A variety of the Solar Systems small bodies, to scale.
Bigger objects are round, but the small ones are anything but!
Wikipedia/Antonio Ciccolella
Gravity: the key to making big things round …
The answer to why the bigger objects are round boils down to the influence ofgravity.
An objects gravitational pull will always point towards the center of its mass.
The bigger something is the more massive it is, and the larger its gravitational pull.
For solid objects, that force is opposed by the strength of the object itself.
However, Earths strength has limits.
As Everest gets taller, its weight increases to the point at which it begins to sink.
The extra weight will push the mountain down into Earths mantle, limiting how tall it can become.
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Any areas, where the water was unusually high would sink, pulled down by Earths gravity.
But the thing is,gravity is surprisingly weak.
When that spherical shape is reached, we say the object is in hydrostatic equilibrium.
But how massive must an object be to achieve hydrostatic equilibrium?
That depends on what its made of.
Saturns moon Mimas, which looks like the Death Star, is spherical and has a diameter of 396km.
Its currently the smallest object we know of that may meet the criterion.
The faster an object in space spins, the more dramatic this effect is.
As a result, it is much less spherical than Earth.
Saturns equatorial diameter is just above 120,500km while its polar diameter is just over 108,600km.
Thats a difference of almost 12,000km!
NASA/JPL-Caltech/Space Science Institute
Some stars are even more extreme.
It spins once every nine hours or so.
Thats so fast that its equatorial diameter is 25% larger than the distance between its poles!
The short answer
The closer you look into a question like this, the more you learn.
