Our Solar System is a mixed bag of objects whizzing round the Sun, varying from the mega gas giants to miniature meteoroids. And we're not particularly special – the same size diversity seems hold for other solar systems too. Why is this?
A couple of engineers ventured into the world of cosmology and, in the Journal of Applied Physics, suggested a reason for this assortment.
The basis of the argument, put forward by Adrian Bejan and Russell Wagstaff from Duke University in the US, is the constructal law – a theory devised by Bejan in 1996 that explains patterns and geometry in nature.
Rain, for instance, doesn't usually get soaked up by the ground. Drops gather to move together across the top of the soil in streams and rivers to empty into lakes and oceans because, according to the theory, water flows better with this design.
The law has also been used to explain topics such as snowflake shapes, city sizes and animal locomotion, but it was only after a conversation with undergraduate student Wagstaff that Bejan applied it to the Solar System.
"This is my first trespass into planetary science," Bejan said.
When the Solar System formed 4.6 billion years ago, it was little more than a disk of gas surrounding our young Sun.
Eventually the disk coalesced into blobs. So why didn't the blobs all remain the same size?
Gravity is to blame, according to Bejan. He and Wagstaff calculated the gravitational pull, or tension, between bodies of the same size suspended in space. The tension, they argue, must be reduced according to the constructal law.
Indeed, they calculated the gravitational tension eased when some of those bodies "snapped" together to coalesce into larger chunks, while most remained relatively small.
"I never thought I would have anything to say about celestial bodies in pure physics, but by chance I realised I have a key to open a new door," he said.
"Everything has evolution and the constructal law can help predict it. The plan is to keep exploring."
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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