The reason why objects in space, including celestial bodies like planets and stars, appear spherical in shape is due to a phenomenon called gravitational equilibrium.
Gravitational equilibrium is the balance between gravity, which pulls matter inward, and the internal forces within the object, which push outward. When an object is massive enough, its gravity will cause it to pull all its constituent parts toward its center, resulting in a spherical shape.
On a smaller scale, the same principle applies to smaller objects like drops of water or even solid objects on Earth. Gravity acts uniformly on all parts of the object, pulling them toward its center. The object's material properties, such as its viscosity or solid composition, determine how the object maintains its shape under the influence of gravity. The forces between molecules or particles within the object push against gravity, resulting in an overall equilibrium that gives the object a spherical shape.
This gravitational equilibrium is also responsible for the spherical shape of Earth. Gravity pulls everything on Earth toward its center, causing the Earth to take on a nearly spherical shape, although it is slightly flattened at the poles and bulges at the equator due to its rotation. The force of gravity acts symmetrically in all directions, which leads to the formation of a sphere.
It's important to note that while large celestial bodies tend to be spherical due to their gravitational forces, smaller objects like asteroids or irregularly shaped moons may deviate from a perfect sphere due to their lower gravitational forces or other factors.