The solar system contains eight planets, classified into two groups: the small, rocky inner planets and the enormous, gaseous outer planets. These worlds vary widely in their physical properties, including size, composition, and distance from the Sun. When determining which planet is the “lightest,” scientists refer to planetary mass, which is the total amount of matter a celestial body contains. This measurement is distinct from density, which measures how tightly that matter is compressed within a given volume.
Mercury: The Least Massive Planet
The planet with the least amount of mass in the solar system is Mercury, the innermost of the four terrestrial, or rocky, planets. Mercury’s mass is approximately \(3.30 \times 10^{23}\) kilograms, making it the smallest in both mass and physical diameter.
Mercury’s mass is only about 5.5% of the mass of Earth. It is also significantly lighter than the next least massive planet, Mars, which contains nearly twice as much mass as Mercury. This tiny world is even less massive than two of the solar system’s largest natural satellites, Jupiter’s Ganymede and Saturn’s Titan. Its small size and low mass are direct results of its formation history near the intense heat of the young Sun.
Internal Structure and Low Mass
Mercury’s low overall mass is tied directly to its small physical size, with a mean radius of only about 2,440 kilometers. This rocky world possesses an unusual internal structure. It is composed of a thin outer layer of silicate rock, known as the crust and mantle, which overlies an exceptionally large metallic core.
The planet’s mantle and crust combined are estimated to be only about 400 kilometers thick. Beneath this thin rocky shell, the iron-rich core extends to a radius of approximately 2,020 kilometers. This means the core accounts for nearly 57% of the planet’s total volume, compared to Earth’s core which makes up only about 17% of its volume. This large metallic core contributes to Mercury’s low overall mass because the lighter silicate material is less abundant than on other terrestrial planets. Scientists theorize that Mercury’s close proximity to the Sun during its formation may have caused much of this lighter, volatile silicate material to be vaporized and stripped away.
Comparing Mass and Density
Although Mercury is the least massive planet, it is not the least dense. Density is calculated by dividing mass by volume, indicating how closely packed the material inside is. Mercury is the second densest planet in the solar system, with a bulk density of \(5.427\) grams per cubic centimeter, only slightly less than Earth’s \(5.515\) grams per cubic centimeter.
This high density is a direct consequence of its giant, iron-rich core, as iron is one of the densest common elements in a planet’s interior. Earth’s density is slightly higher due to the powerful gravitational compression of its greater mass squeezing its core. If the effects of this gravitational compression were removed, Mercury’s constituent materials would actually be considered denser than Earth’s materials. Mars, the next planet up in mass, has a much lower density of \(3.934\) grams per cubic centimeter, reflecting its smaller metallic core and lighter composition.