QUESTION:
If there were only eight planets in the solar system, would that affect 
earth? 

ANSWER from Howard Bushouse on March 22, 1996:
It depends a bit on which one of the nine were missing, but overall it 
would have little to no effect. The sun contains over 99% of the total 
mass in the solar system, therefore it has the dominant effect on all 
the planets. The presence or abscence of one of the planets has very 
little effect on the earth. If you were to remove one of the planets 
closest to the earth, like venus or mars, it might have a very small 
effect on the earth's orbit, but not much.