The icy fringe of the asteroid belt explained

“Primordial objects” may have been captured during the planetary realignment.

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The relocation of the ice giants could explain the frosty exterior of the asteroid belt.
Credit: NASA / JPL-Caltech

A shuffling of the planets at the start of the solar system may explain how ancient balls of ice ended up in the asteroid belt.

This new hypothesis, based on a popular theory of the formation of the solar system known as the “Nice model”, upends an earlier view that the asteroid belt is the remnant of a protoplanetary disk that surrounded the Sun at the time. beginning of its development.

“If the Nice model is true, then 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 an approximately 180 million kilometer wide expanse 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 makeup,” Levison says. On the inner edge of the belt, asteroids appear rocky and scorched, while the outer edge of the belt is filled with icy objects filled with water and organic molecules.

Astronomers believed the belt to be the frozen remnants of the protoplanetary disk that once existed around the Sun. The line between rock and ice, they believed, was a “snow line” beyond which large icy planets could form.

“Orbits have really become kablooy. Harold Levison, Southwestern Research Institute »

But that view posed problems, particularly for Uranus and Neptune, according to Stuart Weidenschilling, a principal investigator at the Planetary Science Institute in Tucson, Arizona, who was not involved in the latest work. At the distances these planets now orbit, the protoplanetary disk would have moved too slowly for them to form properly.

Enter the Nice model. The model, which was introduced four years ago, has Uranus and Neptune forming around half their current distance from the Sun2. Beyond them, the model postulates, lay a vast disk of comet-like balls of ice.

This configuration wasn’t stable, Levison says, and “the orbits really got kablooy.” Jupiter moved inward, while Saturn, Uranus, and Neptune all moved away from the center of the solar system. In doing so, they catapulted icy bodies from the first protoplanetary disk into the inner solar system. The new simulations show that a fraction of them ended up in stable orbits around the outer edge of the asteroid belt, where they reside to this day.

Weidenschelling says the latest work is yet another achievement for Nice’s relatively new model. “It’s another one of those little puzzle pieces that seem to fit together,” he says.

Levison thinks that, if this model is correct, detailed studies of the asteroid belt will tell astronomers more about the evolution of the early solar system. “It’s a bit like a crime scene investigation,” he says. “The way the blood is splattered on the wall says more about what happened than the body itself.”

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Brumfiel, G. Explanation of the Asteroid Belt Icy Fringe.
Nature (2009).

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