Why are planets round?
The Milky Way, our solar system, was formed like many other star systems from a nebula. A nebula is a concentrated area of gases and cosmic dust in the Interstellar Medium or the ISM. These cosmic dusts, and gases swim through space and when a large amount of it gathers, it starts gaining suitable gravity to establish a gravitational field. This clumping creates planets and started the process for star formation.
The clumps of gases and cosmic dusts grow bigger and bigger to form what we know as planets. The force of collision of these clumps cause the newly forming planets to become hot and molten. The gravitational force pull this molten material inside, towards the planet’s center into the shape of a sphere. When these planets cool down, they remain spherical. Since planets also spin on their axis, they are not perfect spheres. The spinning force acts against gravity and cause planets to bulge more around their equators. So, gravity is the main cause of planets being round.