Spherical Celestial Bodies and the Potato Radius

Potato Radius

The potato radius is the approximate size threshold of a celestial object where an object’s gravity compresses the object into a sphere. This is 200 to 300 kilometres in radius depending on the density of the object.

The Earth’s moon, Pluto and Ceres are spherical.

Only Pluto’s largest moon, Charon, is spherical whilst Styx, Nix, Kerberos and Hydra are elongated.

Phobos and Deimos are the two moons of Mars which are irregular rocky satellites. Phobos is 22 km wide with an orbit distance of 6000 km and Deimos is 12 km wide with an orbit distance of 20,000 km.

G: gravitational constant 6.674 m^3 / (kg • s^2) OR 6.674 N • m^2/ kg
σ(y): yield strength of material in Pascals. Yield strength is the stress level at which a material stops stretching elastically and begins to deform
C: sphericity constant. A perfect sphere has a sphericity constant of 1
R(pot): The minimum radius required for the body to become rounded by gravity (the potato radius). R(pot) = C • sqrt(σ(y) / G • ρ^2)

Objects with a radius below this value remain irregular because their material strength resists deformation. Bodies exceeding this radius become spheres or spheroids.

Ceres: This dwarf planet has a mean radius of about 473 to 490 km, which is well above the potato radius, making it distinctly spherical. It is the largest object in the asteroid belt. Density is 2.16 g/cm^3. It contains a high rock and mineral content with 25% water ice content.
Pluto: This larger dwarf planet has a much bigger radius of roughly 1,150 km (or 1,188 km), easily surpassing the threshold and maintaining a round shape. Density is 1.85 g/cm^3. It contains a high proportion of volatile ices (like nitrogen, methane and water ice) and a larger rocky core.

International Astronomical Union (IAU) created the Dwarf Planet category in 2006. A dwarf plant is round celestial body that orbits the Sun but has not cleared the debris and smaller objects out of its orbital path.

A dwarf plent must:

It must orbit the Sun
It must have enough mass (weight and size) for gravity to pull it into a round shape
It must not have cleared its neighborhood of other large objects
t must not be a moon orbiting another world

Five dwarf planets have been recognised.

Ceres: The smallest and closest to the Sun, located in the asteroid belt between Mars and Jupiter
Pluto: Located in the outer Kuiper Belt
Haumea: An egg-shaped world that spins very quickly
Makemake: A bright, icy world in the outer solar system
Eris: The most massive dwarf planet, sitting far out past Pluto

Last updated on Friday 28 August 2026 at 06:25 by administrators

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Spherical Celestial Bodies and the Potato Radius

Potato Radius

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