No planet in the universe is bigger than the Sun. The Sun is a star, and stars are fundamentally different from planets in a way that makes them vastly and inevitably larger. The Sun’s immense size is a direct consequence of the physical processes that define a star, processes that no planet can achieve. The distinction is not merely one of scale but of an entirely different cosmic identity. The largest planets ever discovered, while enormous compared to Earth, are still dwarfed by the smallest known stars. The boundary between these two types of celestial bodies is defined by a critical mass threshold.
The Physical Boundary Between Planets and Stars
The defining characteristic separating a star from a planet is the ability to sustain nuclear fusion in its core. To ignite this reaction, a celestial body must possess a mass equivalent to about 7.5% to 8% of the Sun’s mass. Below this minimum, the core temperature and pressure never reach the millions of degrees Kelvin required to fuse hydrogen atoms into helium.
The Sun generates its own light and heat through self-sustaining hydrogen fusion, allowing it to achieve and maintain its gargantuan size. Planets lack the necessary mass to begin this process. This mass difference inherently limits the maximum possible size of a planet.
The structure of both stars and planets is governed by hydrostatic equilibrium, where outward pressure counteracts the inward pull of gravity. In a star, nuclear fusion provides the necessary outward pressure to resist gravitational collapse. In a gas giant planet, increasing mass beyond a certain point does not lead to continuous expansion. Instead, the growing gravitational force compresses the material, causing the radius to stabilize or even shrink, a phenomenon influenced by core pressure.
The Largest Planets Astronomers Have Found
Astronomers have discovered exoplanets far larger than any in our solar system, often called “Super-Jupiters” or “puffy” planets. These gas giants push the limits of planetary size but remain far smaller than the Sun. For example, the exoplanet WASP-17b has a radius nearly twice that of Jupiter.
Another notable example is TrES-4b, which is approximately 1.7 times the radius of Jupiter. These planets often orbit extremely close to their host stars, and the intense stellar radiation causes their atmospheres to inflate dramatically. This extreme heating lowers the planet’s density, making them “puffy” rather than increasing their mass to stellar levels.
Despite their enormous radii, these largest planets are only a fraction of the Sun’s size. The Sun’s radius is more than five times larger than that of TrES-4b. The Sun’s diameter of nearly 1.4 million kilometers is a scale no known planet can approach.
The Smallest Stars in the Universe
To appreciate the size gap, compare the largest planets to the smallest true stars, known as Red Dwarfs or M-dwarfs. These stars are at the lower end of the stellar mass range, barely exceeding the minimum mass required for sustained hydrogen fusion. Even these low-mass stars are significantly more massive and larger than any planet.
A well-known example is the Red Dwarf TRAPPIST-1, which has a mass of about 8% of the Sun’s mass and a radius only slightly larger than Jupiter’s. The star OGLE-TR-122b, one of the smallest measured main-sequence stars, is only about 20% larger than Jupiter, yet it possesses over 100 times Jupiter’s mass.
In the cosmic inventory, Brown Dwarfs occupy the mass gap between the largest planets and the smallest hydrogen-fusing stars. These objects are often referred to as “failed stars” because they are too massive to be planets but lack the mass to ignite sustained hydrogen fusion. Although they cannot fuse hydrogen, they can briefly fuse a heavier hydrogen isotope called deuterium. The smallest true stars are only marginally larger than the largest planets, but their mass and internal energy production are fundamentally different.
Why Size Is More Than Just Radius
Comparing celestial bodies by radius alone can be misleading because size does not always correlate directly with mass. For gas giants, adding more mass primarily increases the density and internal gravity, rather than continually expanding the radius. A gas giant planet with a mass several times that of Jupiter may have a radius that is only slightly larger, due to immense self-compression.
The radius of a gas giant tends to plateau around the size of Jupiter, even for objects that are many times more massive. This effect of self-compression physically constrains the maximum size of a planet, preventing it from ever growing to the colossal scale of a star. The Sun’s enormous size is a result of its mass being balanced by the extreme outward pressure generated by nuclear fusion, a power source unavailable to any planet.