function sendMinersToPlanet(uint numMiners) public payable {
    require(msg.value >= numMiners * PRICE_TO_MINE, "Not enough paid");
    require(planetPopulation < PLANET_CAPACITY, "Planet is full");
    mint(msg.sender, numMiners);
    for (uint i = 0; i < numMiners; i++) {
      sendSingleMinerToPlanet(msg.sender);
    }
  }