How to define the concept of planet has always been a controversial issue in astronomy.The general assembly of the International Astronomical Union adopted a new definition of"planet"on August 24,2006,which includes the following three points:
1.It must be a celestial body orbiting a star;
2.The mass must be large enough,and its own attraction must be balanced with the rotation speed to make it spherical;
3.It is not affected by other objects around the orbit and can remove other objects near its orbit.
Generally speaking,the diameter of the planet must be more than 800 kilometers and the mass must be more than 5 billion tons.
According to this definition,there are currently eight planets in the solar system,namely mercury,Venus,earth,Mars,Jupiter,Saturn,Uranus and Neptune.