The celestial objects we observe in the night sky often appear similar, leading to common confusion about whether a bright point is a star or a planet. While both are large, spherical bodies in space, their internal mechanics, light sources, and how they interact with their surroundings are vastly different. Understanding these distinctions is the first step in appreciating the true scale and variety of our universe.
The Definition of a Star
A star is defined by its ability to generate its own light and heat through nuclear fusion. Within the star’s core, immense pressure and temperature force hydrogen atoms to fuse into helium, releasing enormous amounts of energy. This thermonuclear reaction results in a self-luminous sphere of plasma, shining by its own power. Our Sun is a star, and every star visible in the night sky is a distant sun. An object must possess enough mass—roughly 0.08 times the mass of the Sun—to initiate and sustain this hydrogen fusion.
The Definition of a Planet
In contrast to a star, a planet does not generate its own light but is only visible because it reflects the light of its parent star. The International Astronomical Union (IAU) defines a planet as a celestial body that orbits a star, is massive enough for its gravity to pull it into a nearly spherical shape, and has cleared the neighborhood around its orbit. Planets are relatively cool, opaque bodies, such as Earth and Mars. They lack the necessary mass to trigger the core nuclear fusion that defines a star, and are illuminated solely by the Sun’s light bouncing off their surfaces or atmospheres.
Observing the Difference in the Night Sky
Twinkling (Scintillation)
The most practical way to distinguish a star from a planet is by observing its visual behavior, particularly its twinkling. Stars appear as distant pinpoints of light, making them highly susceptible to the turbulence of Earth’s atmosphere, which acts like a distorting lens. As starlight passes through varying layers of air density, it is constantly refracted, causing the light to rapidly shift and change in brightness—the effect known as twinkling. Planets are vastly closer to Earth, appearing as tiny discs rather than single points of light. Because the light comes from a broader apparent surface, atmospheric distortions average out, making the planet’s light appear steady and non-twinkling.
Apparent Movement
The second clear difference is their apparent motion across the sky over days and weeks. Stars are so far away that they maintain fixed positions relative to each other, forming the familiar constellations. They appear to move only because of the Earth’s daily rotation. Planets, whose name comes from the ancient Greek word for “wanderer,” exhibit a noticeable movement against the backdrop of these fixed stars. As they orbit the Sun, their position changes day by day, tracing a path known as the ecliptic.