Stars and planets often appear similar to the casual observer, but they are fundamentally distinct objects with separate physical properties and roles in the cosmos. Both are large, rounded astronomical objects held together by gravity, yet they represent two entirely different classes of phenomena. Understanding the difference requires examining their internal mechanics, physical composition, and how they interact within a solar system. The defining line between these two cosmic entities is drawn by a body’s capacity to generate its own energy, its internal state of matter, and its dynamic influence over its orbital path.
The Critical Difference: Energy Generation
The most significant distinction between a star and a planet is the star’s ability to create and sustain its own light and heat through an internal process. Stars are massive spheres of gas that generate tremendous amounts of energy through thermonuclear fusion occurring deep within their cores. This process involves immense gravitational pressure and temperatures, which force hydrogen nuclei to combine and form helium nuclei, releasing a vast output of energy.
A star remains stable for billions of years because the outward pressure from continuous fusion balances the relentless inward pull of its own gravity, a state known as hydrostatic equilibrium. Planets, by contrast, do not have the necessary mass to initiate or sustain this nuclear reaction. They only shine by reflecting the light emitted by their parent star, or they give off negligible amounts of heat remaining from their formation or internal radioactive decay.
The mass threshold required for an object to become a star is the boundary that separates these two categories. A celestial body must possess at least 75 to 80 times the mass of Jupiter, or about 0.08 times the mass of our Sun, to generate the core temperature and pressure needed for sustained hydrogen fusion. Objects below this limit, such as gas giant planets, are sometimes referred to as “failed stars” because they lack the necessary gravitational compression. Objects slightly more massive than Jupiter, but still below the stellar limit, are brown dwarfs; they can briefly fuse deuterium but are not considered true stars.
Physical State and Composition
The internal physical state and chemical makeup of stars and planets also differ dramatically. A star exists primarily as a superheated, ionized gas known as plasma throughout its interior. Plasma is a state of matter where electrons have been stripped from their atoms due to extreme temperatures. This plasma is composed almost entirely of the lightest elements, typically around 75% hydrogen and 25% helium, with only trace amounts of heavier elements.
Planets, however, are made up of differentiated layers that exist in solid, liquid, or gaseous states, but not plasma throughout. Terrestrial planets, like Earth, possess a solid metallic core, a mantle, and a rocky crust. Gas giants are composed mostly of hydrogen and helium but lack the internal conditions for fusion. The composition of planets is far more varied than stars, containing a wide range of heavier elements that condense into rock, ice, and metal, which were synthesized in previous generations of stars. The internal structure of a planet is layered by density, with heavier materials sinking to form a core, a process known as planetary differentiation.
Navigating the Cosmos: Orbital Roles and Definition
The role an object plays within a star system is a key differentiator, reflected in the formal definitions used by astronomers. Stars are the primary, gravitationally dominant bodies around which other objects revolve. A planet is a secondary body that orbits a star, or a stellar remnant, and is not massive enough to initiate thermonuclear fusion.
In 2006, the International Astronomical Union (IAU) established a definitive set of criteria for a body to be classified as a planet. Beyond orbiting the Sun and having sufficient mass for its own gravity to pull it into a nearly round shape, a planet must also have “cleared the neighborhood” around its orbit. This criterion means the object has become gravitationally dominant in its orbital zone, either by accumulating or ejecting other objects of comparable size.
This requirement is irrelevant to stars, which are the central hubs of their systems. It is the point where objects like the dwarf planet Pluto fall short. Pluto shares its orbital region with a population of other Kuiper belt objects, meaning it has not achieved the necessary dynamical dominance to be called a planet. Therefore, a star is a self-luminous, primary body that anchors a system, while a planet is a non-luminous, secondary body that has gravitationally cleared its orbital path.