Every planet in our Solar System, along with detected exoplanets, exhibits a spheroid shape—nearly spherical, with Earth being a prime example. Yet this prevalent form doesn't rule out some degree of flatness in planetary bodies.
For centuries, scientists have confirmed Earth's roundness through observations like ships vanishing hull-first over the horizon, elevated vantage points revealing broader vistas, varying Sun shadow lengths at different latitudes, the Moon's curved shadow during solar eclipses, and direct imagery from space.
While no observed planets are flat, isolated observations could mimic a flat, circular Earth. So, what's the flattest possible planet? One approach might involve crafting a massive solid disk from stone, steel, diamond, or graphene—potentially hundreds of kilometers across, rivaling the Moon's size and surpassing asteroid belt objects.
However, such a disk wouldn't qualify as a planet. In 2006, the International Astronomical Union redefined planets with three criteria: orbiting the Sun or a star; sufficient mass for hydrostatic equilibrium (yielding a quasi-spherical, oblate, or prolate shape due to rotation); and clearing its orbit of comparable bodies.
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A perfectly flat body fails the hydrostatic equilibrium test, disqualifying it as a planet. But significant flatness is achievable while meeting this criterion—through rapid rotation.
Earth rotates once every 24 hours, giving equatorial residents an extra 464 m/s velocity over polar dwellers. This centrifugal force flattens the poles and bulges the equator, forming an oblate spheroid. Earth's equatorial diameter measures 12,756 km, versus 12,714 km at the poles—a 42 km difference, or 21 km closer to the center at the poles.
Gas giants spin faster: Saturn's poles are 10% flatter than its equator. Planetary models suggest even greater extremes. No known planet shows extreme flatness, but Kuiper Belt objects like dwarf planet Haumea set records. Haumea's equatorial diameter along its longest axis is twice its shortest—a 2:1 triaxial ellipsoid, influenced by a past collision that spun it up and birthed moons Hi'iaka and Namaka. Hi'iaka's gravity adds further complexity.
Denser, faster-spinning worlds flatten more. The limit occurs when centrifugal force overcomes gravity at the equator. For an Earth-like planet, that's about 3:1; for a dense uranium world, up to 5:1.
Extreme flatness challenges structural integrity, with friction and differential rotation stressing outer layers—much like Saturn's rings, where outer edges lag inner ones. Physics thus imposes firm limits on planetary flatness, even if flatter worlds than Earth exist.
