The four planets closest to the sun, Mercury, Venus, Earth, and Mars, are known as the inner planets or terrestrial planets. They share these common features:

* Rocky Composition: They are primarily composed of rock and metal, unlike the gas giants further out.

* Solid Surface: They have a solid surface, unlike the gas giants.

* Smaller Size: They are relatively smaller in size and mass compared to the outer planets.

* Few Moons: They have few or no moons, unlike the gas giants.

* No Rings: They don't have prominent ring systems like Saturn.

* High Density: They have a higher density than the outer planets.

* Shorter Orbital Periods: They orbit the Sun much faster and have shorter years than the outer planets.

These similarities are due to their formation closer to the Sun, where the intense heat and solar wind prevented heavier elements from condensing into gas giants.