Are the 8 planets the only planets?

Answer 1

Technically speaking there are only eight planets in our Solar System, those planets being: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

There is however many extrasolar planets (3639) that have been discovered so far. Billions more likely exist in our galaxy alone.

Answer 2

If you're talking about the universe, there are likely billions of planets in each galaxy. These are called exoplanets.

If you're talking about our solar system, then as of 2017, it depends on whether you count major planets only, or all planets.

The 8 major known planets in order from the Sun are:

1. Mercury
   
2. Venus
   
3. Earth
   
4. Mars
   
5. Jupiter
   
6. Saturn
   
7. Uranus
   
8. Neptune
   

If you list ALL 13 known planets in order, including dwarf planets, they are:

1. Mercury
   
2. Venus
   
3. Earth
   
4. Mars
   
5. Ceres
   
6. Jupiter
   
7. Saturn
   
8. Uranus
   
9. Neptune
   
10. Pluto
    
11. Haumea
    
12. Makemake
    
13. Eris
    

There are actually many other objects classified as minor planets, such as Sedna, that may one day be re-classified as dwarf planets. So this number could change.