The inner planets, Mercury, Venus, Earth, and Mars, are all terrestrial planets, meaning they are primarily composed of rock and metal. This gives them a generally spherical shape, though they are not perfectly smooth spheres.

Here's why:

* Gravity: The force of gravity pulls equally in all directions on the planet's mass, leading to a roughly spherical shape.

* Rotation: As planets rotate, they bulge slightly at the equator and flatten slightly at the poles due to centrifugal force. This creates a slightly oblate spheroid shape.

* Internal structure: The inner planets have a core, mantle, and crust, and these layers have different densities and compositions, which can contribute to irregularities in the overall shape.

* Geological activity: Volcanoes, mountains, and other geological features can create local deviations from the overall spherical shape.

Here's a more detailed breakdown:

* Mercury: Being the smallest inner planet, Mercury has a relatively smooth surface with few large features.

* Venus: Venus has a very smooth surface, making it the most spherical of the inner planets. This is because its volcanic activity has smoothed out most surface features.

* Earth: Earth has a somewhat oblate spheroid shape with a noticeable bulge at the equator. This is due to its relatively rapid rotation.

* Mars: Mars has a more irregular shape than the other inner planets, with a large bulge at its north pole and a depression at its south pole. This may be due to its past geological activity.

Overall, the inner planets are all roughly spherical, with some variations in their shape due to their rotation, internal structure, and geological activity.