The universe contains an astonishing array of celestial objects, from tiny dust grains to colossal stars. A fundamental question that often arises is whether stars are inherently larger than planets. Understanding the immense scales involved helps distinguish between these two primary astronomical entities. Exploring their sizes, compositions, and formation processes reveals why stars typically dwarf planets.
The Scale of Stars vs. Planets
Stars are overwhelmingly larger than planets in both diameter and mass. To illustrate this vast difference, consider our Sun, an average-sized star. The Sun has a diameter of approximately 1.4 million kilometers, making it about 109 times wider than Earth. More than 1.3 million Earths could fit inside the Sun.
The largest planet in our solar system, Jupiter, is about 11 times wider than Earth, with a diameter of roughly 142,984 kilometers. Even Jupiter, a gas giant, is still dwarfed by our Sun; approximately 1,000 Jupiters could fit inside the Sun.
The Defining Characteristics of Stars and Planets
The immense size difference between stars and planets stems from their fundamental compositions and formation processes. Stars are celestial bodies that generate their own light and heat through nuclear fusion in their cores. This process involves the fusion of hydrogen nuclei into helium, releasing enormous amounts of energy. For nuclear fusion to ignite and sustain, a star must possess a critical mass, typically at least 7.5% of the Sun’s mass.
Planets, in contrast, do not undergo nuclear fusion and therefore do not produce their own light. They primarily reflect light from their parent stars. The International Astronomical Union (IAU) defines a planet as a celestial body that orbits a star, has sufficient mass to assume a nearly round shape due to its own gravity, and has cleared the neighborhood around its orbit, meaning it has become gravitationally dominant.
Stars form from the gravitational collapse of vast clouds of gas and dust. As this material clumps together, the core heats up under immense pressure until fusion begins. Planets, on the other hand, form from leftover material that did not contribute to the star’s formation, accumulating within a protoplanetary disk orbiting the newly formed star.
Cosmic Giants and Dwarfs: Exploring Size Extremes
While stars are generally much larger than planets, both categories exhibit a wide range of sizes. Among stars, red dwarfs are the smallest, with masses ranging from about 0.08 to 0.6 times that of the Sun. The smallest red dwarfs can be just 9% the radius of the Sun, or even as small as 1.2 times the size of Jupiter. These dim, cool stars are the most common type in the Milky Way galaxy.
At the opposite end of the stellar spectrum are hypergiants, extremely rare and massive stars. Examples like UY Scuti can have a radius around 1,700 times larger than the Sun, making their volume capable of containing billions of Suns. Other red hypergiants, such as VY Canis Majoris, have been measured at over 1,400 times the Sun’s width.
Planets also vary significantly in size. Mercury is the smallest planet in our solar system, with a diameter of about 4,880 kilometers, making it a little more than one-third the width of Earth. Gas giants like Jupiter are the largest planets, but even the largest exoplanets discovered, such as ROXs 42Bb, are around 2.5 times the radius of Jupiter and about nine times its mass. Objects more massive than about 13 times Jupiter’s mass are generally classified as brown dwarfs. These are considered “failed stars” because they have more mass than large planets but not enough to sustain stable hydrogen fusion like true stars, though they can fuse deuterium. Most brown dwarfs are only slightly larger in volume than Jupiter, typically 15-20% larger, despite being significantly more massive.