Many asteroids might be remnants of five destroyed worlds, scientists say
For years, asteroids were thought of as the leftovers of planet formation. But according to a study published Monday in the journal Nature Astronomy, these were once pieces of worlds, too.
A vast majority of the half-million bodies in the inner asteroid belt may in fact be shrapnel from as few as five parent bodies called “planetesimals,” scientists say. But the tangled orbits of those lost worlds meant they were doomed to collide, producing fragments that also collided, producing still more fragments in a cataclysmic cascade that’s been going on for more than four billion years.
The finding doesn’t only illuminate a “mystery” of the asteroid belt, said Katherine Kretke, a planetary scientist at the Southwest Research Institute who was not involved in the study. It could also help resolve a debate about the formation of the eight planets.
The study’s lead author, University of Florida astronomer Stanley Dermott, didn’t necessarily set out to probe a mystery of solar system formation. He and his colleagues were looking at data on the dynamics of bodies in the inner asteroid belt in hopes of figuring out what makes an object leave the belt. But as Dermott began to look through a database of nearEarth objects, he noticed something strange about many large asteroids: their orbits were inclined, or tilted, relative to the plane of the rest of the solar system.
Scientists have previously known that roughly half of inner-belt asteroids belong to five “families.” But Dermott and his colleagues say their analysis suggests that number is as high as 85 per cent. This finding matches other observations of the asteroid belt, said David Nesvorny, a planetary scientist at SWRI who was not involved with Dermott’s study. Asteroids thought to belong to the same family tend to orbit in clusters and have similar chemical compositions. There’s an important, if apparent, implication of the idea that asteroids are actually fragments of larger bodies: “It means asteroids are born big,” Nesvorny said.
According to the traditional story of the origin of the solar system, the planets formed slowly from accretion as particles in the circumstellar disk clumped together to great pebbles, then slightly larger spheres, on and on until they reached their current size. But when scientists try to recreate this story with computer models, it breaks down. Rather than growing, these incipient planets tend to splinter after reaching pebble size. How could this process result in bodies the size of those in the asteroid belt, let alone whole planets?
Enter the “born big” hypothesis. Nesvorny and many others now think that gravity kicks in once clumps in the circumstellar disk reach the pebble stage, swiftly pulling together massive amounts of material to form a huge new planet. In the inner solar system, this produced small, rocky planets such as Earth; farther from the sun, we got gas giants.
“The whole business of formation and evolution of planets and the question of ‘What do we need to form an Earthlike planet elsewhere?’ is something we can finally discuss in meaningful terms,” Dermott said.