Space

Why Do Planet And Stars Come In Different Sizes?

March 1, 2016 | by Alfredo Carpineti

Photo credit: 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. NASA

The evolution of celestial bodies takes a very long time, so our understanding of how planets and stars form is still incomplete. Now a new approach is trying to fill some gaps.

Professor Adrian Bejan of Duke University, North Carolina, and his student, Russell Wagstaff, aimed to answer why planets, stars, and other bodies come in all shapes and sizes, rather than all being the same size if matter is evenly distributed. They suggest that a system reaches equilibirum much faster when objects form to be different sizes, known as hierarchy, and so it will usually go down this route.

To come to this conclusion, they used a model to consider a uniform density of mass, and looked at how it evolved under gravity. The force of gravity between the particles in the material created a volumetric (three-dimensional) tension, and they showed that the difference in body sizes is a natural consequence of the setup.

“Ten years ago, I published this idea in the context of the cracking of mud,” Professor Bejan told IFLScience. “The cracking is hierarchical, meaning you see few large and many small cracks. In the current paper, I talk about these bodies snapping into a hierarchy and not growing through a gradual accretion process.”

The research, published in Journal of Applied Physics, is based on the principle of Constructal Law, a theory of the generation of design in nature proposed by Bejan in 1996.

The theory states that anything that can flow and morph has an evolution, and this idea has been applied to biological and non-biological phenomena, like trees, lightning, rivers, etc. This is the first application of Constructal Law to an astrophysical system, and while it raises interesting points about structure formation, it doesn’t include many important features of a real system.

“I have to acknowledge that this is the simplest imaginable model to approach this question,” said Bejan. “There are other effects which are worth adding to this model.”

The internal gas pressure, the type of elements in the material, the temperature, and the density are all important factors in the formation of structures in the the universe. The model will have to consider these to fully predict the irregularity in sizes as they arise in nature. 

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