#Java, 274 272 264 bytes (includes Pluto!)
Java, 274 272 264 bytes (includes Pluto!)
void p(String p,String l){String q="MeVeEaMaJuSaUrNePl";int w=q.indexOf(p.substring(0,2))/2,e=q.indexOf(l.substring(0,2))/2,m=1,t=e,d[]={5029,4140,7834,55039,64627,144895,162745,140538};long h=0;if(w>e){e=w;w=t;m=-1;}for(;e-->w;)h+=d[e]*1e4;System.out.print(h*m);}
Input/Output:
p("Mercury","Mars") --> 170030000
p("Mars","Mercury") --> -170030000
p("Earth","Earth") --> 0
Spaced and tabbed:
void p(String p,String l){
String q="MeVeEaMaJuSaUrNePl";
int w=q.indexOf(p.substring(0,2))/2,
e=q.indexOf(l.substring(0,2))/2,
m=1,
t=e,
d[]={5029,4140,7834,55039,64627,144895,162745,140538};
long h=0;
if(w>e){
e=w;
w=t;
m=-1;
}
for(;e-->w;)
h+=d[e]*1e4;
System.out.print(h*m);
}