Why celestial bodies come in different sizes

Researchers find that a universe that contains some big objects and many small objects relieves gravitational tension faster than a uniform universe.
By | Published: March 2, 2016 | Last updated on May 18, 2023
Solar system
The solar system, illustrated in this artist’s conception, contains both large and small objects. Researchers from Duke have proposed a new explanation for why the size diversity exists.
Our solar system contains one massive object — the Sun — and many smaller planets and asteroids. Now researchers from Duke University in Durham, North Carolina, have proposed a new explanation for the size diversity, which is found throughout the universe and is called hierarchy.

“Since the 1700s scientists have known that gravity causes objects in the universe to get bigger, but the phenomenon of growth does not explain the hierarchy,” said Adrian Bejan from Duke University. “To my huge surprise, this question has been overlooked.”

Bejan’s specialty is in thermodynamics, and he is the author of the Constructal Law, which states that natural systems evolve to facilitate flow. He had already applied the law of evolution to explain the shape of snowflakes, river basins, lungs and even airplanes, when a conversation with an undergraduate student prompted him to consider how the Constructal Law would manifest itself in the cosmos.

“This is my first trespass into planetary science,” he said.

Bejan and his student, Russell Wagstaff, started by calculating the tension caused by gravitational attraction between bodies of the same size, uniformly distributed in space. They showed that if the bodies coalesce into some large bodies and some small bodies, the tension is reduced faster than if the bodies merged uniformly.

“The discovery is that hierarchy ‘snaps’ from the very beginning, spontaneously,” Bejan said. The break-up of the uniform suspension of bodies into a few large and many small bodies occurs because it’s the fastest way to ease the internal tension caused by gravity, he said.

The natural tendency of a system to evolve toward a state of reduced tension is a manifestation of the Constructal Law, Bejan said, and can be seen in other phenomena, such as soil cracking under a drying wind.

Bejan said he hopes the application of the Constructal Law to the cosmological scale will inspire other researchers to consider how the law could be applied in their own fields.

“I never thought I would have anything to say about celestial bodies in pure physics, but by chance I realized 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.”