A Time Of Tumult

Elizabeth Pearson investigates how the Solar System was transformed 3.9 billion years ago.

- Elizabeth Pearson

When the Solar System first formed 4.5 billion years ago it was a violent place. But quickly, by 4.4 billion years or so ago, the planets had calmed into a familiar configuration – several rocky inner worlds surrounded by gas giants ringed by icy objects. Then, four billion years ago, something catastrophic happened. The planets were thrown once more into chaos, and the face of the Solar System changed forever. This period of upheaval reveals startling truths about the evolution of our home planetary system,and perhaps the origins of life itself.

In the Solar System’s first hundred million years or so, the beginnings of planets clumped together from the dust of a boiling protoplanetary disc around our young star. Frequently the young planets would collide and grow larger, though sometimes they would be destroyed entirely. To start, this early Solar System was much like it is today, but there were several key differences.

“Today, we have giant planets from Jupiter at about 5 AU from the Sun to Neptune at about 30 AU,” says William Bottke from the Southwest Research Institute in Boulder, Colorado; 1 AU is the distance between the Earth and the Sun. “Modelling work shows the ice giants Neptune and Uranus would never have reached their current sizes if they had to form in the current configuration of the planets. Studies suggested instead that all these bodies formed between about 5 and 20 AU. Beyond that lies the Kuiper Belt. What’s interesting about the Kuiper Belt now is that a lot of objects have very special orbits, called resonances. It’s very hard to get them into these resonances.”

Synced surprise