We often take the roundness of planets for granted. Yet the shapes of the bodies that orbit our Sun reveal the powerful forces that shape the cosmos.

In reality, none of the planets or the Sun is a perfect sphere. Each is an oblate spheroid—a shape that bulges around the equator and is slightly flattened at the poles. A convenient way to picture this is a basketball gently pressed in the middle.

Image credit: NASA/ESA/STScI/University of Leicester

For Earth, the polar circumference is 24,812 mi (39,931 km) while the equatorial circumference is 24,900 mi (40,070 km). That means you travel a bit farther around the equator than from pole to pole—an effect of the planet’s equatorial bulge.

Jupiter’s bulge is far more pronounced. The gas giant’s equatorial diameter exceeds its polar diameter by about 7 %—a result of its immense size and rapid spin.

The oblate shape emerges from two competing forces:

• Self‑gravity pulls all matter toward the center. Once an object’s mass is large enough, this inward pull forces the body to collapse into a sphere.
• Rotation pushes outward at the equator. A faster spin amplifies the equatorial bulge.

As Washington State’s Goldendale Observatory director Troy Carpenter explains, “Gravity attempts to crush an object inward in all directions, while rotation tries to flatten it. The balance of these forces creates the oblate spheroid we observe.”

The Sun is nearly spherical because its gravity dominates and its rotation period—about 25 days—is relatively slow. In contrast, many stars spin much faster and exhibit noticeable equatorial bulges. A well‑known example is Altair, a star only 16.8 light‑years away that rotates once every 10.4 hours, making its equatorial diameter roughly 14 % larger than its polar diameter.

Image credit: NASA/JPL/Caltech/Steve Golden

Additional forces also influence planetary shapes. Earth’s oblateness is subtly modified by tidal pulls from the Moon and Sun, as well as by the uneven distribution of mass from plate tectonics. The result is a planet that is “lumpy” rather than perfectly smooth.

Some bodies are far less massive than planets and therefore cannot achieve an oblate shape. Mars’s moons, Phobos and Deimos, are examples of small, potato‑shaped satellites whose self‑gravity is too weak to round them.

In short, the near‑spherical appearance of planets—and even stars—is a direct consequence of gravity’s relentless pull balanced by rotational forces, with subtle adjustments from tidal interactions and internal dynamics.