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 orbital distance of 6000 km and Deimos is 12 km wide with an orbital distance of 20,000 km.

The parameters that determine the potato radius, R(pot), are: ¹

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. For a rocky asteroid, a typical value is 10 MPa with a range of 0.25 MPa for porous asteroids to stony meteorites of 200 MPa. For icy asteroids, the value ranges from approximately 32 MPa to over 120 MPa.
ρ: average density of body. Typical density for rocky asteroids is 1.5 to 3.5 g / cm^3. For icy moons, the density ranges from 0.5 to 1.5 g / cm^3.
C: sphericity constant. A perfect sphere has a sphericity constant of 1.

Multiply g/cm^3 by 1000 to obtain kg/m^3.

Objects with a radius below the potato radius 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 planet must:

Orbit the Sun.
Have enough mass (weight and size) for gravity to pull it into a round shape.
Not have cleared its neighborhood of other large objects.
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 Thursday 3 September 2026 at 20:18 by administrators

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Footnotes

Lineweaver, C. H. & Norman, M. (2010) The Potato Radius: a Lower Minimum Size for Dwarf Planets. eprint arXiv:1004.1091.

Spherical Celestial Bodies and the Potato Radius

Potato Radius

Footnotes

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