Um, why are you wasting a resource? Conquer them, garrison them and let them pay taxes!
Anyway, to your question:
Lasers (and Particle Beams, for that matter) work for planetary bombardement, but their damage is reduced by the atmosphere (Actual Damage = Weapons Damage x (1 - Atmosphere Density), rounded down.
Example: A laser dealing 4 pts of damage will only do 2 pts of damage to a ground target through a 0.5 atmosphere (4 x 0.5 = 2), if the atmosphere would be 0.6, it would only do 1 pt of damage (4 x 0.4 = 1.6, rounded down to 1) This also means, atmospheres of 1.0+ make lasers/Particle Beams useless.
Mesons and Gauss are useless in a bombardement role (gauss, because even a very thin atmosphere reduces their damage to 0 and mesons can´t be used against ground targets at all)
Missiles work best, but leave you with a radioactive wasteland buried under meters of snow (nuclear winter