Particulate Weapon of Absolute Decimation:

Fill a dozen barrels with sand and put it on a ship that can travel AT or NEAR, or a FRACTION of the speed of light. Leave the barrels with open lids so the sand is free to fly out once it's ready. You take the sand and tie it to the front so it doesn't accidentally hit the ship.

Then, you speed the ship RIGHT the hell up and aim yourself at your target. Once you're there, slow down even a single fraction of the speed you're going. The sand is sent FLYING at the speed of light, near light speed, or a fraction of the speed of light. Here's the math:

The speed of light is 1,079,252,848.8 km/hr/299792.458 km/s. A .5 mg grain of sand moving at that speed would have 269,813,212,200 j of force.

For Comparison, a single ton of TNT: 4,184,000,000 j.

Each grain of sand would have more power than 64 tons of TNT. And there's 12 drums full. Like oil drums. Let's say 55 gallon drums, alright. That's 800lbs. Each grain of sand is .5 mg. 

The amount of damage that it could do to a planet is no less than complete destruction.

If my math holds up properly.