Why is Jupiter much larger than Earth? The answer begins with the Big Bang

Earth is the largest of the rocky planets in the solar system. However, its size pales in comparison to the gaseous giant Jupiter, which could fit about 1,321 Earths inside it. This stark difference in size raised a profound question among astronomers: why are gaseous planets so much larger than rocky ones? The answer can be traced back to the origins of our universe and the creation of matter.

In a universe of giants, small worlds can be extraordinary.

The Big Bang left the young universe filled with primordial gas composed of the lightest elements: hydrogen and helium. These two elements still dominate the universe, accounting for about 98% of all visible matter. Our Sun is no exception, being composed largely of these elements, and so are the gaseous planets: Neptune, Uranus, Saturn, and Jupiter. In other words, the universe provides far more raw material for gas giants to grow from.

The heavy elements required for rocky planets like Earth took much longer to appear. They formed under the extreme conditions inside stars that began to emerge roughly 200 million years after the Big Bang. Thermonuclear reactions in stellar cores gradually transformed some of the primordial hydrogen and helium into heavier elements such as silicon and iron. However, this process was demanding and slow, becoming increasingly difficult as the element's atomic number increased.

Periodic table of the chemical elements. Source: Type Calendar.

The creation of the heaviest elements required an even more dramatic event: supernova explosions. When massive stars reached the end of their lives, they exploded and scattered their interiors across space, enriching the universe with newly forged elements. Only after generations of stars lived and died did enough heavy material accumulate for rocky planets to form. Yet even after billions of years of stellar evolution, the universe remains dominated by its simplest ingredients. Hydrogen and helium still vastly outnumber the heavier elements produced in stars.

This cosmic imbalance explains why planets like Jupiter can grow so large. Gas is abundant, rock is rare. During the formation of a solar system, there was far more gas available than the elements needed to build solid worlds. Jupiter’s enormous size is not unique to our solar system; it is a direct result of the universe's beginnings when the Big Bang filled the cosmos with light gases long before the existence of our Sun.

This proves the adage that good things come in small packages. Planets like Earth are made from some of the rarest and most complex elements the universe can produce. Life takes this complexity even further, assembling those elements into extraordinary structures such as DNA. From them emerged a tiny species of human beings capable of looking back across billions of years and discovering that their story began with the Big Bang.