Why the planets are round
The simply answer is gravity. Gravity is the force that pulls every object in the universe towards every other object in the universe. There is gravity on or in every object that exists in the universe.
Think of making a snowball. First you gather up a big hand full of snow. Then cup the snow in your palms and press towards the center, while slowly turning the sphere in your cupped hands. The result is a rounded snowball. Now, think of the planets with all the gases, rotating around the sun with the gravitation force of the sun pulling the gases toward the center. The result is a rounded planet.
Gravity pulls objects toward the center of the earth. If an object, say a planet, is high enough to escape from the atmosphere around the earth, it is then pulled from the sun’s gravity. The object/planet is pulled by the sun’s gravity and goes into orbit around the sun.
Since there is no atmosphere and no friction, the planet will orbit forever. The gravity remains constant pulling the planet towards the center. The gravity pull is equal on all sides of the planet. The matter is balanced by the internal forces. The planet rotates in its’ orbit, and just like the snowball, the planet is a rounded shape.
The planets never escape from the pull or attraction of the sun. They keep moving around it in orbits
Scientists have been studying the solar system for thousands of years. In 1610, Galileo was the first person to use a telescope to study the sky. He agreed with Copernicus that the planets orbited the sun. Until this period of time the people thought the earth was the center of the universe. Galileo’s ideas were controversial. The Catholic Church writings believe he was telling facts contrary to Scriptures. He spent the last years of his life under house arrest. It was not until 1992 that Pope John Paul II vindicated Galileo.
The International Astronomical Union (IAU) was asked to write a definition for a planet. A resolution passed in 2006, said “a classical planet orbiting the sun must have a rounded shape from the effects of its own gravity and must be the dominant object in its region of space.”
The IAU recognized eight classical planets. The ninth planet, Pluto, was called a “dwarf planet.” A dwarf planet does not have massive enough for their gravity to clear other bodies.
These new definitions have met some controversy and opposition. But there you have it; the IAU said a planet is rounded!