Earth, the largest terrestrial planet, provides a natural benchmark for measuring celestial bodies. Pluto, discovered in 1930 and reclassified in 2006, offers a striking contrast to our home planet. Understanding the magnitude of this difference requires examining the precise physical measurements that define their sizes. This comparison reveals just how small Pluto truly is when set against the backdrop of the inner solar system.
Direct Linear Measurement Comparison
The equatorial diameter defines the overall linear dimension of each body. Earth possesses a diameter of approximately 12,742 kilometers (7,918 miles). In stark contrast, Pluto’s diameter is only about 2,377 kilometers (1,473 miles), making it a significantly smaller world.
This linear comparison shows that Pluto is roughly 18.6% the diameter of Earth. If one were to hypothetically walk the entire circumference of Pluto, the total distance traveled would be less than the distance spanning the continental United States. Pluto is also notably smaller than Earth’s own Moon, which has a diameter of about 3,474 kilometers.
This emphasizes that Pluto’s linear size places it among the smaller satellites and icy bodies in the solar system. The sheer difference in circumference means that Earth’s surface area is vastly greater, allowing for the varied geography and biomes we observe.
Mass and Volume Relative Scale
The difference in size becomes far more pronounced when comparing three-dimensional volume and mass. Approximately 170 objects the size of Pluto could be packed inside our home planet. This disparity in volume relates directly to the vast difference in mass, the total matter contained within a body.
Earth is approximately 500 times more massive than Pluto, which profoundly impacts the gravitational pull each body exerts. The low mass of Pluto results in an extremely weak gravitational field at its surface. For instance, a person weighing 150 pounds on Earth would only register about 10 pounds on Pluto.
Pluto has a much lower overall density compared to Earth, which is composed primarily of rock and iron. Pluto is an icy world composed of rock and frozen volatiles like nitrogen, methane, and carbon monoxide.
Contextualizing Pluto’s Dwarf Planet Status
The size discrepancy between Earth and Pluto is fundamentally linked to Pluto’s classification as a dwarf planet. The International Astronomical Union (IAU) established three criteria in 2006 for a body to be considered a full planet. Pluto meets two criteria: it orbits the sun and has achieved hydrostatic equilibrium, meaning its gravity has pulled it into a nearly round shape.
However, it fails the third criterion: it has not “cleared the neighborhood” around its orbit. This means Pluto shares its orbital path with many other significant objects in the distant Kuiper Belt. Pluto is merely one of the largest bodies within this population of icy, trans-Neptunian objects.
Its relatively small size is typical of the icy planetesimals that formed in the outer reaches of the solar system.