Planet busting will depend on the size and mass of the planet. If gravitational binding energy of a planet is sufficient to destroy it (that's from many websites I've found about deathstar energy calcs, don't know if it's necessarily true), then destroying earth requires 2.24E+39 joules of energy to destroy (total destruction).
This comes from a calculation for the energy needed to destroy a solar system and i don't think the link works directly so it will be the first link in this google search (madsci.org):
Okay I didn't expect the link to be that long so I spoilered it.
Using one gram of TNT to equal 4184 joules exactly (wiki TNT equivalent), then in bomb terms, what's needed to essentially vaporize earth is 5.3537285e+23 megatons of TNT or ~9.7 sextillion (21 zeros) Tsar Bombas, the strongest nuclear bomb ever detonated.


@ZomB
Don't exactly know how the bullet works, so I'll assume it's something like a heat seeking missile, except just a projectile.
Actually the more I think about it, the less calculable it is as it's very vague as to how it works, swerving and maneuvering, whether it pierces through shear speed or force...
I'd guesstimate that it has the same energy as 45 armor piercing rounds. Although it might be piercing those people more with heat than speed/force...