Is a planet ever bigger than a star? Generally, the answer is no, but in certain astronomical situations, a planet’s physical size can indeed exceed that of a star. This surprising overlap occurs because “star” and “planet” are fundamentally defined by their mass and the nuclear reactions within their cores, not by their physical radius alone. The size of an object is not always proportional to its mass, especially at the extreme ends of the stellar and planetary scales. Understanding these extremes requires focusing on the underlying physics of mass, density, and fusion.
Defining the Difference Between Stars and Planets
The most significant distinction between a star and a planet lies in nuclear fusion. A true star, like our Sun, is defined by its ability to initiate and sustain the fusion of ordinary hydrogen into helium in its core, which counteracts gravitational collapse. For this sustained reaction to occur, an object must possess a minimum mass of approximately 0.08 times the mass of the Sun, or about 80 times the mass of Jupiter.
Objects that fail to reach this critical mass limit are termed substellar. Within this category, a further division exists based on the fusion of deuterium, a heavier hydrogen isotope. Objects with a mass greater than about 13 Jupiter masses are capable of briefly igniting deuterium fusion in their cores.
This 13 Jupiter mass threshold acts as the formal boundary separating gas giant planets from brown dwarfs, which are often called “failed stars.” Classification is a direct consequence of an object’s mass and the specific nuclear reactions it can perform. The physical radius is a secondary characteristic determined by how the object’s interior structure responds to its total mass.
The Smallest Stars and Star Remnants
The small end of the stellar spectrum includes objects whose physical size is surprisingly small compared to their mass. These objects fall into two main categories: brown dwarfs and the highly compressed stellar remnants known as white dwarfs.
Brown dwarfs exist between the largest planets and the smallest hydrogen-fusing stars. Their radius is typically similar to Jupiter’s, even as their mass increases up to 80 times that of Jupiter. This occurs because tremendous gravity compresses the core as mass is added, preventing the radius from expanding significantly.
The most extreme size reduction is seen in white dwarfs, which are dense stellar remnants left after a star like the Sun exhausts its nuclear fuel. They are the dense, collapsed cores of former stars, not generating energy through fusion. A typical white dwarf contains a mass comparable to the Sun squeezed into a volume roughly the size of Earth. This immense density is maintained by electron degeneracy pressure.
Because of this extreme compression, white dwarfs are physically tiny, with a radius sometimes less than one percent that of the Sun. This means a white dwarf, which is incredibly massive, can be physically smaller than the gas giant planets in our Solar System. The existence of these dense, Earth-sized stellar cores creates the conditions where a planet can be physically larger than a star.
The Largest Gas Giant Planets
The largest known planets are the massive gas giants often referred to as “Super-Jupiters.” These planets are defined by their low mass, falling below the 13 Jupiter mass threshold required for deuterium fusion.
As a gas giant gains mass, its radius initially expands, but this growth does not continue indefinitely. Once a planet reaches a mass near that of Jupiter, increasing gravitational force begins to compress the internal gas layers. This compression means that adding more mass primarily results in a denser planet, not a physically larger one.
The maximum possible size for a planet is capped by this gravitational compression. The largest exoplanets discovered typically have a radius no more than about 1.4 to 1.7 times the radius of Jupiter. Beyond this point, the core becomes so compressed that the physical size starts to shrink, even as its mass climbs toward the brown dwarf limit.
Size Comparison and the Overlap Zone
The synthesis of these limits provides the definitive answer to whether a planet can be bigger than a star. The clearest instance of size reversal involves comparing large gas giants and stellar remnants. A large Super-Jupiter, which can be up to 1.7 times the radius of Jupiter, is physically much larger than a typical Earth-sized white dwarf.
In this scenario, a massive, tiny white dwarf is orbited by a planet physically many times its size. Furthermore, overlap exists even with true, hydrogen-fusing stars. Some of the lowest-mass red dwarf stars can be physically smaller than Jupiter; for example, the red dwarf star 2MASS J0523–1403 has a radius smaller than Jupiter’s, confirming that some planets are physically larger than some true stars.
The size comparison is highly ambiguous near the 13 Jupiter mass boundary separating the largest planets from the smallest brown dwarfs. Since both are governed by similar gravitational compression physics, their radii are often indistinguishable, hovering near Jupiter’s size. Despite the existence of physically larger planets, the fundamental classification remains dependent on mass and the presence or absence of nuclear fusion.