The largest planet in our solar system is JUPITER, which has seven letters. It is the fifth planet from the Sun and is two and a half times more massive than all the other planets combined. JUPITER is composed mostly of hydrogen and helium and is the most massive planet in the Solar System. It has a diameter of 142, 984 km, making it the largest planet in the Solar System. Its strong gravitational field is home to the Galilean moons, a system of four large moons discovered by Galileo Galilei in 1610.