Migrating giants enrich asteroid belt
Jul 17, 2009 1 comment
The Solar System underwent a violent shake-up during its teenage years slinging millions of dark primitive rocks and ice from beyond Neptune into the main asteroid belt, which lies between Mars and Jupiter. This idea contrasts with the standard picture of Solar System formation, which assumes that the entire contents of this belt originate from the same part of the Solar System. It could also yield valuable insights into how the Earth was born from a cloud of interstellar dust nearly 5 billion years ago.
The main asteroid belt in the Solar System is a narrow disc of small objects lying between the orbits of Mars and Jupiter. Images from spectroscopy along with samples gathered from rare collisions with the Earth reveal a wide variety of material from primitive ice-rock mixtures through to igneous rocks. The standard interpretation is that this diversity is a good indicator of what conditions were like in the early solar system before the planets were formed.
A violent youth
In the widely accepted view of how the Solar System evolved, the planets formed in the same orbits they occupy today, by gravitational collapse of a giant molecular cloud some 4.6 billion years ago. In this interpretation, the material in the asteroid belt has also remained in its current location throughout the history of the solar system. Meteorites are studied by geologists to get an idea of what the planet was composed of at the beginning of Earth history.
Now, Harold Levison at the NASA Lunar Science Institute and his colleagues offer a different view of the early Solar System through a simulation they have been developing since 2005. The “Nice model” has all the giant planets forming within a compact configuration between 5 and 15 AU from the Sun — 1 AU is the distance between the Earth and the Sun. Then, after roughly 600 million years, the orbits of these planets started to become unstable, resulting in Uranus and Neptune being scattered outwards into proto-planetary material, which stretches to 30 AU from the Sun.
In astronomical terms this process occurs very rapidly taking just 10,000 years for these planets to migrate to their present orbits. As a result, the arrival of these planets sends material flying in all directions including back towards the inner Solar System. A significant number of these asteroids are then captured in orbits between Mars and Jupiter, or as Trojan asteroids — sharing an orbit with Jupiter. “This is a violent process. I like to use the analogy of a bowling ball colliding with a set of pins,” explained Levison.
Levison and his team estimate that 10–20% of the material in the main asteroid belt could have arrived by this process. He told physicsworld.com that his team intend to develop their research by refining the initial conditions that existed in the proto-planetary disc. “The Nice model is undoubtedly controversial, but there are no other alternative numerical models that can explain how the solar system evolved into its current configuration,” he said.
This research was published in the latest issue of Nature.
About the author
James Dacey is a reporter for physicsworld.com