Asteroid belt may be remains of half a dozen ancient planets

They might seem like boring old rocks, but asteroids and meteorites have some fascinating stories to tell about the history of the solar system. New research from the University of Florida has now traced back the origins of almost all asteroids in the inner belt to just five or six ancient minor planets.

The solar system was far rougher in its youth than it is today. As the huge disc of dust and gas surrounding the Sun started clumping together, planets and moons were born and torn apart as they smashed into each other. The eight planets we know today are the survivors of this time, but other protoplanets were likely jostling for space before or during their reign.

Some of these lost worlds came to an explosive end when they collided with Earth, Mars and Uranus, but they live on in the form of moons of those planets. Meteorites, meanwhile, tell tales of ancient ocean planets and Mercury-sized planetoids that lived long enough for diamonds to form deep below their surface – and died explosively enough for the gems to be cast out into space.

These long-dead bodies seem ephemeral and hard to count, with current classifications involving hundreds of asteroid families. But the new study suggests that going far enough back, these families could be tied together, meaning all of the asteroids in the inner belt might have originated from just a few minor planets.

The University of Florida researchers examined 200,000 asteroids in the inner asteroid belt. For those rocks not assigned families, the team found a correlation between their size and orbit – specifically, the bigger the rock, the more eccentric (oval-shaped) the orbit. The opposite held true between their size and orbital inclination – basically, the smaller the rock, the further tilted its orbit is from the flat plane that most objects orbit along.

The researchers say that these connections suggest that up to 85 percent of the asteroids in the inner belt can be attributed to five known families: Flora, Vesta, Nysa, Polana and Eulalia. The remaining 15 percent could also fall into those same categories, or a few others that are currently unknown.

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