Planets form because of gravity, the force that attracts all matter together. Objects get larger as they collect more matter, and the more massive they get, the stronger their gravitational field. But this explanation does little to explain so-called "hierarchy," or why planets within a solar system have such size diversity. Why don't all the planets form more uniform sizes?

Enter Adrian Bejan, a scientist at Duke University who specializes in thermodynamics. Back in 1996, he proposed a new law of physics called the Constructal Law which helps to explain the shape and form of systems. So far the law has been artfully applied to things like snowflakes, river basins, trees and their branches, lightning, tectonic plates, and even lungs. Now Bejan also believes it can be applied to solve the hierarchy problem, reports Astronomy Magazine.

“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 Bejan. “To my huge surprise, this question has been overlooked.”

The Constructal Law basically pertains to the flow of currents. It states that for any finite-size system to persist in time, it must evolve in such a way that it provides easier access to the imposed currents that flow through it. For instance, river systems form as they do, with their branching streams that turn into rivers, to best facilitate the flow of water.

But what kind of "currents" flow through the solar system to produce the sizes of the planets? The answer, says Bejan, is gravity. Celestial objects within a solar system exert a certain degree of gravitational tension on one another.

Bejan and student Russell Wagstaff started making calculations, and found that if a solar system consisted of uniformly sized planets with equal distances from one another, this system would be extremely tense. The best way to relieve this tension, they found, was for bodies to form in many different sizes, some large and some small, much like how our solar system is constructed now.

It's kind of like why parched soil forms the patterns that it does as it cracks and dries, in order to best relieve tension. Only in the case of the planets, that structural tension is caused by gravity.

Though Bejan's Constructal Law has met with some criticism — for instance, some have suggested the concepts of current and flow within it are vague — it is steadily receiving increased acceptance within the scientific community. The fact that it can be applied to planetary physics only bolsters the case that it may in fact represent a fundamental first principle of physics that had been overlooked until now.

It's an exciting theory, one that suggests all physical systems, whether animate or inanimate, are living systems. They are, like living creatures, constantly shifting and evolving, engaged in a sort of dance to facilitate flow, and to thus survive. It gives a whole new meaning to the catch-phrase, "go with the flow."

“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,” said Bejan. “Everything has evolution, and the Constructal Law can help predict it. The plan is to keep exploring.”