Ever since the discovery of the asteroid Ceres in 1801, these rocky bodies orbiting the Sun at different distances have been considered the residual signatures of the Solar System’s early formative years. Now, a review of past studies, published in Nature on January 30, has revealed how our continuously changing understanding of these signatures is beginning to imply a very turbulent process of planet formation.

Astrophysicists Francesca DeMeo and B. Carry extrapolated the results of studies of asteroids conducted to date to draw a story of great dynamism, including the possibility that a young Jupiter could once have been as close to the Sun as Mars is today.

Their review begins with studies conducted since the 1960s, when astronomers were beginning to classify asteroids in a naïve, taxonomic manner. As DeMeo and Carry point out, the first two real models of asteroid formation combined the bodies’ temperatures and sizes to announce that smaller asteroids in the main belt were colder, while the larger objects were warmer.

However, since the late 1990s, so many ‘rogue’ asteroids were found to flout this hypothesis that they went from being the exception to the rule. Suddenly, astronomers were confronting the implications of a more sophisticated model of formation. It was then that the planetary migration idea was proposed, in the mid-2000s.

Chain of pins

According to it, the planets were in a state of flux even after they had formed, shifting orbits to find their stable locations around the Sun. On the way, astronomers think their gravitational fields could have dragged the asteroids along, like a magnet pulling on a chain of pins, before depositing them in different locations.

The migration model has taken two forms, called the Nice and Grand Tack models. Together, they explain how Jupiter could have been the predominant driver of change in the Solar System.

According to the Grand Tack model, “Jupiter would have moved through the primordial asteroid belt,” getting as close to the Sun as Mars is today, “emptying it and then repopulating it with scrambled material from both the inner and outer Solar System,” before heading back to its current position.

Then, according to the Nice model, Jupiter pulled in asteroids from the Kuiper belt—composed of asteroids from beyond Neptune—into a smaller belt around itself that we now know looks nothing like the main belt. While the Nice model has been found to be plausible, the Grand Tack Model is still evolving.

The next step, DeMeo and Carry suggest, would be for astronomers to move beyond these generic classes and to study the innards of asteroids.

Perhaps that will reveal the greater complexity imbued by their original and current positions, and what more they tell us about how the Solar System formed.

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