The word planet comes from the Greek language. The International Astronomical Union has never officially defined the term planet , and it tries to follow historical guidelines in accepting what is or is not a planet. The object must have a diameter of at least 1 or 2 thousand kilometers. The object must be large enough that its self-gravity will shape it into a sphere. The object must have a dominant mass for its particular distance range from the Sun.
Astronomers often refer to asteroids as minor planets .
Pluto does not fit well into this category, either. These objects have eccentric orbits that cross the orbit of the planet Neptune, although they will never collide with Neptune. Pluto seems to be far larger than other TNOs and it is the only one with a moon, but Pluto does have more in common with these objects than other planets. Other TNO objects are now referred to as Plutinos. Pluto's orbit crosses Neptune's, like a shortcut, bringing it closer to the Sun than the gas giant.