Jupiter holds the title of the largest planet in our solar system. This colossal world is the fifth planet from the Sun and the first of the four outer planets, all classified as gas giants. Its immense size and mass dominate the planetary architecture of the Sun’s family. Jupiter’s composition and structure are fundamentally different from the rocky, inner terrestrial planets, accounting for its gigantic stature compared to Earth, Mars, Venus, and Mercury.
Measuring Jupiter’s Scale
The scale of Jupiter requires comparisons to Earth and the rest of the solar system to provide context. When discussing planetary size, scientists consider both volume, which determines how many Earths could fit inside, and mass. Jupiter’s mean radius is nearly 70,000 kilometers, making it about 11 times wider than Earth. Its volume is so great that over 1,300 Earths could be packed inside the gas giant.
Its mass is approximately 318 times the mass of Earth. Jupiter’s gravitational influence is a primary force in the outer solar system, containing more than twice the mass of all the other planets, moons, asteroids, and comets combined. The planet’s mass is nearly one-thousandth that of the Sun, which holds 99.8% of the solar system’s total mass. The gravitational center of the Sun and Jupiter, known as the barycenter, lies slightly above the Sun’s visible surface due to this immense concentration of matter.
The Internal Structure and Composition
Jupiter’s massive size is directly related to its classification as a gas giant, dominated by the lightest elements, primarily hydrogen and helium. Moving inward from the atmosphere, pressure and temperature increase dramatically. This causes the gaseous hydrogen to transition into a liquid state without a distinct surface boundary, creating what is often referred to as the largest ocean in the solar system—an ocean made of liquid hydrogen.
At depths about halfway to the center, extreme pressure compresses the liquid hydrogen into a state known as metallic hydrogen. In this exotic state, electrons are stripped from the hydrogen atoms, allowing the material to conduct electricity like a metal. The planet’s rapid rotation, completing a turn in under ten hours, drives electrical currents within this layer, generating Jupiter’s powerful magnetic field. This magnetic field is the strongest of any planet in the solar system.
Deeper still lies the core, thought to be a dense region of heavier elements like rock and ice. Early models suggested a solid, compact core, but data from the Juno mission indicates the core may be larger than previously expected, perhaps even diffuse or partially dissolved into the metallic hydrogen layer. The core is estimated to contain a mass equivalent to about 8 to 15 Earths, consisting of elements such as oxygen, carbon, and nitrogen. Jupiter also generates more heat internally than it receives from the Sun, a remnant of its formation and gravitational contraction.
Size Comparisons Beyond Our Solar System
While Jupiter reigns supreme in our local planetary neighborhood, its size is not the upper limit for planets found elsewhere in the galaxy. Exoplanet discoveries reveal a class of worlds known as “super-Jupiters,” which are more massive than our gas giant. Some of these are “Hot Jupiters,” giant planets orbiting extremely close to their parent stars, often with masses several times that of Jupiter.
Planetary size eventually reaches a point where the object begins to blur the line between a planet and a failed star. This boundary is defined by the mass required to initiate a small amount of fusion in the core. Objects more than about 13 times the mass of Jupiter can fuse deuterium, a heavier isotope of hydrogen, and are classified as brown dwarfs.
Brown dwarfs are often called “sub-stellar objects” because they are too massive to be considered planets but lack the mass to sustain the hydrogen fusion characteristic of a true star. Jupiter stands at the pinnacle of planetary size in our solar system, but it would need to accumulate at least 12 times more mass to cross the threshold into the realm of brown dwarfs. It remains the largest object orbiting the Sun that is not the Sun itself.