As a planet, three conditions must be met:
Orbit around the star in an approximately circular orbit.
The mass is large enough to make the celestial body spherical by its own gravity.
It can gradually clear the celestial bodies near its orbit.
Pluto does not meet the third rule. Pluto's satellite (Phobos) is too large, forming a two-planet system. So according to this definition, Pluto is excluded from the big planet and is called a dwarf planet.
The memory method of the eight planets:
1, simple notation. Five elements (fire, water and earth in Jin Mu), land, sea and air (Neptune, earth and heavenly king).
2. The first word of the name. Water, gold, fire, wood, earth and sea. It's a little long but easy to remember.
3. Imagine the memory. Although far-fetched, the memory is preserved for a long time. Crystal ball, burning wood, became the earth, the ends of the earth.
4. Poetry memorization. Water overflows Jinshan, wood burns the earth, the sky and the sea are integrated, and the solar system is vast. Burning wood scorches the earth, so there is an asteroid belt between Mars and Jupiter.