Stars are overwhelmingly larger than planets. The fundamental difference between these celestial bodies is not merely size, but the physical process that powers them. Stars are self-luminous, generating immense light and heat, while planets are non-luminous bodies that orbit a star. This distinction, determined by mass and internal conditions, establishes a vast difference in their typical sizes.
Fundamental Differences in Formation
The minimum size of a star is dictated by the precise amount of mass required to initiate a specific thermonuclear reaction in its core. For a gas cloud to become a true, stable star—a main-sequence star—it must accumulate enough mass to generate the pressure and temperature needed for sustained hydrogen fusion. This threshold is calculated to be approximately 80 times the mass of Jupiter, or about 7.5% of the Sun’s mass.
Planets, conversely, are formed through a process of accretion where material in a protoplanetary disk gradually clumps together. Even the largest gas giants, which are primarily composed of hydrogen and helium, never achieve the necessary mass to ignite their cores. The requirement for fusion ensures that the smallest star is still vastly more massive and larger than the largest known planet.
Understanding the Scale Comparison
The sheer magnitude of the size difference is best illustrated by direct comparison using our own solar system. Jupiter, the largest planet, has a diameter roughly 11 times that of Earth. The Sun, a relatively average star, has a diameter about 10 times that of Jupiter.
This means that approximately 1,300 Earths could fit inside the volume of Jupiter, yet over 1,300 Jupiters could be compressed into the volume of the Sun. While stellar sizes vary dramatically—from small red dwarfs to enormous supergiant stars—the smallest, dimmest red dwarfs maintain a radius that is still significantly larger than any known planet.
When Size Boundaries Blur
While the mass difference remains absolute, the physical radius comparison can sometimes blur the boundary between the largest planets and the smallest stellar objects. This occurs primarily with two specific astronomical objects.
Brown dwarfs, often called “failed stars,” are substellar objects that fall into the mass gap between the largest planets and the smallest true stars, typically ranging from 13 to 80 Jupiter masses. Despite their far greater mass, brown dwarfs are only slightly larger in radius than Jupiter. This is because their immense gravity compresses their internal material to a high density, preventing a large increase in physical size.
Certain stellar remnants, such as white dwarfs, can be comparable in size to Earth. These objects are the dense cores of stars that have completed their life cycle and are no longer fusing hydrogen, yet they are still classified by their stellar origin.