Large planets tend toward nearly round shapes because gravity pulls their material toward the center from every direction. Smaller asteroids can remain irregular because their self-gravity is too weak to overcome the strength of their material.
Gravity smooths large-scale shapes
As a body accumulates enough material, its interior can deform over geological timescales under its own weight. The result approaches a shape near gravitational equilibrium.
Why Earth is not a perfect sphere
Rotation makes Earth bulge slightly at the equator. Mountains, ocean basins and differences in internal density introduce additional small deviations from a mathematically perfect sphere.
What about gas giants?
Jupiter and Saturn are also nearly round, with more noticeable equatorial bulges because of their rapid rotation. Their outer materials are fluids, so rotation has a pronounced effect on overall shape.
How this relates to dwarf planets
Being nearly round is one criterion in the IAU definition of a planet or dwarf planet. Pluto is round but does not meet the separate orbital-dominance criterion used to call a world a major planet.
Frequently asked question
Does every object in space eventually become round?
No. Many small moons, comets and asteroids remain lumpy or elongated because their gravity is too weak.
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