Attacking a planet uses up all Bacteria and Nanobots, and resets colonies and bacteria upgrades. Destroying planets unlocks new upgrades and features, such as Colony irradiation or Nanobot production.
A planet counts as destroyed if its remaining population has been reduced to 0. Killing a percentage of a planet's population earns Gene Strands and there is no limit to how many times the player can attack one planet to destroy it. However, it's not possible to attack a planet if the player cannot gain even 1 Gene Strand or if he has less than 1 Bacteria.