The International Astronomical Union defines a planet as a celestial body that orbits around a star (like our sun) and is big enough that it forms into a shape of a sphere by its own gravity. But a planet cannot be big enough that it can cause thermonuclear fusion. It also has to have a clear orbit with no other bodies of comparable size influencing its gravitational force.
Known dwarf planets are smaller than Earth's moon.
Pluto is a dwarf planet. So are Cere, Eris, Makemake, & Haumea. A dwarf planet is nearly round & orbits the Sun, but its orbit is not clear of other bodies.
Did You Know There Are Two Categories of Planets?
A Planet falls into two categories. It is either a Terrestrial Planet or a Gas Giant. Planets that have a body composed mainly of rock (like the Earth) is called a terrestrial planet.
Terrestrial Planets:
This would include Mercury, Venus, Mars and Earth.
Gas Giants:
planets that are made of gas. They do not have a solid surface crust. Jupiter and Saturn falls into this category. Uranus and Neptune are also Gas Giants, but they also fall into sub category called Ice Giants because they contain a huge proportion of rock and ice as well.
