Planets are emphatically smaller than stars. This size difference is a fundamental distinction dictated by the laws of physics and the ultimate fate of the celestial body. The vast disparity in scale reflects a difference in their internal composition and energy generation. Understanding the relationship between mass and size reveals why stars are enormous while planets are comparatively tiny.
The Physical Difference Between Stars and Planets
The distinction between a star and a planet rests on mass, which determines if an object can generate energy through nuclear fusion. Stars begin as massive clouds of gas and dust that collapse under their own gravity. This gravitational compression heats the core to high temperatures and pressures.
Once the object’s mass exceeds a threshold—approximately 80 times the mass of Jupiter—the core becomes hot enough to ignite stable, sustained nuclear fusion of hydrogen into helium. This process releases tremendous amounts of light and heat, defining the object as a star. This internal energy source fights against gravity, stabilizing the star’s size for billions of years.
Planets never reach this critical mass threshold. Even the largest gas giants lack the necessary internal pressure to initiate hydrogen fusion. They cool and contract over time, radiating only residual heat left over from their formation and gravitational compression. Without an internal furnace to maintain their size, they remain significantly smaller than their stellar counterparts.
Scaling the Solar System: Size Comparisons
The size difference between a planet and a star is difficult to visualize without concrete comparisons. Our Sun, an average-sized star, has a radius about 109 times greater than Earth’s. Over one million Earths could fit inside the volume of the Sun.
Even Jupiter, the largest planet in our solar system, is dwarfed by the Sun. Jupiter has a diameter approximately 11 times that of Earth. However, the Sun’s diameter is still about 10 times larger than Jupiter’s.
Roughly 1,000 planets the size of Jupiter could be packed inside the Sun. This disparity highlights how mass dictates scale in the cosmos, creating objects fundamentally different in their dimensions. The Sun contains over 99.8% of the total mass of the entire solar system, emphasizing its dominance in size over all the orbiting planets combined.
The Edge Cases: Brown Dwarfs and Super-Jupiters
Classification becomes complex at the margins, where objects blur the line between the largest planets and the smallest stars. Brown dwarfs, often called “failed stars,” occupy this transitional mass range. These objects are too massive to be considered planets but lack the mass required for stable, sustained hydrogen fusion like true stars.
A brown dwarf must possess a mass between about 13 and 80 times that of Jupiter. They are massive enough to ignite the fusion of deuterium, a heavier isotope of hydrogen, which occurs at a lower temperature and pressure than standard hydrogen fusion. This short-lived fusion process distinguishes them from planets.
The largest planets, known as “Super-Jupiters,” can reach masses up to the 13-Jupiter-mass limit. Curiously, a brown dwarf that is 80 times the mass of Jupiter can have a physical radius nearly identical to Jupiter itself. This is because the immense gravity of the higher-mass object compresses it so tightly that the size does not increase proportionally with the mass, a phenomenon related to electron degeneracy pressure within the core. While the size of the largest planet and the smallest brown dwarf may be similar, their internal physics and mass clearly define the star-planet boundary.