the orbits of Mars and Jupiter that contains millions of objects of various shapes and sizes. "The surprising thing about these objects is that they show a wide range of diversity in their chemical composition," says Levison. On the belt's inner edge, the asteroids appear rocky and scorched, whereas the outer edge of the belt is filled with icy objects full of water and organic molecules.
Planetary crime scene
Astronomers had believed the belt to be the frozen remains of the protoplanetary disc that once existed around the Sun. The line between rock and ice, they reasoned, was a 'snow line' beyond which large, icy planets could form.
The orbits really just went kablooy. Harold Levison , Southwest Research Institute
But there were problems with this view, particularly for Uranus and Neptune, according to Stuart Weidenschilling, a senior researcher at the Planetary Science Institute in Tucson, Arizona, who was not involved in the latest work. At the distances at which those planets now orbit, the protoplanetary disc would have been moving too slowly for them to form properly.
Enter the Nice model. The model, which was introduced four years ago, has Uranus and Neptune forming at roughly half of their present-day distance from the Sun2. Beyond them, the model postulates, lay a vast disc of comet-like balls of ice.
This set-up wasn't stable, Levison says, and "the orbits really just went kablooy". Jupiter moved inwards, while Saturn, Uranus and Neptune all moved away from the Solar System's centre. As they did so, they catapulted icy bodies from the early protoplanetary disc into the inner Solar System. The new simulations show that a fraction of these ended up in stable orbits around the outer edge of the asteroid belt, where they reside to this day.
Weidenschelling says that the latest work is yet another accomplishment for the relatively new Nice model. "It's another one of these little puzzle pieces that seem to fit in," he says.
Levison believes that, if this model is correct, detailed studies of the asteroid belt will tell astronomers more about how the early Solar System evolved. "It's sort of like a crime scene investigation," he says. "How the blood is splattered on the wall tells you more about what happened than the body itself."
Levison, H. F. et al. Nature 460, 364-366 (2009).
Tsiganis, K., Gomes, R., Morbidelli, A. & Levison, H. F. Nature 435, 459-461 (2005).
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Brumfiel, G. Asteroid belt's icy fringe explained. Nature (2009). https://doi.org/10.1038/news.2009.693