The shape of objects in the Solar System is primarily determined by their mass and the resulting gravitational forces they exert. For celestial bodies that are large enough—such as planets, moons, and stars—gravity pulls matter into a shape that minimizes surface area, which generally leads to a roughly spherical shape. This is especially true for objects with sufficient mass to achieve hydrostatic equilibrium, where their own gravity overcomes structural forces (like material strength) to create a rounded shape.
While other factors like composition (which affects the strength and behavior of materials), temperature (which can influence material states and physical properties), and distance from the Sun (which can affect thermal dynamics and historical processes) play roles in the evolution and characteristics of the objects, they do not fundamentally determine their shapes in the same way that mass does.
In summary, while all these factors can influence features, the most critical determinant of the shape of large celestial objects is their mass.